Argo Group International (ARGO-PA) 10-K 27 Feb 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Feb. 21, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Trading Symbol	AGII
Entity Registrant Name	Argo Group International Holdings, Ltd.
Entity Central Index Key	1,091,748
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		29,360,313
Entity Public Float			$ 1,787.4

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets
Fixed maturities available-for-sale, at fair value (cost: 2017 - $3,320.6; 2016 - $2,938.8)	$ 3,343.4	$ 2,932.4
Equity securities available-for-sale, at fair value (cost: 2017 - $338.2; 2016 - $335.2)	487.4	447.4
Other investments (cost: 2017 - $534.1; 2016 - $527.6)	543.6	535
Short-term investments, at fair value (cost: 2017 - $368.5; 2016 - $405.5)	368.5	405.5
Total investments	4,742.9	4,320.3
Cash	176.6	86
Accrued investment income	23.5	20.7
Premiums receivable	598.6	463.8
Reinsurance recoverables	2,093.3	1,385.6
Goodwill	161.4	152.2
Intangible assets, net of accumulated amortization	96.8	67.7
Current income taxes receivable, net	1.4	0
Deferred acquisition costs, net	160.4	139.1
Ceded unearned premiums	399.5	302.8
Other assets	309.6	266.8
Total assets	8,764	7,205
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	4,201	3,350.8
Unearned premiums	1,207.7	970
Accrued underwriting expenses	115.3	115
Ceded reinsurance payable, net	734	466.6
Funds held	42.7	77.1
Senior unsecured fixed rate notes	139.6	139.5
Other indebtedness	184.5	55.4
Junior subordinated debentures	256.6	172.7
Current income taxes payable, net	0	8.1
Deferred tax liabilities, net	31.3	24.1
Other liabilities	31.6	33
Total liabilities	6,944.3	5,412.3
Commitments and contingencies (Note 17)	0	0
Shareholders' equity:
Common shares - $1.00 par, 500,000,000 shares authorized; 40,385,309 and 40,042,330 shares issued at December 31, 2017 and December 31, 2016, respectively	40.4	40
Additional paid-in capital	1,129.1	1,123.3
Treasury shares (10,785,007 and 10,028,755 shares at December 31, 2017 and December 31, 2016, respectively)	(423.4)	(378.2)
Retained earnings	977	959.9
Accumulated other comprehensive income, net of taxes	96.6	47.7
Total shareholders' equity	1,819.7	1,792.7
Total liabilities and shareholders' equity	$ 8,764	$ 7,205

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Available-for-sale cost	$ 3,320.6	$ 2,938.8
Equity securities cost	338.2	335.2
Other investments cost	534.1	527.6
Short-term investments, Cost	$ 368.5	$ 405.5
Common shares, par value	$ 1	$ 1
Common shares, shares authorized	500,000,000	500,000,000
Common shares, shares issued	40,385,309	40,042,330
Treasury shares, shares	10,785,007	10,028,755

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Premiums and other revenue:
Earned premiums	$ 1,572.3		$ 1,410.8		$ 1,371.9
Net investment income	140		115.1		88.6
Fee and other income	22.5		24.5		22.2
Net realized investment and other gains	39.3		26.1		24.1
Total revenue	1,774.1		1,576.5		1,506.8
Expenses:
Losses and loss adjustment expenses	1,050.2		810.1		766.1
Underwriting, acquisition and insurance expenses	635.4		547		536.7
Interest expense	27.7		19.6		19
Fee and other expense	14.6		22.4		25.8
Foreign currency exchange losses (gains)	6.3		(4.5)		(18.3)
Total expenses	1,734.2		1,394.6		1,329.3
Income before income taxes	39.9		181.9		177.5
Income tax (benefit) provision	(10.4)		35.2		14.3
Net income	$ 50.3		$ 146.7		$ 163.2
Net income per common share:
Basic	$ 1.68	[1]	$ 4.86	[1]	$ 5.31
Diluted	1.64	[1]	4.75	[1]	5.20
Dividend declared per common share	$ 1.08		$ 0.86		$ 0.73
Weighted average common shares:
Basic	29,962,524		30,166,440		30,769,089
Diluted	30,757,234		30,845,710		31,385,460
Net realized investment and other gains before other-than-temporary impairment losses	$ 41.8		$ 36.3		$ 36
Other-than-temporary impairment losses recognized in earnings:
Other-than-temporary impairment losses on fixed maturities	(0.8)		(1.7)		(2.2)
Other-than-temporary impairment losses on equity securities	(1.7)		(8.5)		(9.7)
Impairment losses recognized in earnings	(2.5)		(10.2)		(11.9)
Net realized investment and other gains	$ 39.3		$ 26.1		$ 24.1
[1]	Basic and diluted net income per common share are computed independently for each quarter and full year based on the respective average number of common shares outstanding; therefore, the sum of the quarterly net income per common share data may not equal the net income per common share for the year.

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 50.3	$ 146.7	$ 163.2
Other comprehensive income:
Foreign currency translation adjustments	(1.4)	4	(6)
Defined benefit pension plans:
Net gain (loss) arising during the year	1.2	(0.3)	0.4
Unrealized gains (losses) on securities:
Gains (losses) arising during the year	105.7	58.1	(121.1)
Reclassification adjustment for gains included in net income	(41.6)	(23.2)	(7.2)
Other comprehensive income (loss) before tax	63.9	38.6	(133.9)
Defined benefit pension plans:
Net gain (loss) arising during the year	0.4	(0.1)	0.3
Unrealized gains (losses) on securities:
Gains (losses) arising during the year	28	15.7	(31.3)
Reclassification adjustment for gains included in net income	(13.4)	(13.2)	(6.3)
Income tax provision (benefit) related to other comprehensive income	15	2.4	(37.3)
Other comprehensive income (loss), net of tax	48.9	36.2	(96.6)
Comprehensive income	$ 99.2	$ 182.9	$ 66.6

Consolidated Statements of Shar

Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Common Shares [Member]	Additional Paid-In Capital [Member]	Treasury Shares [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Beginning Balance at Dec. 31, 2014	$ 1,646.7	$ 34.3	$ 836.3	$ (301.4)	$ 969.4	$ 108.1
Net income	163.2	0	0	0	163.2	0
Other comprehensive income (loss), net of tax	(96.6)	0	0	0	0	(96.6)
Repurchase of common shares (575,155, 847,111 and 756,252 at a weighted average price of $51.58, $55.64 and $59.76 for 2015, 2016, and 2017 respectively)	(29.7)	0	0	(29.7)	0	0
Activity under stock incentive plans	9.2	0.3	8.9	0	0	0
Retirement of common shares (tax payments on equity compensation)	(4.1)	(0.1)	(4)	0	0	0
Deferred tax - share-based payments	0.6	0	0.6	0	0	0
Employee stock purchase plan	1.5	0	1.5	0	0	0
10% stock dividend	0	2.6	121.6	0	(124.2)	0
Cash dividends declared - common shares	(22.7)	0	0	0	(22.7)	0
Ending Balance at Dec. 31, 2015	1,668.1	37.1	964.9	(331.1)	985.7	11.5
Net income	146.7	0	0	0	146.7	0
Other comprehensive income (loss), net of tax	36.2	0	0	0	0	36.2
Repurchase of common shares (575,155, 847,111 and 756,252 at a weighted average price of $51.58, $55.64 and $59.76 for 2015, 2016, and 2017 respectively)	(47.1)	0	0	(47.1)	0	0
Activity under stock incentive plans	15.4	0.2	15.2	0	0	0
Retirement of common shares (tax payments on equity compensation)	(2.1)	0	(2.1)	0	0	0
Deferred tax - share-based payments	0.7	0	0.7	0	0	0
Employee stock purchase plan	1.4	0	1.4	0	0	0
10% stock dividend	0	2.7	143.2	0	(145.9)	0
Cash dividends declared - common shares	(26.6)	0	0	0	(26.6)	0
Ending Balance at Dec. 31, 2016	1,792.7	40	1,123.3	(378.2)	959.9	47.7
Net income	50.3	0	0	0	50.3	0
Other comprehensive income (loss), net of tax	48.9	0	0	0	0	48.9
Repurchase of common shares (575,155, 847,111 and 756,252 at a weighted average price of $51.58, $55.64 and $59.76 for 2015, 2016, and 2017 respectively)	(45.2)	0	0	(45.2)	0	0
Activity under stock incentive plans	12.4	0.4	12	0	0	0
Retirement of common shares (tax payments on equity compensation)	(8.2)	(0.1)	(8.1)	0	0	0
Employee stock purchase plan	2	0.1	1.9	0	0	0
10% stock dividend	4.4
Cash dividends declared - common shares	(33.2)	0	0	0	(33.2)	0
Ending Balance at Dec. 31, 2017	$ 1,819.7	$ 40.4	$ 1,129.1	$ (423.4)	$ 977	$ 96.6

Consolidated Statements of Sha7

Consolidated Statements of Shareholders' Equity (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Stockholders Equity [Abstract]
Common shares repurchased	756,252	847,111	575,155
Repurchase of common shares, weighted average price	$ 59.76	$ 55.64	$ 51.58
Stock dividend declared		10.00%	10.00%
Cash dividend declared - common shares, per share	$ 1.08	$ 0.86	$ 0.73

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 50.3	$ 146.7	$ 163.2
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Amortization and depreciation	33.8	35.4	38.7
Share-based payments expense	12.3	19.8	29.1
Deferred income tax (benefit) provision, net	(17.9)	(1.1)	8.3
Net realized investment and other gains	(39.3)	(26.1)	(24.1)
Undistributed earnings from alternative investment portfolio	(49.5)	(23.9)	(3)
Loss (gains) on disposals of fixed assets, net	2.1	(0.1)	0.2
Change in:
Accrued investment income	(2.7)	0.9	0.5
Receivables	(602.7)	(318)	(182.6)
Deferred acquisition costs	(11.5)	(6.4)	(8.5)
Ceded unearned premiums	(4.2)	(51.2)	(45.9)
Reserves for losses and loss adjustment expenses	653.9	220.2	94.3
Unearned premiums	85.5	80.1	76.5
Ceded reinsurance payable and funds held	88.8	153.6	157.2
Income taxes	(8.9)	19.9	3.3
Accrued underwriting expenses	(24.4)	(18.9)	(24.6)
Other, net	(0.6)	(48.9)	0.6
Cash provided by operating activities	165	182	283.2
Cash flows from investing activities:
Sales of fixed maturity investments	1,433.3	1,138.4	787.5
Maturities and mandatory calls of fixed maturity investments	678.3	1,002.7	843.9
Sales of equity securities	201.1	208.3	122
Sales of other investments	95.5	96.8	58.4
Purchases of fixed maturity investments	(2,464.1)	(2,137.3)	(1,842.8)
Purchases of equity securities	(157.7)	(146.9)	(114.1)
Purchases of other investments	(39)	(96.3)	(77.2)
Change in foreign regulatory deposits and voluntary pools	(7.2)	(1.3)	3.5
Change in short-term investments	306.7	(193.9)	46.1
Settlements of foreign currency exchange forward contracts	(2.9)	(5.4)	(10.1)
Acquisition of subsidiaries, net of cash acquired	(105.2)	0	0
Purchases of fixed assets	(30.6)	(32.1)	(18.9)
Other, net	(29.5)	21.9	8.1
Cash used in investing activities	(121.3)	(145.1)	(193.6)
Cash flows from financing activities:
Additional long-term borrowings	125	0	0
Activity under stock incentive plans	1.4	1	1.8
Repurchase of Company's common shares	(45.2)	(47.1)	(29.7)
Payment of cash dividends to common shareholders	(33.2)	(26.6)	(22.7)
Cash provided by (used in) financing activities	48	(72.7)	(50.6)
Effect of exchange rate changes on cash	(1.1)	0.1	1.7
Change in cash	90.6	(35.7)	40.7
Cash, beginning of year	86	121.7	81
Cash, end of period	$ 176.6	$ 86	$ 121.7

Business and Significant Accoun

Business and Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Business and Significant Accounting Policies	1. Business and Significant Accounting Policies Business Argo Group International Holdings, Ltd. (“Argo Group,” “we” or the “Company”) is an international underwriter of specialty insurance and reinsurance products in the property and casualty market. Argo Group US, Inc. (“Argo Group US”) is a subsidiary of Argo Financial Holding (Ireland) UC (“Argo Ireland”). Argo Underwriting Agency Limited (“Syndicate 1200”) is a subsidiary of Argo International Holdings, Ltd. Argo Re, Ltd. (“Argo Re”), a Bermuda based company, is the parent of both Argo Ireland and Argo International Holdings, Ltd. Argo Re is directly owned by Argo Group. Effective February 6, 2017, we acquired all of the issued and outstanding capital stock of Maybrooke Holdings, S.A. (“Maybrooke”), a holding company, and its subsidiaries, which operates under the name “Ariel Re.” As of December 31, 2017, Ariel Reinsurance, Ltd. was dissolved and merged into Argo Re, unifying the two reinsurance platforms as one functional unit. See Note 2, “Acquisition of Maybrooke” for additional detail related to the acquisition. During the first quarter of 2017, we evaluated and modified the presentation of our reportable segments to better reflect our new operating framework and management structure. Under this model, Argo Group’s chief operating decision maker – Mark E. Watson III, President and Chief Executive Officer – evaluates performance and allocates resources based on the review of the U.S. Operations and the International Operations. The business unit that produces the risk and not the location of the underlying exposure is the primary characteristic in distinguishing operating and reportable segments. For example, a U.S. property exposure underwritten through our Syndicate platform would be included in International Operations. Consistent with prior periods, the Run-off Lines and Corporate segments include all other activity of Argo Group and are included in our consolidated financial results. Segment results for the years ended December 31, 2016 and 2015 have been reclassified to conform to the current presentation. The two segments are further described below. U.S. Operations is comprised of the former Excess and Surplus Lines and Commercial Specialty reporting segments. Excess and Surplus Lines products are underwritten by Colony Insurance Company (“Colony”). Commercial Specialty represents our admitted specialty businesses and consists of the following operations: Argo Insurance, Rockwood Casualty Insurance Company (“Rockwood”), Argo Pro, Argo Surety, Trident Insurance Services, Alteris and ARIS Title Insurance Corporation (“ARIS”). International Operations is comprised of the former Syndicate 1200 and International Specialty reporting segments, and the recently acquired Ariel Re business. International Specialty products are provided by our Bermuda operations, which include Argo Re and Argo Insurance – Casualty and Professional Lines and Argo Seguros Brasil S.A. based in Brazil. Syndicate 1200 products are underwritten by Argo Underwriting Agency Limited based in London, on behalf of one underwriting syndicate under the Lloyd’s of London (“Lloyd’s”) global franchise. Ariel Re products are primarily underwritten by Ariel Corporate Member Limited in London, on behalf of Syndicate 1910 under Lloyd’s. Our Run-off Lines Basis of Presentation and Use of Estimates The consolidated financial statements of Argo Group and its subsidiaries have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”). The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenues and expenses during the reporting period. The major estimates reflected in our consolidated financial statements include, but are not limited to, reserves for losses and loss adjustment expenses; reinsurance recoverables, including the reinsurance recoverables allowance for doubtful accounts; estimates of written and earned premiums; reinsurance premium receivable; fair value of investments and assessment of potential impairment; valuation of goodwill and other intangibles and our deferred tax asset valuation allowance. Actual results could differ from those estimates. Specifically, estimates for reserves for losses and loss adjustment expenses are based upon past claim experience modified for current trends as well as prevailing economic, legal and social conditions. Although management believes that amounts included in the accompanying consolidated financial statements are reasonable, such estimates may be more or less than the amounts ultimately paid when the claims are settled. The estimates are continually reviewed and any changes are reflected in current operating results. Further, the nature of loss exposures involves significant variability due to the nature of the long-tailed payments on certain claims. As such, losses and loss adjustment expenses could vary significantly from the recorded amounts. The consolidated financial statements include the accounts and operations of Argo Group and its subsidiaries. All material intercompany accounts and transactions have been eliminated. Certain amounts in prior years’ financial statements have been reclassified to conform to the current presentation. Amounts related to trade capital providers, who are third-party capital participants that provide underwriting capital to the Syndicate 1200 segment, are included in the balance sheet. Trade capital providers participate on a quota share basis, assuming 100% of their contractual participation in the underwriting syndicate results and with such results settled on a year of account basis. We have evaluated our investment in our twelve statutory trusts (collectively, the “Trusts”) and two charitable foundations (collectively, the “Foundations”) under the Financial Accounting Standards Board’s (“FASB’s”) provisions for consolidation of variable interest entities under Accounting Standards Codification (“ASC”) Topic 810-10, “Consolidation,” as amended. We determined that the Trusts and Foundations are variable interest entities due to the fact that the Trusts and Foundations do not have sufficient equity to finance their activities without additional subordinate financial support from other parties. We do not have any power to direct the activities that impact the Trusts’ or Foundations’ economic performance. We are not entitled to receive a majority of the residual returns of the Trusts and U.S. charitable foundations. Additionally, we are not responsible for absorbing the majority of the expected losses of the Trusts or U.S. charitable foundations; therefore, we are not the primary beneficiary and, accordingly, the Trusts and U.S. charitable foundations are not included in our consolidated financial statements. The expenses and donations of the charitable foundations in Bermuda are paid by Argo Group and have been included in the consolidated results. We have used a series of special purpose reinsurance companies to provide reinsurance coverage through a series of transactions, including insurance-linked securities. Under the provisions of ASC Topic 810-10, these reinsurance companies are variable interest entities. However, we do not have a variable interest in these entities, and therefore are not required to consolidate them in our consolidated financial statements. Stock Dividends On February 20, 2018, our Board of Directors declared a 15% stock dividend, payable on March 21, 2018, to shareholders of record at the close of business on March 7, 2018. The share numbers and per share amounts disclosed in the Consolidated Financial Statements and Notes to the Consolidated Financial Statements have not been retroactively adjusted to give effect to the stock dividend. For additional details regarding the stock dividend, see Note 25, “Subsequent Event.” On May 3, 2016, our Board of Directors declared a 10% stock dividend, payable on June 15, 2016, to shareholders of record at the close of business on June 1, 2016. As a result of the stock dividend, 2,735,542 additional shares were issued. Cash was paid in lieu of fractional shares of our common shares. All references to share and per share amounts in this document and related disclosures have been adjusted to reflect the stock dividend for all periods presented. On February 17, 2015, our Board of Directors declared a 10% stock dividend, payable on March 16, 2015, to shareholders of record at the close of business on March 2, 2015. As a result of the stock dividend, 2,554,506 additional shares were issued. Cash was paid in lieu of fractional shares of our common shares. All references to share and per share amounts in this document and related disclosures have been adjusted to reflect the stock dividend for all periods presented. Cash Cash consists of cash deposited in banks, generally in concentration and operating accounts. Interest-bearing cash accounts are classified as short-term investments. Investments Investments in fixed maturities at December 31, 2017 and 2016 include bonds and structured securities. Equity securities include common stocks, preferred stocks and mutual funds. Other investments consist of foreign regulatory deposits, hedge funds, private equity funds, private equity direct investments, and voluntary pools. Short-term investments consist of money market funds, certificates of deposit, bonds, sovereign debt and interest-bearing cash accounts. Investments maturing in less than one year are classified as short-term investments in our consolidated financial statements. The amortized cost of fixed maturity securities is adjusted for amortization of premiums and accretion of discounts. This amortization or accretion is included in “Net investment income” in our Consolidated Statements of Income. For the structured securities portion of the fixed maturity securities portfolio, we recognize income using a constant effective yield based on anticipated prepayments and the estimated economic life of the securities. Premium or discount on high investment grade securities (rated AA or higher) is amortized into income using the retrospective method. Premium or discount on lower grade securities (rated less than AA) is amortized into income using the prospective method. Our investments in fixed maturities and equity securities are considered available-for-sale and are carried at fair value. Changes in the fair value of investments classified as available-for-sale are not recognized in income during the period, but rather are recognized as a separate component of shareholders’ equity until realized. Fair value of these investments is estimated using prices obtained from third-party pricing services, where available. For securities where we were unable to obtain fair values from a pricing service or broker, fair values were estimated using information obtained from investment advisors. We performed several processes to ascertain the reasonableness of these investment values by i) obtaining and reviewing internal control reports for our service providers that obtain fair values from third-party pricing services, ii) discussing with our investment managers their process for reviewing and validating pricing obtained from outside services and obtaining values for all securities from our investment managers and iii) comparing the security pricing received from the investment managers with the prices used in the consolidated financial statements and obtaining additional information for variances that exceeded a certain threshold. As of December 31, 2017, investments we hold for which we did not receive a fair value from a pricing service or broker accounted for less than 1% of our investment portfolio. The actual value at which these securities could actually be sold or settled with a willing buyer or seller may differ from our estimated fair values depending on a number of factors including, but not limited to, current and future economic conditions, quantity sold or settled, presence of an active market and availability of a willing buyer or seller. The cost of securities sold is based on the specific identification method. Changes in the value of other investments consisting of hedge funds, private equity funds, private equity direct investments and voluntary pools are principally recognized to income during the period using the equity method of accounting. Our foreign regulatory deposits are assets held in trust in jurisdictions where there is a legal and regulatory requirement to maintain funds locally in order to protect policyholders. Lloyd’s is the appointed investment manager for the funds. The underlying assets are invested in government securities, agency securities and corporate bonds whose values are obtained from Lloyd’s. Foreign currency future contracts held by us are valued by our counterparties using market driven foreign currency exchange rates. We regularly evaluate our investments for other-than-temporary impairment. For fixed maturity securities, the evaluation for a credit loss is generally based on the present value of expected cash flows of the security as compared to the amortized book value. For structured securities, frequency and severity of loss inputs are used in projecting future cash flows of the securities. Loss frequency is measured as the credit default rate, which includes such factors as loan-to-value ratios and credit scores of borrowers. Loss severity includes such factors as trends in real estate values and proceeds at foreclosure. We also recognize other-than-temporary losses on our fixed maturity securities that we intend to sell. All investment balances include amounts relating to trade capital providers. The results of operations and other comprehensive income exclude amounts relating to trade capital providers. Trade capital providers’ participation in the syndicate results are included in reinsurance recoverable for ceded losses and reinsurance payable for ceded premiums. Receivables Premiums receivable, representing amounts due from insureds, are presented net of an allowance for doubtful accounts. The allowances for doubtful accounts were $0.6 million and $2.7 million at December 31, 2017 and 2016, respectively. Premiums receivable include amounts relating to the trade capital providers’ quota share. Reinsurance recoverables represent amounts of paid losses and loss adjustment expenses, case reserves and incurred but not reported (“IBNR”) amounts ceded to reinsurers under reinsurance treaties. Reinsurance recoverables also reflect amounts that are due from trade capital providers. Reinsurance recoverables are presented in our Consolidated Balance Sheets net of an allowance for doubtful accounts of $2.1 million and $2.1 million at December 31, 2017 and 2016, respectively (see Note 4, “Reinsurance” for related disclosures). An estimate of amounts that are likely to be charged off is established as an allowance for doubtful accounts as of the balance sheet date. Our estimate includes specific insured and reinsurance balances that are considered probable to be charged off after all collection efforts have ceased and in accordance with historical write-off trends based on aging categories. Premiums receivable and reinsurance recoverables on paid losses written off, net of recoveries against the allowance for doubtful accounts or directly to the income statement are as follows: For the Years Ended December 31, (in millions) 2017 2016 2015 Premiums receivable $ 1.5 $ 1.1 $ 1.0 Reinsurance recoverables — — 0.2 Net written off $ 1.5 $ 1.1 $ 1.2 Recoveries occur when subsequent collection or litigation results in the receipt of amounts previously written off. Amounts recovered are applied against the bad debt expense account. Earned Premiums Premium revenue is generally recognized ratably over the policy period. Premiums that have yet to be earned are reported as “Unearned premiums” in our Consolidated Balance Sheets. Unearned premium balances include cessions to reinsurers including trade capital providers, while the earned premium recognized in our Consolidated Statements of Income excludes amounts relating to trade capital providers. The trade capital providers’ quota share amount is included in “Ceded reinsurance payable, net”. Assumed reinstatement premiums that reinstate coverage are written and earned at the time the associated loss event occurs. The original premium is earned over the remaining exposure period of the contract. Reinstatement premiums are estimated based upon contract terms for reported losses and estimated for incurred but not reported losses. Retrospectively Rated Policies We have written a number of workers compensation, property and other liability policies that are retrospectively rated. Under this type of policy, the policyholder or coverholder may be entitled, subsequent to coverage expiration, to a refund or may owe additional premiums based on the amount of losses incurred under the policy. The retrospective premium adjustments on certain policies are limited to a minimum or maximum premium adjustment, which is calculated as a percentage of the standard amount of premium charged during the life of the policy. Accrued retrospectively rated premiums have been determined based on estimated ultimate loss experience of the individual policyholder accounts. The estimated liability for return of premiums under retrospectively rated policies is included in “Unearned premiums” in our Consolidated Balance Sheets and was $7.9 million and $5.7 million at December 31, 2017 and 2016, respectively. The estimated amount included in premiums receivables for additional premiums due under retrospectively rated policies was $0.1 million and $0.1 million at December 31, 2017 and 2016, respectively. Deferred Acquisition Costs Policy acquisition costs, which include commissions, premium taxes, fees and certain other costs of underwriting policies, are deferred, when such class of policies are profitable, and amortized over the same period in which the related premiums are earned. To qualify for capitalization, the policy acquisition cost must be directly related to the successful acquisition of an insurance contract. Anticipated investment income is considered in determining whether the deferred acquisition costs are recoverable and whether a premium deficiency exists. We continually review the methods of making such estimates and establishing the deferred costs with any adjustments made in the accounting period in which the adjustment arose. The 2017 and 2016 net amortization of policy acquisition costs will not equal the change in our Consolidated Balance Sheets as the trade capital providers’ share is not reflected in our Consolidated Statements of Income and differences arise from foreign currency exchange rates applied to deferred acquisition costs which are treated as a nonmonetary asset. Reserves for Losses and Loss Adjustment Expenses Liabilities for unpaid losses and loss adjustment expenses include the accumulation of individual case estimates for claims reported as well as estimates of IBNR claims and estimates of claim settlement expenses. Reinsurance recoverables on unpaid claims and claim expenses represent estimates of the portion of such liabilities that will be recoverable from reinsurers. Amounts recoverable from reinsurers are recognized as assets at the same time and in a manner consistent with the unpaid claims liabilities associated with the reinsurance policy. Reinsurance In the normal course of business, our insurance and reinsurance subsidiaries cede risks above certain retention levels to other insurance companies. Reinsurance recoverables include claims we paid and estimates of unpaid losses and loss adjustment expenses that are subject to reimbursement under reinsurance and retrocessional contracts. The method for determining reinsurance recoverables for unpaid losses and loss adjustment expenses involves reviewing actuarial estimates of gross unpaid losses and loss adjustment expenses to determine our ability to cede unpaid losses and loss adjustment expenses under our existing reinsurance contracts. This method is continually reviewed and updated and any resulting adjustments are reflected in earnings in the period identified. Reinsurance premiums, commissions and expense reimbursements are accounted for on a basis consistent with those used in accounting for the original policies issued and the term of the reinsurance contracts. Amounts recoverable from reinsurers for losses and loss adjustment expenses for which our insurance and reinsurance subsidiaries have not been relieved of their legal obligations to the policyholder are reported as assets. Goodwill and Intangible Assets Goodwill and intangible assets are allocated to the segment in which the results of operations for the acquired company are reported (see Note 19, “Segment Information” for further discussion). Intangible assets with a finite life are amortized over the estimated useful life of the asset. Goodwill and intangible assets with an indefinite useful life are not amortized. Goodwill and intangible assets are tested for impairment on an annual basis or more frequently if events or changes in circumstances indicate that the carrying amount may not be recoverable. For goodwill, we may perform a qualitative test to determine whether it is more-likely-than-not that the fair value of a reporting unit is less than its carrying amount. A quantitative impairment test was performed in 2017. The first step of the quantitative test is to identify if a potential impairment exists by comparing the fair value of a reporting unit with its carrying amount, including goodwill (“Step 1”). If the fair value of a reporting unit exceeds its carrying value amount, goodwill of the reporting unit is not considered to have a potential impairment and the second step is not necessary. However, if the carrying amount of the reporting unit exceeds its fair value, the second step (“Step 2”) is performed to determine if goodwill is impaired and to measure the amount of impairment loss to recognize, if any. Step 2 compares the implied fair value of goodwill with the carrying amount of goodwill. If the implied value of goodwill is less than the carrying amount of goodwill, it is written down to its fair value with a corresponding expense reflected in the Consolidated Statements of Income. The implied goodwill is calculated based on a hypothetical purchase price allocation, similar to the requirements in the accounting guidance for business combinations, whereby the implied fair value of the reporting unit is allocated to the fair value of the assets and liabilities of the reporting unit. We perform our goodwill impairment test on the first day of the fourth quarter of each year, or October 1 of each year. In performing Step 1 of the impairment test, we estimated the fair value of reporting units using an average of three valuation methods: a comparable company analysis, a precedent transaction analysis and a discounted cash flow analysis. All three methods require management to make various judgments and assumptions. The discounted cash flow analysis included projections of earned premiums, loss ratios, expense growth and discount rates for each reporting unit. Assumptions about such future cash flows are based on our budgets, business plans, economic projections, anticipated future cash flows and market data. Finally, the comparable company analysis and precedent transaction analysis required judgment in selecting comparable companies and comparable transactions for use in the calculations. In all instances, future changes in these judgments and assumptions could cause impairment of goodwill. For the years ended December 31, 2017, 2016 and 2015, all of our reporting units passed Step 1 of the goodwill impairment analysis as the fair value of each reporting unit were in excess of their carrying values. Therefore, Step 2 of the goodwill impairment analysis was not required. Any future decline in the fair value of these reporting units could result in the carrying value of the reporting unit being in excess of fair value, triggering Step 2 of the impairment testing model, which could result in an impairment of goodwill. As noted above, we have elected to make the first day of the fourth quarter the annual impairment assessment date for goodwill and indefinite-lived intangible assets. A quantitative impairment analysis subsequent to this date has not been performed as we have concluded that no additional indicators of impairment have arisen through December 31, 2017. The following table presents our intangible assets and accumulated amortization at December 31: December 31, 2017 December 31, 2016 (in millions) Gross Carrying Amount Accumulated Amortization Gross Carrying Amount Accumulated Amortization Lloyd's capacity $ 89.0 n/a $ 60.5 n/a Distribution network 50.2 44.1 45.5 38.5 Additional Lloyd's capacity 4.8 4.8 4.8 4.8 Other 3.3 1.6 1.4 1.2 $ 147.3 $ 50.5 $ 112.2 $ 44.5 Included in the table above is the $35.1 million of intangible assets acquired from the acquisition of Maybrooke (see Note 2, “Acquisition of Maybrooke”). The weighted average useful life by category at December 31, 2017 was 8.9 years for the distribution network, 5.0 years for the additional Lloyd’s capacity and 6.4 years for other. The weighted average useful life for all categories was 8.5 years at December 31, 2017. During the years ended December 31, 2017, 2016 and 2015, amortization expense was $5.9 million, $5.5 million and $7.5 million, respectively, and is included in “Underwriting, acquisition and insurance expenses” in our Consolidated Statements of Income . The estimated amortization expense for the years ended December 31, 2018, 2019, 2020, 2021 and 2022 is $3.5 million, $1.7 million, $1.3 million, $1.3 million and $0.1 million, respectively. Property and Equipment Property and equipment used in operations, including certain costs incurred to develop or obtain computer software for internal use, are capitalized and carried at cost less accumulated depreciation and are reported in “Other assets” in our Consolidated Balance Sheets. Depreciation is calculated using a straight-line method over the estimated useful lives of the assets, generally three to thirty nine years. The accumulated depreciation for property and equipment was $124.1 million and $101.6 million at December 31, 2017 and 2016, respectively. The net book value of our property and equipment at December 31, 2017 and 2016 was $151.0 million and $142.2 million, respectively. The depreciation expense at December 31, 2017, 2016 and 2015 was $24.1 million, $20.9 million and $17.5 million, respectively. Derivative Instruments We enter into short-term, currency spot and forward contracts to mitigate foreign exchange rate exposure in our non-U.S. Dollar denominated fixed maturity investments. The forward contracts used are typically thirty to ninety days and are renewed as long as the non-U.S. Dollar denominated fixed maturity investments are held in our portfolio. Forward contracts are designated as hedges for accounting purposes. We also enter into foreign currency exchange forward contracts to manage currency exposure on losses related to global catastrophe events. These foreign currency forward contracts are carried at fair value in our Consolidated Balance Sheets in “Other liabilities” and “Other assets” at December 31, 2017 and 2016, respectively. The realized and unrealized gains and losses are included in “Net realized investment and other gains” in our Consolidated Statements of Income. Share-Based Payments Compensation expense for share-based payments is recognized based on the measurement-date fair value for awards that will settle in shares. Awards that are expected to be settled in cash are accounted for as liability awards, resulting in the fair value of the award being measured at each reporting date until the award is exercised, forfeited or expires unexercised. Compensation expense for awards that are settled in equity are recognized on a straight line pro rata basis over the vesting period, adjusted for expected forfeitures. See Note 13, “Share-based Compensation” for related disclosures. Foreign Currency Exchange Gain (Loss) The U.S. Dollar is the functional currency of all but two of our foreign operations. Monetary assets and liabilities in foreign operations that are denominated in foreign currencies are revalued at the exchange rates in effect at the balance sheet date. The resulting gains and losses from changes in the foreign exchange rates are reflected in net income. Revenues and expenses denominated in foreign currencies are translated at the prevailing exchange rate during the period with the resulting foreign exchange gains and losses included in net income for the period. In the case of our foreign currency denominated available-for-sale investments, the change in exchange rates between the local currency and our functional currency at each balance sheet date represents an unrealized appreciation or depreciation in value of these securities and is included as a component of accumulated other comprehensive gain. Translation gains and losses related to our operations in Brazil and Malta are recorded as a component of shareholders’ equity in our Consolidated Balance Sheets. At December 31, 2017 and 2016, the foreign currency translation adjustments were a loss of $19.0 million and $17.6 million, respectively. Income Taxes On December 22, 2017, the Tax Cuts and Jobs Act (“TCJA”) was enacted. Among many changes resulting from TCJA, the new law (i) reduces the corporate tax rate to 21% effective January 1, 2018, (ii) eliminates the corporate alternative minimum tax for tax years beginning after December 31, 2017, (iii) allows businesses to immediately expense, for tax purposes, the cost of new investments in certain qualified depreciable assets, (iv) modifies the computation of loss reserve discounting for tax purposes, (v) modifies the recognition of income rules by requiring the recognition of income for certain items no later than the tax year in which an item is taken into account as income on an applicable financial statement and (vi) significantly modifies the United States international tax system. Deferred tax assets and liabilities are recognized for the estimated future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates in effect for the year in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in net income in the period in which the change is enacted. We recognize potential accrued interest and penalties within our global operations in “Interest expense” and “Underwriting, acquisition and insurance expenses,” respectively, in our Consolidated Statements of Income related to unrecognized tax benefits. Supplemental Cash Flow Information Income taxes paid. We paid income taxes of $16.5 million, $16.6 million and $10.9 million in 2017, 2016 and 2015, respectively. Income taxes recovered. We recovered income taxes of $2.5 million, $0.5 million and $11.7 million in 2017, 2016 and 2015, respectively. Interest paid as follows: For the Years Ended December 31, (in millions) 2017 2016 2015 Senior unsecured fixed rate notes $ 9.3 $ 9.3 $ 9.3 Junior subordinated debentures 12.6 7.7 7.0 Other indebtedness 4.9 2.3 2.4 Revolving credit facility 0.3 — — Total interest paid $ 27.1 $ 19.3 $ 18.7 Recently Issued Accounting Pronouncements On February 14, 2018, the Financial Accounting Standards Board (“FASB”) On December 22, 2017, the SEC staff issued Staff Accounting Bulletin No. 118 (SAB 118) which provides guidance on accounting for the effects of the TCJA. We have adopted this guidance within our 2017 consolidated financial statements. In May 2017, the FASB issued ASU 2017-09, “Compensation – Stock Compensation” (Topic 718): Scope of Modification Accounting. ASU 2017-09 clarifies when changes to the terms or conditions of a share-based payment award must be accounted for as modifications. The guidance requires entities to apply the modification accounting guidance if the value, vesting conditions or classification of the

Acquisition of Maybrooke

Acquisition of Maybrooke	12 Months Ended
	Dec. 31, 2017
Business Combinations [Abstract]
Acquisition of Maybrooke	2. Acquisition of Maybrooke Effective February 6, 2017, we acquired all of the issued and outstanding capital stock of Maybrooke, a holding company, and its subsidiaries, which operates under the name “Ariel Re.” The purchase price of $235.3 million was paid in cash from funds on hand and available under our credit facility (see Note 8, “Other Indebtedness”). Ariel Re is a global underwriter of specialty insurance and reinsurance business written primarily through its Lloyd’s Syndicate 1910. Ariel Re provides Argo Group with a number of strategic advantages, including enhanced scale in its London- and Bermuda-based platforms. Subsequently, we placed Maybrooke into liquidation and began to collapse its subsidiaries into our structure to create operating efficiencies, including merging Ariel Reinsurance, Ltd. into Argo Re, Ltd. effective December 31, 2017. The restructuring is expected to be completed in the first half of 2018. The acquisition is being accounted for in accordance with ASC 805, “Business Combinations.” Purchase accounting, as defined by ASC 805, requires that the assets acquired and liabilities assumed be recognized at their fair values as of the acquisition date. The fair values disclosed herein were determined based on management’s best estimates and the finalization of certain valuation analyses during the fourth quarter of 2017. Provisional fair values were recorded in the Company’s interim consolidate financial statements and notes for the periods ending March 31, June 30 and September 30, 2017. The following table presents the fair value of the assets acquired and liabilities assumed relating to the acquisition of Maybrooke: (in millions) Assets: Investments $ 340.7 Cash 130.1 Premiums receivable 157.6 Reinsurance balances 172.8 Other assets 35.6 Total assets 836.8 Liabilities: Reserves for losses and loss adjustment expenses 197.0 Unearned premiums 152.5 Ceded reinsurance payable, net 144.2 Junior subordinated debentures 83.6 Accrued underwriting expenses 29.0 Other liabilities 39.5 Total liabilities 645.8 Net assets acquired 191.0 Purchase price 235.3 Excess purchase price $ 44.3 The excess of the purchase price over the fair value of the net assets acquired has been allocated $9.2 million to goodwill and $35.1 million to intangible assets. The goodwill and intangible assets are included in the International Operations segment in Note 19, “Segment Information”. We recorded an amortization expense of $1.2 million related the intangible assets for the year ended December 31, 2017. Goodwill is calculated as the excess of the purchase price over the fair value of the net assets recognized, including identifiable intangible assets. Goodwill is primarily attributable to the anticipated synergies and economies of scale expected from the operations at Lloyd’s and Bermuda, and skilled assembled workforce. Goodwill is not deductible for tax purposes. A summary of the acquired intangible assets is shown below: Fair Value (in millions) Estimated Useful Life Stamp capacity $ 28.5 n/a Distribution network 4.7 5 years Trade name 1.9 5 years Subtotal - intangible assets 35.1 Goodwill 9.2 n/a Total goodwill and intangible assets $ 44.3 Through the acquisition of Maybrooke, we assumed unsecured junior subordinated debentures with a principal balance of $91.8 million (“the Maybrooke debt”). The Maybrooke debt is carried on our consolidated balance sheet at $83.9 million, which represents the debt’s fair value of $83.6 million at the date of acquisition plus accumulated accretion of discount to par value (see Note 7, “Junior Subordinated Debentures”). We recognized approximately $2.5 million of acquisition-related transaction costs in our Consolidated Statements of Income, all of which were recorded during the first quarter of 2017. Of these amounts, $2.2 million were reported in “Underwriting, acquisition and insurance expenses” and $0.3 million in “Interest expense” related to the borrowings under our credit facility to help fund the acquisition. No material transaction costs were incurred after the first quarter of 2017. Maybrooke’s Contribution to Argo Group’s Revenue and Income The following selected financial information summarizes the results of Maybrooke from the date of acquisition that have been included in our Consolidated Statement of Income: (in millions) For the Year Ended December 31, 2017 Revenues $ 127.6 Net loss $ (13.1 ) Unaudited Pro forma Results of Operations The following unaudited pro forma financial information has been provided to present a summary of the combined results of Argo Group’s operations with Maybrooke’s as if the acquisition had occurred on January 1, 2016. The unaudited pro forma financial information is for informational purposes only and is not necessarily indicative of what the results would have been had the acquisition been completed at the date indicated above, as it may not include all necessary adjustments. Future changes to Maybrooke’s business, such as, but not limited to, the impact from underwriting decisions, changes in risk selection, or retention rates, could result in a material favorable or unfavorable impact on Argo Group’s future results of operations and financial position. The unaudited pro forma results for year ended December 31, 2017 include favorable development from prior accident years of $11.2 million, including $6.2 million relating to one specific claim in January 2017. In addition, the $2.5 million of nonrecurring transaction costs directly attributable to the acquisition for year ended December 31, 2017, as disclosed above, have also been removed from the unaudited pro forma results in the table below. For the Years Ended December 31, (in millions, except per share data) 2017 2016 Pro forma revenues $ 1,787.9 $ 1,793.1 Pro forma net income 58.7 167.9 Pro forma net income per share - basic 1.96 5.57 Pro forma net income per share - diluted 1.91 5.44

Investments

Investments	12 Months Ended
	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Investments	3. Investments Composition of Invested Assets The amortized cost, gross unrealized gains, gross unrealized losses and fair value of investments as of December 31 were as follows: December 31, 2017 (in millions) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Fixed maturities U.S. Governments $ 419.9 $ 0.2 $ 5.0 $ 415.1 Foreign Governments 229.0 6.7 2.5 233.2 Obligations of states and political subdivisions 327.7 9.3 1.1 335.9 Corporate bonds 1,514.5 24.4 13.2 1,525.7 Commercial mortgage-backed securities 136.3 0.1 1.5 134.9 Residential mortgage-backed securities 309.3 2.8 2.7 309.4 Asset-backed securities 161.3 0.7 0.8 161.2 Collateralized loan obligations 222.6 5.9 0.5 228.0 Total fixed maturities 3,320.6 50.1 27.3 3,343.4 Equity securities 338.2 154.0 4.8 487.4 Other investments 534.1 9.6 0.1 543.6 Short-term investments 368.5 — — 368.5 Total investments $ 4,561.4 $ 213.7 $ 32.2 $ 4,742.9 December 31, 2016 (in millions) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Fixed maturities U.S. Governments $ 275.1 $ 0.6 $ 4.5 $ 271.2 Foreign Governments 244.2 1.1 8.0 237.3 Obligations of states and political subdivisions 375.7 8.9 1.8 382.8 Corporate bonds 1,316.9 23.3 19.5 1,320.7 Commercial mortgage-backed securities 154.9 0.4 1.6 153.7 Residential mortgage-backed securities 174.8 3.7 1.7 176.8 Asset-backed securities 127.6 0.1 2.1 125.6 Collateralized loan obligations 269.6 3.8 9.1 264.3 Total fixed maturities 2,938.8 41.9 48.3 2,932.4 Equity securities 335.2 117.9 5.7 447.4 Other investments 527.6 7.5 0.1 535.0 Short-term investments 405.5 — — 405.5 Total investments $ 4,207.1 $ 167.3 $ 54.1 $ 4,320.3 Included in “Total investments” in our Consolidated Balance Sheets at December 31, 2017 and 2016 is $130.8 million and $131.9 million, respectively, of assets managed on behalf of the trade capital providers, who are third-party participants that provide underwriting capital to Syndicate 1200. Contractual Maturity The amortized cost and fair values of fixed maturity investments as of December 31, 2017, by contractual maturity, were as follows: (in millions) Amortized Cost Fair Value Due in one year or less $ 262.3 $ 261.9 Due after one year through five years 1,432.2 1,440.0 Due after five years through ten years 623.3 630.6 Thereafter 173.3 177.4 Structured securities 829.5 833.5 Total $ 3,320.6 $ 3,343.4 The expected maturities may differ from the contractual maturities because debtors may have the right to call or prepay obligations. Other Invested Assets Details regarding the carrying value, redemption characteristics and unfunded investment commitments of the other invested assets portfolio as of December 31, 2017 and 2016 were as follows: December 31, 2017 (in millions) Carrying Value Unfunded Commitments Investment Type Hedge funds $ 163.6 $ — Private equity 179.2 129.9 Long only funds 196.5 — Other investments 4.3 — Total other invested assets $ 543.6 $ 129.9 December 31, 2016 (in millions) Carrying Value Unfunded Commitments Investment Type Hedge funds $ 180.9 $ — Private equity 179.0 93.4 Long only funds 170.7 — Other investments 4.4 — Total other invested assets $ 535.0 $ 93.4 The following describes each investment type: • Hedge funds: Hedge funds include funds that primarily buy and sell stocks including short sales, multi-strategy credit, relative value credit and distressed credit. • Private equity: Private equity includes buyout funds, real asset/infrastructure funds, credit special situations funds, mezzanine lending funds and direct investments and strategic non-controlling minority investments in private companies that are principally accounted for using the equity method of accounting. • Long only funds: Our long only funds include a fund that primarily owns international stocks, funds that primarily own investment-grade corporate and sovereign fixed income securities and participations in investment pools. • Other investments: Other investments include our participation in a trust preferred investment pool and options. Unrealized Losses and Other-than-temporary Impairments An aging of unrealized losses on our investments in fixed maturities, equity securities, other investments and short-term investments is presented below: December 31, 2017 Less Than One Year One Year or Greater Total (in millions) Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses Fixed maturities U.S. Governments $ 313.7 $ 1.9 $ 83.7 $ 3.1 $ 397.4 $ 5.0 Foreign Governments 175.2 2.0 35.9 0.5 211.1 2.5 Obligations of states and political subdivisions 33.3 0.5 22.4 0.6 55.7 1.1 Corporate bonds 674.1 9.9 77.7 3.3 751.8 13.2 Commercial mortgage-backed securities 58.2 0.4 37.8 1.1 96.0 1.5 Residential mortgage-backed securities 164.4 1.6 52.4 1.1 216.8 2.7 Asset-backed securities 85.4 0.4 31.9 0.4 117.3 0.8 Collateralized loan obligations (2) 34.6 0.5 0.9 — 35.5 0.5 Total fixed maturities 1,538.9 17.2 342.7 10.1 1,881.6 27.3 Equity securities 46.9 4.8 — — 46.9 4.8 Other investments — — 0.3 0.1 0.3 0.1 Total $ 1,585.8 $ 22.0 $ 343.0 $ 10.2 $ 1,928.8 $ 32.2 December 31, 2016 Less Than One Year One Year or Greater Total (in millions) Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses Fixed maturities U.S. Governments $ 183.4 $ 4.5 $ — $ — $ 183.4 $ 4.5 Foreign Governments 201.2 8.0 — — 201.2 8.0 Obligations of states and political subdivisions 72.6 1.7 1.8 0.1 74.4 1.8 Corporate bonds 490.5 17.7 50.6 1.8 541.1 19.5 Commercial mortgage-backed securities 70.6 1.5 7.1 0.1 77.7 1.6 Residential mortgage-backed securities (2) 87.5 1.7 4.4 — 91.9 1.7 Asset-backed securities 69.7 1.4 8.2 0.7 77.9 2.1 Collateralized loan obligations 122.5 8.6 16.9 0.5 139.4 9.1 Total fixed maturities 1,298.0 45.1 89.0 3.2 1,387.0 48.3 Equity securities 62.1 5.7 — — 62.1 5.7 Other investments 0.3 0.1 — — 0.3 0.1 Short-term investments (1) 4.8 — — — 4.8 — Total $ 1,365.2 $ 50.9 $ 89.0 $ 3.2 $ 1,454.2 $ 54.1 (1) Unrealized losses less than one year are less than $0.1 million. (2) Unrealized losses one year or greater are less than $0.1 million. We regularly evaluate our investments for other than temporary impairment. For fixed maturity securities, the evaluation for a credit loss is generally based on the present value of expected cash flows of the security as compared to the amortized book value. For structured securities, frequency and severity of loss inputs are used in projecting future cash flows of the securities. Loss frequency is measured as the credit default rate, which includes such factors as loan-to-value ratios and credit scores of borrowers. For equity securities and other investments, the length of time and the amount of decline in fair value are the principal factors in determining other-than-temporary impairment. We also recognize other-than-temporary losses on fixed maturity securities that we intend to sell. We hold a total of 8,084 securities, of which 2,273 were in an unrealized loss position for less than one year and 357 were in an unrealized loss position for a period one year or greater as of December 31, 2017. Unrealized losses greater than twelve months on fixed maturities were the result of a number of factors, including increased credit spreads, foreign currency fluctuations and higher market yields relative to the date the securities were purchased, and for structured securities, by the performance of the underlying collateral as well. In considering whether an investment is other-then-temporarily impaired or not, we also considered that we do not intend to sell the investment and it is unlikely that we will be required to sell the investment before recovery of its amortized cost basis, which may be maturity. In situations where we did not recognize other-than-temporary losses on investments in our equity portfolio, we have evaluated the near-term prospects of the investment in relation to the severity and duration of the impairment and based on that evaluation, have the ability and intent to hold these investments until a recovery of the cost basis. We do not consider these investments to be other-than-temporarily impaired at December 31, 2017. We recognized other-than-temporary losses on our fixed maturities and equity portfolios as follows: For the Years Ended December 31, (in millions) 2017 2016 2015 Other-than-temporary impairment: Obligations of states and political subdivisions $ (0.1 ) $ — $ — Corporate bonds (0.7 ) (1.7 ) (2.2 ) Equity securities (1.7 ) (8.5 ) (9.7 ) Other-than-temporary impairment losses $ (2.5 ) $ (10.2 ) $ (11.9 ) Net Investment Income and Realized Gains and Losses Investment income and expenses were as follows: For the Years Ended December 31, (in millions) 2017 2016 2015 Investment income: Interest on fixed maturities $ 97.1 $ 88.9 $ 78.2 Dividends on equity securities 13.9 15.6 16.3 Income on alternative investments 49.5 29.9 10.0 Income on short-term and other investments 7.7 0.4 0.2 Investment income 168.2 134.8 104.7 Investment expenses (28.2 ) (19.7 ) (16.1 ) Net investment income $ 140.0 $ 115.1 $ 88.6 The following table presents our gross realized investment gains (losses) and other: For the Years Ended December 31, (in millions) 2017 2016 2015 Realized gains Fixed maturities $ 25.7 $ 21.5 $ 12.3 Equity securities 51.3 57.5 40.5 Other investments 25.7 47.5 37.6 Short-term investments 0.7 0.5 1.2 Other assets — 1.2 — Gain on sale of real estate holdings — — 0.3 Gross realized investment and other gains 103.4 128.2 91.9 Realized losses Fixed maturities (20.0 ) (35.9 ) (23.5 ) Equity securities (5.2 ) (9.7 ) (6.6 ) Other investments (36.2 ) (46.0 ) (24.0 ) Short-term investments (0.2 ) (0.3 ) (1.8 ) Other-than-temporary impairment losses on fixed maturities (0.8 ) (1.7 ) (2.2 ) Other-than-temporary impairment losses on equity securities (1.7 ) (8.5 ) (9.7 ) Gross realized investment and other losses (64.1 ) (102.1 ) (67.8 ) Net realized investment and other gains before income taxes 39.3 26.1 24.1 Income tax expense (12.0 ) (11.6 ) (10.0 ) Net realized investment and other gains, net of income taxes $ 27.3 $ 14.5 $ 14.1 Changes in unrealized appreciation (depreciation) related to investments are summarized as follows: For the Years Ended December 31, (in millions) 2017 2016 2015 Change in unrealized gains (losses) Fixed maturities $ 25.4 $ 36.5 $ (65.3 ) Equity securities 36.6 (2.6 ) (64.1 ) Other investments 2.1 0.6 1.5 Short-term investments — 0.4 (0.4 ) Net unrealized investment and other gains (losses) before income taxes 64.1 34.9 (128.3 ) Income tax (expense) benefit (14.6 ) (2.4 ) 37.6 Net unrealized investment and other gains (losses), net of income taxes $ 49.5 $ 32.5 $ (90.7 ) Foreign Currency Exchange Forward Contracts We entered into foreign currency exchange forward contracts to manage operational currency exposure on our Canadian dollar (“CAD”) investment portfolio, minimize negative impacts to investment portfolio returns, and gain exposure to a total return strategy which invests in multiple currencies. The currency forward contracts are carried at fair value in our Consolidated Balance Sheets in “Other liabilities” and “Other assets” at December 31, 2017 and 2016, respectively. The gains and losses are included in “Net realized investment and other gains” in our Consolidated Statements of Income. The fair value of our foreign currency exchange forward contracts as of December 31 was as follows: (in millions) December 31, 2017 December 31, 2016 Operational currency exposure $ (0.2 ) $ — Asset manager investment exposure (0.9 ) 0.7 Total return strategy 0.7 3.3 Total $ (0.4 ) $ 4.0 The following table presents our gross investment realized gains and losses on our foreign currency exchange forward contracts: For the Years Ended December 31, (in millions) 2017 2016 2015 Realized gains Global catastrophe (1) $ — $ — $ 0.5 Operational currency exposure 12.4 10.9 26.1 Asset manager investment exposure 1.5 9.0 8.5 Total return strategy 10.4 25.6 1.4 Gross realized investment gains 24.3 45.5 36.5 Realized losses Global catastrophe — — (2.3 ) Operational currency exposure (13.8 ) (18.0 ) (12.9 ) Asset manager investment exposure (11.3 ) (4.5 ) (2.3 ) Total return strategy (7.6 ) (21.0 ) (3.8 ) Gross realized investment losses (32.7 ) (43.5 ) (21.3 ) Net realized investment (losses) gains on foreign currency exchange forward contracts $ (8.4 ) $ 2.0 $ 15.2 (1) Regulatory Deposits, Pledged Securities and Letters of Credit We are required to maintain assets on deposit with various regulatory authorities to support our insurance and reinsurance operations. We maintain assets pledged as collateral in support of irrevocable letters of credit issued under the terms of certain reinsurance agreements for reported loss and loss expense reserves. The following table presents our components of restricted assets at December 31: (in millions) December 31, 2017 December 31, 2016 Securities on deposit for regulatory and other purposes $ 175.3 $ 168.7 Securities pledged as collateral for letters of credit 78.1 35.9 Securities and cash on deposit supporting Lloyd’s business 355.5 161.8 Total restricted investments $ 608.9 $ 366.4 Fair Value Measurements Fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. Fair value measurement assumes that the transaction to sell the asset or transfer the liability occurs in the principal market for the asset or liability, or in the absence of a principal market, the most advantageous market. Market participants are buyers and sellers in the principal (or most advantageous) market that are independent, knowledgeable, able to transact for the asset or liability and willing to transfer the asset or liability. Valuation techniques consistent with the market and income approach are used to measure fair value. The inputs of these valuation techniques are categorized into three levels. • Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities that can be accessed at the reporting date. We define actively traded as a security that has traded in the past seven days. We receive one quote per instrument for Level 1 inputs. • Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. We receive one quote per instrument for Level 2 inputs. • Level 3 inputs are unobservable inputs. Unobservable inputs reflect our own assumptions about the assumptions market participants would use in pricing the asset or liability based on the best information available in the circumstances. We receive fair value prices from third-party pricing services and our outside investment managers. These prices are determined using observable market information such as dealer quotes, market spreads, cash flows, yield curves, live trading levels, trade execution data, market consensus prepayment speeds, credit information and the security’s terms and conditions, among other things. We have reviewed the processes used by the third-party providers for pricing the securities, and have determined that these processes result in fair values consistent with GAAP requirements. In addition, we review these prices for reasonableness, and have not adjusted any prices received from the third-party providers as of December 31, 2017 and 2016. A description of the valuation techniques we use to measure assets at fair value is as follows: Fixed Maturities (Available-for-Sale) Levels 1 and 2: • United States Treasury securities are typically valued using Level 1 inputs. For these securities, we obtain fair value measurements from third-party pricing services using quoted prices (unadjusted) in active markets at the reporting date. • United States Government agencies, non-U.S. Government securities, obligations of states and political subdivisions, credit securities and foreign denominated government and credit securities are reported at fair value using Level 2 inputs. For these securities, we obtain fair value measurements from third-party pricing services. Observable data may include dealer quotes, market spreads, yield curves, live trading levels, trade execution data, credit information and the security’s terms and conditions, among other things. • Asset and mortgage-backed securities and collateralized loan obligations are reported at fair value using Level 2 inputs. For these securities, we obtain fair value measurements from third-party pricing services. Observable data may include dealer quotes, market spreads, cash flows, yield curves, live trading levels, trade execution data, market consensus prepayment speeds, credit information and the security’s terms and conditions, among other things. Fixed Maturities Level 3: • We own a $1.9 million term loan that is valued using unobservable inputs. Equity Securities Level 1: Equity securities are principally reported at fair value using Level 1 inputs. For these securities, we obtain fair value measurements from a third-party pricing service using quoted prices (unadjusted) in active markets at the reporting date. Equity Securities Level 2: We own interests in a mutual fund that is reported at fair value using Level 2 inputs. The valuation is based on the funds’ net asset value per share, at the end of each month. The underlying assets in the funds are valued primarily on the basis of closing market quotations or official closing prices on each valuation day. Equity Securities Level 3: We own certain equity securities that are reported at fair value using Level 3 inputs. The valuation techniques for these securities include the following: • Fair value measurements obtained from the National Association of Insurance Commissioners’ Security Valuation Office at the reporting date. • Fair value measurements for an investment in an equity fund obtained by applying final prices provided by the administrator of the fund, which is based upon certain estimates and assumptions. • Fair value measurements from a broker and an independent valuation service, both based upon estimates and assumptions. Other Investments Level 2: Foreign regulatory deposits are assets held in trust in jurisdictions where there is a legal and regulatory requirement to maintain funds locally in order to protect policyholders. Lloyd’s is the appointed investment manager for the funds. These assets are invested in short-term government securities, agency securities and corporate bonds and are valued using Level 2 inputs based upon values obtained from Lloyd’s. Short-term Investments: Short-term investments are principally reported at fair value using Level 1 inputs, with the exception of short-term corporate and governmental bonds reported at fair value using Level 2 inputs as described in the fixed maturities section above. Values for the investments categorized as Level 1 are obtained from various financial institutions as of the reporting date. Transfers Between Level 1 and Level 2 Securities: There were no transfers between Level 1 and Level 2 securities during 2017 or 2016. Based on an analysis of the inputs, our financial assets measured at fair value on a recurring basis have been categorized as follows: Fair (in millions) December 31, 2017 Level 1 (a) Level 2 (b) Level 3 (c) Fixed maturities U.S. Governments $ 415.1 $ 410.6 $ 4.5 $ — Foreign Governments 233.2 — 233.2 — Obligations of states and political subdivisions 335.9 — 335.9 — Corporate bonds 1,525.7 — 1,523.8 1.9 Commercial mortgage-backed securities 134.9 — 134.9 — Residential mortgage-backed securities 309.4 — 309.4 — Asset-backed securities 161.2 — 161.2 — Collateralized loan obligations 228.0 — 228.0 — Total fixed maturities 3,343.4 410.6 2,930.9 1.9 Equity securities 487.4 483.0 2.1 2.3 Other investments 108.8 — 108.8 — Short-term investments 368.5 333.7 34.8 — $ 4,308.1 $ 1,227.3 $ 3,076.6 $ 4.2 (a) Quoted prices in active markets for identical assets (b) Significant other observable inputs (c) Significant unobservable inputs Fair Value Measurements at Reporting Date Using (in millions) December 31, 2016 Level 1 (a) Level 2 (b) Level 3 (c) Fixed maturities U.S. Governments $ 271.2 $ 228.0 $ 43.2 $ — Foreign Governments 237.3 — 237.3 — Obligations of states and political subdivisions 382.8 — 382.8 — Corporate bonds 1,320.7 — 1,318.7 2.0 Commercial mortgage-backed securities 153.7 — 153.7 — Residential mortgage-backed securities 176.8 — 176.8 — Asset-backed securities 125.6 — 125.6 — Collateralized loan obligations 264.3 — 264.3 — Total fixed maturities 2,932.4 228.0 2,702.4 2.0 Equity securities 447.4 444.9 2.1 0.4 Other investments 91.5 — 91.5 — Short-term investments 405.5 375.1 30.4 — $ 3,876.8 $ 1,048.0 $ 2,826.4 $ 2.4 (a) Quoted prices in active markets for identical assets (b) Significant other observable inputs (c) Significant unobservable inputs The fair value measurements in the tables above do not equal “Total investments” on our Consolidated Balance Sheets as they exclude certain other investments that are accounted for under the equity-method of accounting. A reconciliation of the beginning and ending balances for the investments categorized as Level 3 are as follows: Fair Value Measurements Using Observable Inputs (Level 3) (in millions) Credit Financial Equity Securities Total Beginning balance, January 1, 2017 $ 2.0 $ 0.4 $ 2.4 Transfers into Level 3 — — — Transfers out of Level 3 — — — Total gains or losses (realized/unrealized): Included in net income (loss) — — — Included in other comprehensive income (loss) (0.1 ) 0.2 0.1 Purchases, issuances, sales, and settlements: Purchases — 1.7 1.7 Issuances — — — Sales — — — Settlements — — — Ending balance, December 31, 2017 $ 1.9 $ 2.3 $ 4.2 Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at December 31, 2017 $ — $ — $ — (in millions) Credit Financial Equity Securities Total Beginning balance, January 1, 2016 $ — $ 0.7 $ 0.7 Transfers into Level 3 — — — Transfers out of Level 3 — — — Total gains or losses (realized/unrealized): Included in net income (loss) — — — Included in other comprehensive income (loss) — — — Purchases, issuances, sales, and settlements: Purchases 2.0 — 2.0 Issuances — — — Sales — (0.3 ) (0.3 ) Settlements — — — Ending balance, December 31, 2016 $ 2.0 $ 0.4 $ 2.4 Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at December 31, 2016 $ — $ — $ — At December 31, 2017 and 2016, we did not have any financial assets or financial liabilities measured at fair value on a nonrecurring basis or any financial liabilities on a recurring basis.

Reinsurance

Reinsurance	12 Months Ended
	Dec. 31, 2017
Insurance [Abstract]
Reinsurance	4. Reinsurance We reinsure certain risks with other insurance companies. Such arrangements serve to limit our maximum loss on certain individual risks as well as on catastrophes and large or unusually hazardous risks. We are liable to our insureds for reinsurance ceded in the event our reinsurers do not meet their obligations. Thus, a credit exposure exists with respect to reinsurance ceded to the extent that any reinsurer is unable or unwilling to meet the obligations assumed under the reinsurance contracts. Our allowance for uncollectible reinsurance balances receivable on paid losses and incurred claims was $2.1 million as of December 31, 2017 and 2016. Under certain reinsurance agreements, collateral, including letters of credit, is held to secure performance of reinsurers in meeting their obligations. The amount of such collateral was $907.7 million and $492.6 million at December 31, 2017 and 2016, respectively. The collateral we hold does not apply to our entire outstanding reinsurance recoverable. Rather, collateral is provided on an individual contract basis as appropriate. For each individual reinsurer, the collateral held may exceed or fall below the total outstanding recoverable from that individual reinsurer. The long-term nature of the reinsurance contracts creates a credit risk to us over time arising from potentially uncollectible reinsurance. To mitigate that counterparty risk, we evaluate our reinsurers to assess their financial condition. The factors that underlie these reviews include a financial risk assessment as well as an internal assessment of the capitalization and the operational risk of the reinsurer. As a result of these reviews, we may make changes to the approved markets that are used in both our treaty and facultative reinsurance programs. Estimated losses recoverable from reinsurers and the ceded portion of unearned premiums are reported as assets in our Consolidated Balance Sheets. Included in “Reinsurance recoverables” are paid loss recoverables of $380.3 million and $215.0 million as of December 31, 2017 and 2016, respectively. “Earned Premiums” and “Losses and loss adjustment expenses” are reported net of reinsurance in our Consolidated Statements of Income. Losses and loss adjustment expenses of $1,050.2 million, $810.1 million and $766.1 million for the years ended December 31, 2017, 2016 and 2015, respectively, are net of amounts ceded to reinsurers of $992.6 million, $419.3 million and $284.6 million, respectively. We are required to accept certain assigned risks and other legally mandated reinsurance obligations. Prior to the mid-1980s, we assumed various forms of casualty reinsurance for which we continue to maintain reserves for losses and loss adjustment expenses (see Note 6, “Run-off Lines”). For such assumed reinsurance transactions, we engage in various monitoring steps that are common with assumed reinsurance such as ongoing claims reviews. We currently assume property related reinsurance primarily through our subsidiaries, Argo Re and Ariel Re, and casualty related reinsurance primarily through Syndicate 1200. Premiums were as follows: For the Years Ended December 31, (in millions) 2017 2016 2015 Direct written premiums $ 2,029.2 $ 1,792.5 $ 1,651.4 Reinsurance ceded to other companies (1,043.7 ) (724.6 ) (610.0 ) Reinsurance assumed from other companies 668.0 372.3 360.7 Net written premiums $ 1,653.5 $ 1,440.2 $ 1,402.1 Direct earned premiums $ 1,912.2 $ 1,722.8 $ 1,602.2 Reinsurance ceded to other companies (1,033.6 ) (675.8 ) (563.7 ) Reinsurance assumed from other companies 693.7 363.8 333.4 Net earned premiums $ 1,572.3 $ 1,410.8 $ 1,371.9 Percentage of reinsurance assumed to net earned premiums 44.1 % 25.8 % 24.3 %

Reserves for Losses and Loss Ad

Reserves for Losses and Loss Adjustment Expenses	12 Months Ended
	Dec. 31, 2017
Insurance [Abstract]
Reserves for Losses and Loss Adjustment Expenses	5. Reserves for Losses and Loss Adjustment Expenses The following table provides a reconciliation of reserves for losses and loss adjustment expenses (“LAE”): For the Twelve Months Ended December 31, (in millions) 2017 2016 2015 Net reserves beginning of the year $ 2,180.2 $ 2,133.3 $ 2,137.1 Net Maybrooke reserves acquired 131.8 — Add: Losses and LAE incurred during current calendar year, net of reinsurance: Current accident year 1,058.4 843.4 798.5 Prior accident years (8.2 ) (33.3 ) (32.4 ) Losses and LAE incurred during calendar year, net of reinsurance 1,050.2 810.1 766.1 Deduct: Losses and LAE payments made during current calendar year, net of reinsurance: Current accident year 289.6 178.9 169.0 Prior accident years 599.8 537.6 564.5 Losses and LAE payments made during current calendar year, net of reinsurance: 889.4 716.5 733.5 Change in participation interest (1) (23.2 ) (36.3 ) (1.2 ) Foreign exchange adjustments 38.4 (10.4 ) (35.2 ) Net reserves - end of year 2,488.0 2,180.2 2,133.3 Add: Reinsurance recoverables on unpaid losses and LAE, end of year 1,713.0 1,170.6 990.3 Gross reserves - end of year $ 4,201.0 $ 3,350.8 $ 3,123.6 (1) Amount represents (decrease) increase in reserves due to change in our Syndicate 1200 participation. Reserves for losses and LAE represent the estimated indemnity cost and related adjustment expenses necessary to investigate and settle claims. Such estimates are based upon individual case estimates for reported claims, estimates from ceding companies for reinsurance assumed and actuarial estimates for losses that have been incurred but not yet reported to the insurer. Any change in probable ultimate liabilities is reflected in current operating results. The impact from the (favorable) unfavorable development of prior accident years’ losses and LAE reserves on each reporting segment is presented below: For the Twelve Months Ended December 31, (in millions) 2017 2016 2015 U.S. Operations $ (38.7 ) $ (35.9 ) $ (23.0 ) International Operations 13.2 (16.0 ) (18.0 ) Run-off Lines 17.3 18.6 8.6 Total unfavorable (favorable) prior-year development $ (8.2 ) $ (33.3 ) $ (32.4 ) The following describes the primary factors behind each segment’s prior accident year reserve development for the years ended December 31, 2017, 2016 and 2015: Year ended December 31, 2017: • U.S. Operations: Favorable development in our general liability, workers compensation, surety and commercial automobile lines. • International Operations: Unfavorable development in the property and liability lines, primarily due to the first quarter 2017 Ogden rate change and claims from Hurricane Matthew. Partially offsetting this unfavorable development was favorable development on the property reinsurance lines. • Run-off Lines: Unfavorable development on prior accident years driven by our asbestos exposure due to increasing defense costs and an increase in the time claims remain open, and in other run-off lines, partially offset by favorable development in the run-off risk management lines. Year ended December 31, 2016: • U.S. Operations: Favorable development in our commercial automobile, workers compensation, surety and commercial multiple peril lines, partially offset by unfavorable development in our general liability lines. • International Operations: Favorable development in the property reinsurance lines, as well as general liability and property facultative business underwritten by our Lloyd’s Syndicate 1200. • Run-off Lines: Unfavorable development from our asbestos exposure, run-off risk management lines and other run-off lines. The unfavorable development from our asbestos exposure was due to increased defense costs and a final settlement with a large primary insured. Year ended December 31, 2015: • U.S. Operations: Favorable development in the general liability, commercial auto liability and surety lines, partially offset by unfavorable development in the commercial multiple peril and workers compensation lines. • International Operations: Favorable development in general liability lines, various liability lines underwritten by our Lloyd’s Syndicate 1200 and the property reinsurance lines. • Run-off Lines: Unfavorable development in our run-off workers compensation lines due to increasing medical costs on older claims and the unwind of reserve discount, asbestos and environmental exposures driven by increased defense costs in our primary asbestos exposure and unfavorable development in our other assumed business. Partially offsetting this unfavorable development was favorable development in our run-off property catastrophe reinsurance lines. In the opinion of management, our reserves represent the best estimate of our ultimate liabilities, based on currently known facts, current law, current technology and assumptions considered reasonable where facts are not known. Due to the significant uncertainties and related management judgments, there can be no assurance that future favorable or unfavorable loss development, which may be material, will not occur. Short-Duration Contract Disclosures ASU 2015-09, “Disclosures about Short-Duration Contracts” (Topic 944), requires insurers to make disclosures about their liability for unpaid claims and claim adjustment expenses for short-duration insurance contracts. These disclosures include tables showing incurred and paid claims development information (net of reinsurance and excluding unallocated loss adjustment expenses) which are disaggregated based on the characteristics of the insurance contracts that the insurer writes and other factors specific to the reporting entity. The information should be disclosed by accident year for the number of years claims typically remain outstanding, but need not be more than 10 years, including a reconciliation of the disaggregated information to the consolidated statement of financial position. The basis for our disaggregation of this information is by each of our two ongoing reporting segments, U.S. Operations and International Operations, further disaggregated within each segment by our operating divisions and the primary insurance and reinsurance lines of business we write. We’ve chosen to disaggregate the data in this way so as to not obscure useful information by otherwise aggregating items with significantly different characteristics. See Note 19, “Segment Information,” for additional information regarding our two ongoing reporting segments. Operating Divisions Our U.S. Operations reporting segment is comprised of two primary operating divisions, Excess and Surplus Lines and Commercial Specialty, while International Operations includes the International Specialty, Syndicate 1200 and Ariel Re operating divisions. Each of our operating divisions are further described below. Excess and Surplus Lines The Excess and Surplus Lines division focuses on U.S.-based risks that the standard (admitted) market is unwilling or unable to underwrite. The standard market’s limited appetite for such coverage is often driven by the insured’s unique risk characteristics, the perils involved, the nature of the business, and/or the insured’s loss experience. We are often able to underwrite these risks with more flexible policy terms through our Excess and Surplus Lines division. We underwrite this business on both an admitted and non-admitted basis. Commercial Specialty This Commercial Specialty division provides coverages designed to meet the specialized insurance needs of U.S.-based businesses within certain well-defined markets. It targets business classes and industries with distinct risk profiles that can benefit from specially designed insurance programs, tailored loss control and expert claims handling. This division serves its targeted niche markets with a narrowly focused underwriting profile and specialized knowledge of the businesses it serves. International Specialty International Specialty includes Argo’s Bermuda insurance and reinsurance platform, which specialize in specialty property catastrophe reinsurance, excess casualty insurance and professional liability insurance. ArgoGlobal SE, our Malta-based unit, underwrites various insurance products and services in continental Europe. Argo Seguros is our property and casualty insurance company based in Sao Paolo, Brazil, which is focused on serving that country’s domestic commercial insurance market. Syndicate 1200 The Syndicate 1200 division is focused on underwriting worldwide property, specialty and non-U.S. liability insurance through Argo Underwriting Agency, Ltd. on behalf of Lloyd’s Syndicate 1200 within the Lloyd’s of London global franchise. Ariel Re Ariel Re is a global underwriter of specialty insurance and reinsurance business written primarily through its Lloyd’s Syndicate 1910. Lines of Business We use an underwriting committee structure to monitor and evaluate the operating performance of our lines of business. The underwriting committees are organized to allow products or coverages with similar characteristics to be managed and evaluated in distinct groups. Using this approach, our insurance business is categorized into underwriting groups, which are Liability, Professional, Property and Specialty. Noted below are descriptions of the types of characteristics considered to disaggregate our business into these groups, as well as other qualitative factors to consider when using the information contained in the following incurred and paid claims development tables. Liability Our Liability business generally covers exposures where most claims are reported without a significant time lag between the event that gives rise to a claim and the date the claim is reported to us. However, since facts and information are frequently not complete at the time claims are reported to us, and because protracted litigation is sometimes involved, it can be several years before the ultimate value of these claims is determined. In our International Specialty division, much of the business covers higher layers, potentially increasing the time it takes to fully determine our exposure. Professional Much of our Professional business is written on a claims-made basis resulting in coverage only for claims that are reported to us during the year in which the policy is effective, thus reducing the number of claims that will become known to us after the end of the policy expiration date. However, facts and information are frequently not complete at the time claims are reported to us, and protracted litigation is sometimes involved. It can be several years before the ultimate value of these claims is determined. In our International Specialty division, much of the business covers higher layers, potentially increasing the time it takes to fully determine our exposure. Property Property losses are generally reported within a short period of time from the date of loss, and in most instances, property claims are settled and paid within a relatively short timeframe. However, Property can be impacted by catastrophe losses which can be more complex than non-catastrophe Property claims due to factors such as difficulty accessing impacted areas and other physical, legal and regulatory impediments potentially extending the period of time it takes to settle and pay claims. The impacts of catastrophe losses can be more significant in our International Specialty and Syndicate 1200 divisions. Specialty Specialty lines losses are generally reported within a short period of time from the date of loss, and in most instances, Specialty lines claims are settled and paid within a relatively short timeframe. However, Specialty lines can be impacted by larger losses where facts and information are frequently not complete at the time claims are reported to us. These large losses can be more complex than smaller Specialty claims due to factors such as difficulty determining actual damages and other physical, legal and regulatory impediments potentially extending the period of time it takes to settle and pay claims. Descriptions of the primary types of coverages included in each of the lines of business for each operating division are noted below: Excess and Surplus Lines • Liability: primary and excess specialty casualty, contract liability, commercial multi-peril, product liability, environmental liability, and auto liability Commercial Specialty • Liability: workers compensation, general liability, auto liability, and various public entity liability risks • Professional: management liability and errors and omissions liability International Specialty • Liability: long-tail casualty and general liability • Property: catastrophe reinsurance and direct and facultative excess reinsurance Syndicate 1200 • Liability: general liability, international casualty and motor treaties • Professional: professional indemnity, directors and officer’s liability, and medical malpractice • Property: direct and facultative excess reinsurance, North American and international binders, and residential collateral protection for lending institutions • Specialty: personal accident, aviation, cargo, yachts, and onshore and offshore marine Ariel Re • Property: property catastrophe reinsurance and excess property direct and facultative insurance Run-off Lines Segment We have a Run-off Lines segment for certain products that we no longer underwrite, including asbestos and environmental claims. We have excluded the Run-off Lines segment from the following disaggregated short-duration contract disclosures due to its insignificance to our consolidated financial position and results of operations, both quantitatively and qualitatively. Gross reserves for losses and LAE in Run-off Lines account for less than 7% of our consolidated gross reserves for losses and LAE, and are primarily related to accident years prior to the mid-1990s. As such, claims development tables for the most recent 10 accident years would not provide meaningful information to users of our financial statements, as the majority of the remaining reserves for losses and LAE would be for accident years not separately presented. See Note 6, “Run-off Lines,” for further information on this segment, including discussion of prior accidents years’ development. Accident Years Presented Based on the previous operating structure and management of our business prior to calendar year 2011, we did not track ultimate claims and claim adjustment expenses by accident year at a level of detail consistent with the current segmentation of our business, including our operating divisions, with the exception of the business in Syndicate 1200. As a result, it is impracticable to obtain the information necessary to provide historical ultimate claims and claim adjustment expense estimates prior to December 31, 2011 in the following incurred and paid claims development tables for all disaggregation categories except those associated with Syndicate 1200. Syndicate 1200 ultimate claims and claim adjustment expenses are provided beginning with accident year 2010 due to the retroactive whole account quota share contract we entered into on December 31, 2012. As a result of this transaction, reserves for losses and LAE prior to accident year 2010 were legally transferred to another syndicate within the Lloyd’s market. Under this quota share contract, we did not retain any direct indemnity or credit risk for the reserves prior to accident year 2010. Prior to the acquisition in February 2017, Ariel Re’s ultimate claims and claim adjustment expense data was not historically available by accident years and line of business. As a result, it is not practical, nor would it be consistent to provide information for calendar years 2016 and prior by accident year at our line of business level. For the 2017 calendar year, we have begun accumulating such claims information by accident year and line of business, and have included such in the tables below. Foreign Currency Portions of the business we write in the Syndicate 1200, International Specialty and Ariel Re divisions are denominated in foreign currencies. We have used the December 31, 2017 balance sheet foreign exchange rates to recast the incurred and paid claims information for all periods presented in the following claims development tables in order to eliminate the effects of changes in foreign currency translation rates. Reserves for Incurred But Not Reported (“IBNR”) Claims Reserves for IBNR claims are based on the estimated ultimate cost of settling claims, including the effects of inflation and other social and economic factors, using past experience adjusted for current trends and any other factors that would modify past experience. We use a variety of statistical and actuarial techniques to analyze current claims costs, including frequency and severity data and prevailing economic, social and legal factors. Each such method has its own set of assumptions and outputs, and each has strengths and weaknesses in different areas. Since no single estimation method is superior to another method in all situations, the methods and assumptions used to project loss reserves will vary by coverage and product. We use what we believe to be the most appropriate set of actuarial methods and assumptions for each product line grouping and coverage. While the loss projection methods may vary by product line and coverage, the general approach for calculating IBNR remains the same: ultimate losses are forecasted first, and that amount is reduced by the amount of cumulative paid claims and case reserves. Reserves established in prior years are adjusted as loss experience develops and new information becomes available. Adjustments to previously estimated reserves are reflected in the results of operations in the year in which they are made. As described above, various actuarial methods are utilized to determine the reserves for losses and LAE recorded in our Consolidated Balance Sheets. Weightings of methods at a detailed level may change from evaluation to evaluation based on a number of observations, measures, and time elements. There were no significant changes to the methods and assumptions underlying our consolidated reserve estimations and selections as of December 31, 2017. Incurred & Paid Claims Development Disclosures The following tables provide information about incurred and cumulative paid losses and allocated loss adjustment expenses (“ALAE”), net of reinsurance. The following tables also include IBNR reserves plus expected development on reported claims and the cumulative number of reported claims as of December 31, 2017. Reporting Segment: U.S. Operations Operating Division: Excess and Surplus Lines Line of Business: Liability (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2011 $ 202.9 $ 206.0 $ 205.8 $ 200.0 $ 193.5 $ 192.8 $ 189.0 2012 189.6 196.0 189.7 183.6 184.4 182.1 2013 217.9 222.6 224.3 227.2 220.4 2014 213.0 215.1 213.1 211.9 2015 232.3 237.0 228.6 2016 246.5 250.9 2017 260.6 Total $ 1,543.5 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2011 $ 17.6 $ 53.8 $ 91.0 $ 122.9 $ 146.6 $ 162.4 $ 170.0 2012 17.2 52.8 89.1 120.8 142.4 157.5 2013 17.6 60.2 100.4 135.2 163.7 2014 15.0 52.2 95.9 131.6 2015 16.5 51.9 91.4 2016 17.4 52.8 2017 11.5 $ 778.5 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance 70.1 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 835.1 As of December 31, 2017 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ 189.0 $ 12.4 8,404 2012 182.1 15.1 7,322 2013 220.4 34.9 7,180 2014 211.9 48.0 6,306 2015 228.6 92.3 5,661 2016 250.9 151.4 4,969 2017 260.6 218.4 3,801 (1) Information presented for calendar years prior to 2017 is required supplementary information and is unaudited. (2) The cumulative number of reported claims is measured at a coverage level by occurrence. Reported occurrences that do not result in a liability are included as reported claims. Reporting Segment: U.S. Operations Operating Division: Commercial Specialty Line of Business: Liability (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2011 $ 140.6 $ 155.1 $ 159.0 $ 157.5 $ 158.2 $ 154.0 $ 153.7 2012 140.3 146.3 149.7 152.2 151.5 147.7 2013 126.6 133.2 135.1 133.2 131.1 2014 115.5 118.6 116.9 114.5 2015 102.6 106.7 101.6 2016 95.9 99.4 2017 $ 114.2 Total $ 862.2 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2011 $ 23.1 $ 57.5 $ 85.9 $ 111.3 $ 126.1 $ 135.1 $ 139.8 2012 20.1 51.0 80.7 105.3 120.8 127.9 2013 18.9 49.4 73.6 93.6 102.8 2014 17.4 38.4 58.7 75.2 2015 16.4 35.0 48.8 2016 11.1 31.7 2017 16.3 Total $ 542.5 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance 55.3 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 375.0 As of December 31, 2017 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ 153.7 $ 6.8 28,153 2012 147.7 9.6 23,594 2013 131.1 13.4 18,901 2014 114.5 17.3 16,261 2015 101.6 27.4 14,379 2016 99.4 38.1 11,225 2017 114.2 64.8 10,897 (1) Information presented for calendar years prior to 2017 is required supplementary information and is unaudited. (2) The cumulative number of reported claims is measured by individual claimant at a coverage level. Reported occurrences that do not result in a liability are included as reported claims. Reporting Segment: U.S. Operations Operating Division: Commercial Specialty Line of Business: Professional (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2011 $ 35.0 $ 35.0 $ 35.0 $ 32.5 $ 28.2 $ 26.9 $ 26.6 2012 27.8 28.3 28.6 25.8 24.0 24.5 2013 20.9 21.5 21.1 19.0 19.8 2014 22.4 22.4 26.0 33.7 2015 29.9 29.5 33.2 2016 44.2 44.8 2017 60.1 Total $ 242.7 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2011 $ 3.2 $ 11.8 $ 17.8 $ 22.0 $ 24.0 $ 25.4 $ 25.7 2012 2.3 8.6 16.9 19.9 21.4 22.6 2013 1.9 6.3 10.9 14.2 17.6 2014 2.3 5.4 15.1 24.1 2015 1.8 8.3 15.6 2016 2.4 11.9 2017 3.5 Total $ 121.0 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance 0.2 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 121.9 As of December 31, 2017 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ 26.6 $ 0.3 818 2012 24.5 0.2 637 2013 19.8 1.1 616 2014 33.7 2.1 1,041 2015 33.2 6.5 1,802 2016 44.8 19.8 3,190 2017 60.1 50.4 3,272 (1) Information presented for calendar years prior to 2017 is required supplementary information and is unaudited. (2) The cumulative number of reported claims is measured by individual claimant at a coverage level. Reported occurrences that do not result in a liability are included as reported claims. Reporting Segment: International Operations Operating Division: International Specialty Line of Business: Liability (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2011 $ 6.6 $ 6.6 $ 6.6 $ 4.4 $ 2.2 $ 1.6 $ 1.0 2012 7.4 7.4 7.4 5.6 4.5 1.7 2013 9.1 9.5 9.5 6.5 6.2 2014 10.7 10.9 10.7 7.2 2015 12.6 15.3 25.7 2016 15.2 15.3 2017 18.4 Total $ 75.5 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2011 $ — $ — $ — $ — $ — $ — $ — 2012 — — — 0.1 0.1 0.1 2013 0.4 0.7 0.8 0.9 3.4 2014 0.2 0.7 0.8 1.0 2015 0.3 0.6 16.8 2016 0.4 0.9 2017 0.2 Total $ 22.4 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance 0.4 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 53.5 As of December 31, 2017 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ 1.0 $ 1.0 1,422 2012 1.7 1.6 1,403 2013 6.2 2.7 1,306 2014 7.2 3.1 1,465 2015 25.7 0.7 1,720 2016 15.3 14.1 2,148 2017 18.4 18.0 1,927 (1) Information presented for calendar years prior to 2017 is required supplementary information and is unaudited. (2) The cumulative number of reported claims is measured by individual claimant at a coverage level. Reported occurrences that do not result in a liability are included as reported claims. Reporting Segment: International Operations Operating Division: International Specialty Line of Business: Property (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2011 $ 118.0 $ 107.4 $ 109.6 $ 107.6 $ 106.9 $ 106.8 $ 107.1 2012 47.4 51.7 50.6 51.8 46.7 46.6 2013 35.5 39.8 39.2 35.1 34.6 2014 28.3 28.5 26.5 25.6 2015 28.4 24.7 23.5 2016 46.2 40.1 2017 54.5 Total $ 332.0 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2011 $ 41.1 $ 67.5 $ 89.3 $ 97.6 $ 100.0 $ 101.3 $ 102.3 2012 12.4 31.3 40.8 49.8 44.2 44.5 2013 5.2 21.7 32.2 32.5 33.2 2014 3.3 14.0 20.6 23.5 2015 4.6 11.9 15.5 2016 14.7 22.8 2017 12.1 Total $ 253.9 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance 4.0 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 82.1 As of December 31, 2017 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ 107.1 $ — 400 2012 46.6 0.8 322 2013 34.6 0.5 357 2014 25.6 1.5 355 2015 23.5 3.7 420 2016 40.1 8.7 647 2017 54.5 4.2 786 (1) Information presented for calendar years prior to 2017 is required supplementary information and is unaudited. (2) The cumulative number of reported claims is measured by individual claimant at a coverage level. Reported occurrences that do not result in a liability are included as reported claims. Reporting Segment: International Operations Operating Division: Syndicate 1200 Line of Business: Liability (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2010 $ 5.9 $ 5.0 $ 4.7 $ 6.4 $ 6.2 $ 6.1 $ 6.0 $ 6.1 2011 8.5 9.0 11.4 11.5 10.8 10.6 11.2 2012 9.1 11.3 15.5 14.9 14.4 15.2 2013 23.7 27.8 27.4 25.6 25.7 2014 38.8 38.0 35.2 34.6 2015 35.9 30.9 31.0 2016 26.6 27.9 2017 25.3 Total $ 177.0 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2010 $ — $ 0.2 $ 0.6 $ 1.1 $ 2.1 $ 2.9 $ 3.9 $ 4.2 2011 0.3 0.8 1.7 3.5 5.7 7.3 8.2 2012 0.4 1.2 2.7 6.0 8.6 10.5 2013 1.6 3.4 7.2 11.8 16.2 2014 2.0 4.8 10.2 14.1 2015 0.8 5.3 7.6 2016 1.9 5.8 2017 1.8 Total $ 68.4 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 108.6 As of December 31, 2017 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims 2010 $ 6.1 $ 0.1 2011 11.2 0.6 2012 15.2 1.5 2013 25.7 3.9 2014 34.6 7.2 2015 31.0 11.1 2016 27.9 14.1 2017 25.3 19.8 (1) Information presented for calendar years prior to 2017 is required supplementary information and is unaudited. Reporting Segment: International Operations Operating Division: Syndicate 1200 Line of Business: Professional (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2010 $ 16.1 $ 13.2 $ 13.1 $ 11.6 $ 10.4 $ 9.7 $ 9.7 $ 9.8 2011 19.9 21.8 19.2 16.1 15.3 15.3 15.8 2012 14.5 14.4 14.7 14.6 14.6 15.7 2013 23.6 23.6 23.7 23.3 23.6 2014 36.4 37.6 37.9 41.4 2015 39.6 39.1 40.7 2016 34.2 28.5 2017 25.5 Total $ 201.0 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2010 $ 0.1 $ 1.0 $ 2.0 $ 3.1 $ 3.9 $ 5.3 $ 6.7 $ 7.0 2011 1.1 2.6 4.3 6.8 8.6 11.0 11.7 2012 0.6 1.9 4.5 6.0 8.5 9.9 2013 1.7 3.9 7.4 12.1 16.0 2014 1.7 6.7 15.3 24.7 2015 2.3 8.6 15.3 2016 2.1 5.9 2017 1.1 Total $ 91.6 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 109.4 As of December 31, 2017 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims 2010 $ 9.8 $ 0.2 2011 15.8 0.9 2012 15.7 1.8 2013 23.6 4.2 2014 41.4 9.0 2015 40.7 15.1 2016 28.5 14.8 2017 25.5 20.1 (1) Information presented for calendar years prior to 2017 is required supplementary information and is unaudited. Reporting Segment: International Operations Operating Division: Syndicate 1200 Line of Business: Property (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2010 $ 51.1 $ 59.0 $ 55.3 $ 54.8 $ 52.1 $ 51.8 $ 51.3 $ 51.9 2011 109.8 115.1 109.1 96.2 94.3 93.7 93.6 2012 89.5 89.3 93.5 92.6 91.6 91.3 2013 84.0 79.8 78.8 77.3 76.9 2014 71.2 64.5 65.9 66.1 2015 54.9 66.3 73.2 2016 70.7 86.3 2017 83.9 Total $ 623.2 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2010 $ 1.8 $ 19.8 $ 28.4 $ 34.2 $ 36.2 $ 40.5 $ 42.0 $ 43.0 2011 23.7 47.8 62.6 74.4 80.3 81.9 83.3 2012 29.9 48.2 63.5 74.6 77.0 77.9 2013 44.8 57.2 69.7 73.8 74.2 2014 29.7 51.8 57.8 59.5 2015 23.0 42.9 51.9 2016 39.1 62.3 2017 27.4 Total $ 479.5 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 143.7 As of December 31, 2017 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims 2010 $ 51.9 $ — 2011 93.6 — 2012 91.3 — 2013 76.9 — 2014 66.1 — 2015 73.2 3.7 2016 86.3 8.6 2017 83.9 33.6 (1) Information presented for calendar years prior to 2017 is required supplementary information and is unaudited. Reporting Segment: International Operations Operating Division: Syndicate 1200 Line of Business: Specialty (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2010 $ 13.1 $ 16.0 $ 13.8 $ 13.3 $ 12.3 $ 12.3 $ 12.2 $ 12.2 2011 39.0 40.7 39.4 34.6 33.8 33.8 33.6 2012 53.1 57.0 61.4 49.9 59.7 59.4 2013 76.7 82.2 83.5 83.1 82.6 2014 93.2 99.6 101.0 101.9 2015 90.5 88.7 94.4 2016 86.6 85.1 2017 80.1 Total $ 549.3 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2010 $ 1.9 $ 8.8 $ 9.0 $ 9.4 $ 9.1 $ 9.9 $ 10.1 $ 10.2 2011 11.7 19.8 24.0 27.5 28.9 29.5 29.8 2012 18.2 27.9 39.4 46.3 49.3 50.3 2013 31.4 53.1 69.6 77.0 78.6 2014 38.2 72.3 83.5 88.8 2015 31.4 54.8 65.7 2016 37.9 57.9 2017 18.8 Total $ 400.1 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 149.2 As of December 31, 2017 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims 2010 $ 12.2 $ — 2011 33.6 — 2012 59.4 — 2013 82.6 — 2014 101.9 2.6 2015 94.4 7.3 2016 85.1 13.0 2017 80.1 41.0 (1) Information presented for calendar years prior to 2017 is required supplementary information and is unaudited. Reporting Segment: International Operations Operating Division: Ariel Re Line of Business: Property Products (in millions, except number of claims reported) For the Year Ended December 31, 2017 Accident Year Incurred Losses & ALAE, Net of Reinsura

Run-off Lines

Run-off Lines	12 Months Ended
	Dec. 31, 2017
Discontinued Operations And Disposal Groups [Abstract]
Run-off Lines	6 . Run-off Lines We have discontinued active underwriting of certain lines of business, including those lines that were previously recorded in Argo Group’s Risk Management segment. All current activity within these lines is related to the management of claims and other administrative functions. Also included in Run-off Lines are other liability reserves, which include exposure to claims for asbestos and environmental liabilities written in past years. The other liability reserves are often characterized by long elapsed periods between the occurrence of a claim and ultimate payment to resolve the claim. We use a specialized staff dedicated to administer and settle these claims. The following table presents our gross reserves for Run-off Lines as of December 31: December 31, (in millions) 2017 2016 Asbestos and Environmental: Reinsurance assumed $ 29.3 $ 28.4 Other 26.6 20.0 Total Asbestos and Environmental 55.9 48.4 Risk management 219.6 236.9 Run-off reinsurance lines 1.8 1.9 Other run-off lines 4.6 3.7 Gross reserves - Run-off Lines $ 281.9 $ 290.9 We have received asbestos and environmental liability claims arising from other liability coverage primarily written in the 1960s, 1970s and into the mid-1980s. Asbestos and environmental claims originate from policies directly underwritten by us and from reinsurance assumed during this period, including a portion assumed from the London market. The following table represents the total gross reserves for our asbestos exposure: December 31, (in millions) 2017 2016 2015 Direct written Case reserves $ 2.1 $ 2.8 $ 2.0 Unallocated loss adjustment expense ("ULAE") 0.5 0.5 0.5 Incurred but not reported ("IBNR") 18.8 12.1 8.2 Total direct written reserves 21.4 15.4 10.7 Assumed domestic Case reserves 9.8 10.5 12.2 ULAE 0.8 0.8 0.8 IBNR 13.7 10.5 13.7 Total assumed domestic reserves 24.3 21.8 26.7 Assumed London Case reserves 2.3 3.3 4.0 ULAE — — — IBNR 0.6 1.4 1.4 Total assumed London reserves 2.9 4.7 5.4 Total asbestos reserves $ 48.6 $ 41.9 $ 42.8 The following table presents our underwriting losses for Run-off Lines: For the Years Ended December 31, (in millions) 2017 2016 2015 Asbestos and Environmental: Reinsurance assumed $ (8.7 ) $ (1.3 ) $ (1.0 ) Other (6.7 ) (9.6 ) (3.4 ) Total Asbestos and Environmental (15.4 ) (10.9 ) (4.4 ) Risk management (8.8 ) (13.1 ) (8.2 ) Run-off reinsurance lines (0.1 ) 0.1 2.0 Other run-off lines (1.4 ) (1.2 ) (3.5 ) Total underwriting loss - Run-off Lines $ (25.7 ) $ (25.1 ) $ (14.1 ) Reserves for asbestos and environmental claims cannot be estimated with traditional loss reserving techniques that rely on historical accident year loss development factors. The uncertainty in the asbestos and environmental reserves estimates arises from several factors including lack of actuarially credible historical data, inapplicability of standard actuarial projection techniques, uncertainty with regards to claim costs, coverage interpretations and judicial, statutory and regulatory provisions under which the claims may be ultimately resolved. It is impossible to predict how the courts will interpret coverage issues and these resolutions may have a material impact on the ultimate resolution of the asbestos and environmental liabilities. We use a variety of estimation methods to calculate reserves as a whole; however, reserves for asbestos and environmental claims were determined utilizing a variety of methods which rely on historical claim reporting and average claim cost information. We apply greatest weight to the method that projects future calendar period claims and average claim costs because it best captures the unique claim characteristics of our underlying exposures. Although management has recorded its best estimate of loss reserves, due to the uncertainties of estimation of liability that may arise as discussed herein, further deterioration of claims could occur in the future. Please see Note 5, “Reserves for Losses and Loss Adjustment Expenses” for further discussion.

Junior Subordinated Debentures

Junior Subordinated Debentures	12 Months Ended
	Dec. 31, 2017
Junior Subordinated Debentures [Member]
Junior Subordinated Debentures	7. Junior Subordinated Debentures Through a series of trusts, that are wholly-owned subsidiaries (non-consolidated), we issued debt. The debentures are variable with the rate being reset quarterly and subject to certain interest rate ceilings. Interest payments are payable quarterly. The debentures are all unsecured and are subordinated to other indebtedness. At December 31, 2017 and 2016, all debentures were eligible for redemption subject to certain terms and conditions at a price equal to 100% of the principal plus accrued and unpaid interest. A summary of our outstanding junior subordinated debentures is presented below: (in millions) Issue Date Trust Preferred Pools Maturity Rate Structure Interest Rate at December 31, 2017 Amount Argo Group 05/15/2003 PXRE Capital Statutory Trust II 05/15/2033 3M LIBOR + 4.10% 5.52% $ 18.1 11/06/2003 PXRE Capital Trust VI 09/30/2033 3M LIBOR + 3.90% 5.59% 10.3 Argo Group US 05/15/2003 Argonaut Group Statutory Trust I 05/15/2033 3M LIBOR + 4.10% 5.52% 15.5 12/16/2003 Argonaut Group Statutory Trust III 01/08/2034 3M LIBOR + 4.10% 5.46% 12.3 04/29/2004 Argonaut Group Statutory Trust IV 04/29/2034 3M LIBOR + 3.85% 5.27% 13.4 05/26/2004 Argonaut Group Statutory Trust V 05/24/2034 3M LIBOR + 3.85% 5.30% 12.3 05/12/2004 Argonaut Group Statutory Trust VI 05/12/2034 3M LIBOR + 3.80% 5.40% 13.4 09/17/2004 Argonaut Group Statutory Trust VII 12/15/2034 3M LIBOR + 3.60% 5.19% 15.5 09/22/2004 Argonaut Group Statutory Trust VIII 09/22/2034 3M LIBOR + 3.55% 5.21% 15.5 10/22/2004 Argonaut Group Statutory Trust IX 12/15/2034 3M LIBOR + 3.60% 5.19% 15.5 09/15/2005 Argonaut Group Statutory Trust X 09/15/2035 3M LIBOR + 3.40% 4.99% 30.9 Total Outstanding $ 172.7 (in millions) Issue Date Trust Preferred Pools Maturity Rate Structure Interest Rate at December 31, 2016 Amount Argo Group 05/15/2003 PXRE Capital Statutory Trust II 05/15/2033 3M LIBOR + 4.10% 5.00% $ 18.1 11/06/2003 PXRE Capital Trust VI 09/30/2033 3M LIBOR + 3.90% 4.90% 10.3 Argo Group US 05/15/2003 Argonaut Group Statutory Trust I 05/15/2033 3M LIBOR + 4.10% 5.00% 15.5 12/16/2003 Argonaut Group Statutory Trust III 01/08/2034 3M LIBOR + 4.10% 4.98% 12.3 04/29/2004 Argonaut Group Statutory Trust IV 04/29/2034 3M LIBOR + 3.85% 4.76% 13.4 05/26/2004 Argonaut Group Statutory Trust V 05/24/2034 3M LIBOR + 3.85% 4.77% 12.3 05/12/2004 Argonaut Group Statutory Trust VI 05/12/2034 3M LIBOR + 3.80% 4.79% 13.4 09/17/2004 Argonaut Group Statutory Trust VII 12/15/2034 3M LIBOR + 3.60% 4.56% 15.5 09/22/2004 Argonaut Group Statutory Trust VIII 09/22/2034 3M LIBOR + 3.55% 4.55% 15.5 10/22/2004 Argonaut Group Statutory Trust IX 12/15/2034 3M LIBOR + 3.60% 4.56% 15.5 09/15/2005 Argonaut Group Statutory Trust X 09/15/2035 3M LIBOR + 3.40% 4.36% 30.9 Total Outstanding $ 172.7 Junior Subordinated Debentures from Maybrooke Acquisition Unsecured junior subordinated debentures with a principal balance of $91.8 million were assumed through the acquisition of Maybrooke (“the acquired debt”). As part of the ongoing liquidation of the Maybrooke holding company, which began subsequent to our acquisition in 2017, the acquired debt was ultimately assigned to Argo Re and is carried on our consolidated balance sheet at $83.9 million, which represents the debt’s fair value at the date of acquisition plus accumulated accretion of discount to par value, as required by accounting for business combinations under ASC 805 (see Note 2, “Acquisition of Maybrooke”). At December 31, 2017, the acquired debt was eligible for redemption at par. Interest accrues on the acquired debt based on a variable rate, which is reset quarterly. Interest payments are payable quarterly. A summary of the terms of the acquired debt outstanding at December 31, 2017 is presented below: (in millions) Issue Date Maturity Rate Structure Interest Rate at December 31, 2017 Principal at December 31, 2017 Carrying Value at December 31, 2017 9/15/2007 9/15/2037 3 month LIBOR + 3.15% 4.74 % $ 91.8 $ 83.9

Other Indebtedness

Other Indebtedness	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Other Indebtedness	8. Other Indebtedness Our Consolidated Balance Sheets includes various long-term debt instruments under the caption “Other indebtedness,” as detailed in the table below. Information regarding the terms and principal amounts of each of these debt instruments is also provided. (in millions) Debt Type December 31, 2017 December 31, 2016 Floating rate loan stock $ 58.9 $ 54.8 Term loan 125.0 — Other debt 0.6 0.6 Total other indebtedness $ 184.5 $ 55.4 Floating Rate Loan Stock This debt was assumed through the acquisition of Argo Underwriting Agency, Ltd. These notes are unsecured. At December 31, 2017 and 2016, all notes were eligible for redemption subject to certain terms and conditions at a price equal to 100% of the principal plus accrued and unpaid interest. Interest on the U.S. Dollar and Euro notes is due semiannually and quarterly, respectively. A summary of the notes outstanding at December 31, 2017 and 2016 is presented below: (in millions) Issue Date Currency Maturity Rate Structure Interest Rate at December 31, 2017 Amount 12/08/2004 U.S. Dollar 11/15/2034 6 month LIBOR + 4.2% 5.66% $ 6.5 09/06/2005 Euro 08/22/2035 3 month LIBOR + 4.0% 3.67% 14.1 10/31/2006 U.S. Dollar 01/15/2036 6 month LIBOR + 4.0% 5.46% 10.0 10/31/2006 Euro 11/22/2036 3 month LIBOR + 4.0% 3.67% 12.4 06/08/2007 Euro 09/15/2037 3 month LIBOR + 3.9% 3.57% 15.9 $ 58.9 (in millions) Issue Date Currency Maturity Rate Structure Interest Rate at December 31, 2016 Amount 12/08/2004 U.S. Dollar 11/15/2034 6 month LIBOR + 4.2% 5.18% $ 6.5 09/06/2005 Euro 08/22/2035 3 month LIBOR + 4.0% 3.70% 12.8 10/31/2006 U.S. Dollar 01/15/2036 6 month LIBOR + 4.0% 4.98% 10.0 10/31/2006 Euro 11/22/2036 3 month LIBOR + 4.0% 3.70% 11.2 06/08/2007 Euro 09/15/2037 3 month LIBOR + 3.9% 3.58% 14.3 $ 54.8 No principal payments have been made since the acquisition of Argo Underwriting Agency, Ltd. The floating rate loan stock denominated in Euros fluctuates due to foreign currency translation. The outstanding balance on these loans was $42.4 million Borrowing Under Revolving Credit Facility On March 3, 2017, each of Argo Group, Argo Group US, Inc., Argo International Holdings Limited and Argo Underwriting Agency Limited (the “Borrowers”) entered into a $325.0 million Credit Agreement (“New Credit Agreement”) with JPMorgan Chase Bank, N.A., as administrative agent. The New Credit Agreement replaced and terminated the previous $175.0 million Credit Agreement (“Prior Agreement”). The New Credit Agreement provides for a $200.0 million revolving credit facility with a maturity date of March 3, 2022 unless extended in accordance with the terms of the New Credit Agreement. In addition, the New Credit Agreement includes a $125.0 million term loan borrowing, which Argo Group used to pay off in its entirety the $125.0 million borrowing drawn on January 31, 2017 under the Prior Agreement to help fund the acquisition of Maybrooke. Interest accrues based on a variable rate, which resets and is payable based on reset options selected by Argo Group pursuant to the terms of the New Credit Agreement. A summary of the terms of the outstanding balance at December 31, 2017 is presented below: (in millions) Issue Date Maturity Rate Structure Interest Rate at December 31, 2017 Amount 3/3/2017 3/3/2019 2 month LIBOR + 1.5% 2.94% $ 125.0 Borrowings under the New Credit Agreement may be used for general corporate purposes, including working capital, permitted acquisitions and letters of credit, and each of the Borrowers has agreed to be jointly and severally liable for the obligations of the other Borrowers under the New Credit Agreement. The New Credit Agreement contains customary events of default. If an event of default occurs and is continuing, the Borrowers could be required immediately to repay all amounts outstanding under the New Credit Agreement. Lenders holding at least a majority of the loans and commitments under the New Credit Agreement could elect to accelerate the maturity of the loans and/or terminate the commitments under the New Credit Agreement upon the occurrence and during the continuation of an event of default. Included in the New Credit Agreement is a provision that allows up to $200.0 million of the revolving credit facility to be used for LOCs, subject to availability. On March 3, 2017, the $0.2 million LOC outstanding under the Prior Credit Agreement was transferred to the New Credit Agreement. At December 31, 2017 and 2016, there were no borrowings outstanding under the revolving portions of the credit facilities. At December 31, 2017 and 2016, there were $0.5 million and $0.2 million, respectively in LOCs against the New and Prior Credit Agreement, respectively. Other Debt As part of the ARIS Title Insurance Corporation (“ARIS”) acquisition, at December 31, 2017 and 2016, we had a note payable for $0.6 million. The note had a variable interest rate of 2.00% above 30-day LIBOR, with the variable interest rate being reset quarterly and subject to certain interest rate ceilings. Interest payments are payable quarterly. The note payable matures on April 1, 2019.

Disclosures about Fair Value of

Disclosures about Fair Value of Financial Instruments	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Disclosures about Fair Value of Financial Instruments	9. Disclosures about Fair Value of Financial Instruments Cash. The carrying amount approximates fair value. Investment securities and short-term investments . See Note 3, “Investments,” for additional information. Premiums receivable and reinsurance recoverables on paid losses. The carrying value of current receivables and reinsurance recoverables on paid losses approximates fair value. Debt. At December 31, 2017 and 2016, the fair value of our Junior subordinated debentures, Senior unsecured fixed rate notes and Other indebtedness was estimated using appropriate market indices or quoted prices from external sources based on current market conditions. All of these debt instruments would be in Level 3 of the fair value hierarchy, as the fair value estimates shown below were calculated using unobservable inputs reflecting our assumptions about the assumptions market participants would use in pricing the liabilities. A summary of our financial instruments whose carrying value did not equal fair value is shown below: December 31, 2017 December 31, 2016 (in millions) Carrying Amount Fair Value Carrying Amount Fair Value Junior subordinated debentures Trust preferred debentures $ 172.7 $ 172.9 $ 172.7 $ 162.4 Subordinated debentures acquired with Maybrooke 83.9 85.0 — — Senior unsecured fixed rate notes 139.6 141.2 139.5 139.3 Floating rate loan stock 58.9 59.0 54.8 51.5

Shareholders' Equity

Shareholders' Equity	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Shareholders' Equity	10. Shareholders’ Equity On February 20, 2018, our Board of Directors declared a 15% stock dividend, payable on March 21, 2018, to shareholders of record at the close of business on March 7, 2018. For additional details regarding the aforementioned stock dividend, see Note 25, “Subsequent Event.” In addition, our Board of Directors declared a quarterly cash dividend in the amount of $0.27 on each share of common stock outstanding, on a post-stock dividend basis. The cash dividend will be paid on March 23, 2018, to shareholders of record at the close of business on March 7, 2018. On May 3, 2016, our Board of Directors declared a 10% stock dividend, payable on June 15, 2016, to shareholders of record at the close of business on June 1, 2016. As a result of the stock dividend, 2,735,542 additional shares were issued. Cash was paid in lieu of fractional shares of our common shares. All references to share and per share amounts in this document and related disclosures have been adjusted to reflect the stock dividend for all periods presented. On February 17, 2015, our Board of Directors declared a 10% stock dividend, payable on March 16, 2015, to shareholders of record at the close of business on March 2, 2015. As a result of the stock dividend, 2,554,506 additional shares were issued. Cash was paid in lieu of fractional shares of our common shares. All references to share and per share amounts in this document and related disclosures have been adjusted to reflect the stock dividend for all periods presented. During 2017, our Board of Directors declared quarterly cash dividends totaling $1.08 on each share of common stock outstanding to our shareholders of record. For the year ended December 31, 2017, we paid cash dividends totaling $33.2 million to our shareholders. During 2016, our Board of Directors declared quarterly cash dividends totaling $0.86 on each share of common stock outstanding. For the year ended December 31, 2016, we paid cash dividends totaling $26.6 million to our shareholders. During 2015, our Board of Directors declared quarterly cash dividends totaling $0.80 on each share of common stock outstanding or $0.73 on each share outstanding adjusted for the 2016 stock dividends to our shareholders of record. For the year ended December 31, 2015, we paid cash dividends totaling $22.7 million to our shareholders. We are authorized to issue 30 million shares of $1.00 par value preferred shares. As of December 31, 2017 and 2016, no preferred shares were issued and outstanding. On May 3, 2016, our Board of Directors authorized the repurchase of up to $150.0 million of our common shares (“2016 Repurchase Authorization”). The 2016 Repurchase Authorization supersedes all the previous Repurchase Authorizations. As of December 31, 2017, availability under the 2016 Repurchase Authorization for future repurchases of our common shares was $85.0 million. For the year ended December 31, 2017 and 2016, we repurchased 756,252 common shares and 847,111 common shares, respectively for $45.2 million and $47.1 million, respectively A summary of common shares repurchased for the twelve months ended December 31, 2017 is shown below: Repurchase Type Date Trading Plan Initiated 2017 Purchase Period Number of Shares Repurchased Average Price of Shares Repurchased Total Cost (in millions) Repurchase Authorization Year 10b5-1 Trading Plan 6/14/2017 06/20/2017-08/10/2017 156,570 $ 59.53 $ 9.4 2016 10b5-1 Trading Plan 9/15/2017 09/18/2017-11/07/2017 84,711 $ 60.71 5.1 2016 10b5-1 Trading Plan 12/15/2017 12/27/2017 1,420 $ 60.53 0.1 2016 Open Market N/A 08/11/2017-09/15/2017 421,976 $ 59.75 25.2 2016 Open Market N/A 11/08/2017-11/21/2017 91,575 $ 59.28 5.4 2016 Total 756,252 $ 59.76 $ 45.2 At December 31, 2017, we had the following authorized, unissued common shares reserved for future issuance: Reserve Name Shares Reserved 2014 Long-Term Incentive Plan 3,178,971 2016 Employee Share Purchase Plan 492,082 Total 3,671,053

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income (Loss)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Accumulated Other Comprehensive Income (Loss)	11. Accumulated Other Comprehensive Income (Loss) A summary of changes in accumulated other comprehensive income (loss), net of taxes (where applicable) by component is presented below: (in millions) Foreign Translation Adjustments Unrealized Holding Gains on Securities Defined Benefit Pension Plans Total Balance, January 1, 2016 $ (21.6 ) $ 40.0 $ (6.9 ) $ 11.5 Other comprehensive income (loss) before reclassifications 4.0 42.4 (0.2 ) 46.2 Amounts reclassified from accumulated other comprehensive income (loss) — (10.0 ) — (10.0 ) Net current-period other comprehensive income (loss) 4.0 32.4 (0.2 ) 36.2 Balance, December 31, 2016 (17.6 ) 72.4 (7.1 ) 47.7 Other comprehensive income (loss) before reclassifications (1.4 ) 77.7 0.8 77.1 Amounts reclassified from accumulated other comprehensive income (loss) — (28.2 ) — (28.2 ) Net current-period other comprehensive income (loss) (1.4 ) 49.5 0.8 48.9 Balance, December 31, 2017 $ (19.0 ) $ 121.9 $ (6.3 ) $ 96.6 The amounts reclassified from accumulated other comprehensive income (loss) shown in the above table have been included in the following captions in our Consolidated Statements of Income: For the Years Ended December 31, (in millions) 2017 2016 2015 Unrealized gains and losses on securities: Net realized investment gains $ (41.6 ) $ (23.2 ) $ (7.2 ) Provision for income taxes 13.4 13.2 6.3 Net of taxes $ (28.2 ) $ (10.0 ) $ (0.9 )

Net Income Per Common Share

Net Income Per Common Share	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Net Income Per Common Share	12. Net Income Per Common Share The following table presents the calculation of net income per common share on a basic and diluted basis: For the Years Ended December 31, (in millions, except number of shares and per share amounts) 2017 2016 2015 Net income $ 50.3 $ 146.7 $ 163.2 Weighted average common shares outstanding - basic 29,962,524 30,166,440 30,769,089 Effect of dilutive securities: Equity compensation awards 794,710 679,270 616,371 Weighted average common shares outstanding - diluted 30,757,234 30,845,710 31,385,460 Net income per common share: Basic $ 1.68 $ 4.86 $ 5.31 Diluted $ 1.64 $ 4.75 $ 5.20 Excluded from the weighted average common shares outstanding calculation at December 31, 2017, 2016 and 2015 are 10,785,007 shares, 10,028,755 shares and 9,181,644 shares, respectively, which are held as treasury shares. The shares are excluded as of their repurchase date. In 2017, 2016 and 2015, there were no anti-dilutive shares of common stock to be excluded from the computation of diluted net income per common share. On February 20, 2018, our Board of Directors declared a 15% stock dividend payable on March 21, 2018, to shareholders of record at the close of business on March 7, 2018. The share numbers and per share amounts shown in the table above have not been retroactively adjusted to give effect to the stock dividend. For additional details regarding the stock dividend, see Note 25, “Subsequent Event.”

Share-based Compensation

Share-based Compensation	12 Months Ended
	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Share-based Compensation	13. Share-based Compensation The fair value method of accounting is used for equity-based compensation plans. Under the fair value method, compensation cost is measured based on the fair value of the award at the measurement date and recognized over the requisite service period. We use the Black-Scholes model to estimate the fair values on the measurement date for share options and share appreciation rights (“SARs”). The Black-Scholes model uses several assumptions to value a share award. The volatility assumption is based on the historical change in our stock price over the previous five years preceding the measurement date. The risk-free rate of return assumption is based on the five-year U.S. Treasury constant maturity rate on the measurement date. The expected award life is based upon the average holding period over the history of the incentive plan. The expected dividend yield is based on our history and expected dividend payouts. The following table summarizes the assumptions we used: For the Years Ended December 31, 2017 2016 2015 Risk-free rate of return 1.83% to 2.22% 1.00% to 2.02% 1.44% to 1.81% Expected dividend yields 1.63% to 1.72% 1.62% to 1.70% 1.46% to 1.60% Expected award life (years) 4.48 to 4.49 4.50 to 4.61 4.62 to 4.71 Expected volatility 18.13% to 18.70% 18.73% to 19.70% 20.04% to 22.09% All outstanding awards were adjusted to reflect the 10% stock dividend in 2016, resulting in a 10% increase to the number of awards outstanding and a 9.09% reduction in the exercise price. We estimate forfeitures based on historical forfeitures patterns, thereby recognizing expense only for those awards that are expected to vest. The estimate of forfeitures is adjusted as actual forfeitures differ from our estimate, resulting in recognition of compensation expense only for those awards that actually vest. The compensation expense recognized under all our share-based payment plans was $12.3 million ($10.3 million, net of tax), $19.8 million ($15.5 million, net of tax) and $29.1 million ($23.7 million, net of tax) for the years ended December 31, 2017, 2016 and 2015, respectively. The compensation expense is included in “Underwriting, acquisition and insurance expenses” in our Consolidated Statements of Income. We present all tax benefits resulting from the exercise of stock options and vesting of non-vested shares as cash flows from financing activities. Excess tax benefits are realized tax benefits from tax deductions for exercised options and vested shares in excess of the deferred tax asset attributable to stock compensation costs for such options. Such tax benefits and cash flows were immaterial for all reporting periods. Argo Group’s Long-Term Incentive Plans In November 2007, our shareholders approved the 2007 Long-Term Incentive Plan (the “2007 Plan”), which provided for an aggregate of 4.5 million shares of our common stock that may be issued to executives, non-employee directors, and other key employees. As of May 2014, 1.5 million shares remained available for grant under the 2007 Plan. In May 2014, our shareholders approved the 2014 Long-Term Incentive Plan (the “2014 Plan”), which provides for an additional 2.8 million shares of our common stock to be available for issuance to executives, non-employee directors and other key employees. The share awards may be in the form of share options, SARs, restricted shares, restricted share awards, restricted share units awards, performance awards, other share-based awards and other cash-based awards. Shares issued under this plan may be shares that are authorized and unissued or shares that we reacquired, including shares purchased on the open market. Share options and SARs will count as one share for the purposes of the limits under the incentive plans; restricted shares, restricted share units, performance units, performance shares or other share-based incentive awards which settle in common shares will count as 2.75 shares for purpose of the limits under the 2014 Plan. Share options may be in the form of incentive share options, non-qualified share options and restorative options. Share options are required to have an exercise price that is not less than the market value on the date of grant. We are prohibited from repricing the options. The term of the share options cannot exceed seven years from the grant date. A summary of restricted share activity as of December 31, 2017 and changes during the year then ended is as follows: Shares Weighted-Average Grant Date Fair Value Outstanding at January 1, 2017 702,030 $ 42.69 Granted 235,431 $ 62.85 Vested and issued (183,458 ) $ 41.96 Expired or forfeited (72,334 ) $ 50.53 Outstanding at December 31, 2017 681,669 $ 49.02 As of December 31, 2017, there was $25.6 million of total unrecognized compensation cost related to restricted share compensation arrangements granted by Argo Group. The weighted-average period over which this unrecognized expense is expected to be recognized is 2.0 years. The total fair value of shares vested during the year ended December 31, 2017 was $7.7 million. A summary of stock-settled SARs activity as of December 31, 2017 and changes during the year then ended is as follows: Shares Weighted-Average Exercise Price Outstanding at January 1, 2017 1,982,695 $ 34.80 Exercised (486,454 ) $ 30.77 Expired or forfeited (186,089 ) $ 40.75 Outstanding at December 31, 2017 1,310,152 $ 35.45 Vested or expected to vest as of end of year 1,236,514 $ 19.80 Exercisable at end of year 891,067 $ 22.80 The stock-settled SARs vest over a one to four year period. Upon exercise of the stock-settled SARs, the employee is entitled to receive shares of our common stock equal to the appreciation of the stock as compared to the exercise price. For the year ended December 31, 2017, 486,454 stock-settled SARs were exercised resulting in 252,580 shares being issued. As of December 31, 2017, there was $3.6 million of total unrecognized compensation cost related to stock-settled SARs outstanding. The weighted-average period over which this unrecognized expense is expected to be recognized is 1.0 years. Aggregate intrinsic value of the stock-settled SARs at December 31, 2017 was $26.2 million. A summary of cash-settled SARs activity as of December 31, 2017 and changes during the year then ended is as follows: Shares Weighted-Average Exercise Price Outstanding at January 1, 2017 500,486 $ 32.08 Exercised (318,562 ) $ 31.87 Expired or forfeited (14,628 ) $ 23.40 Outstanding at December 31, 2017 167,296 $ 33.23 As of December 31, 2017, all the cash-settled SARs are fully vested. Upon exercise of the cash-settled SARs, the employee is entitled to receive cash payment for the appreciation in the value of our common stock over the exercise price. We account for the cash-settled SARs as liability awards, which require the awards to be revalued at each reporting period. For the year ended December 31, 2017, 318,562 cash-settled SARs were exercised resulting in $10.2 million in cash payments. Included in the total shares outstanding at December 31, 2017 are 406,712 restricted shares whose vesting is contingent on the employee meeting defined performance conditions. Employees have a specified time period in which to meet the performance condition (typically one year) and forfeit the grant (on a pro rata basis) if the performance conditions are not met in the specified time frame. We evaluate the likelihood of the employee completing the performance condition and include this estimate in the determination of the forfeiture factor for the grants. Employees Share Purchase Plans We have established an employee stock purchase plan for eligible employees (Argo Group’s 2007 Employee Share Purchase Plan). Under this plan, newly issued shares of our common stock may be purchased over an offering period of three months at 85% of the lower of the market value on the first day of the offering period or on the designated purchase date at the end of the offering period. We have also established a Save As You Earn Plan for our United Kingdom employees (Argo Group’s Save As You Earn Plan). Under this plan, newly issued shares of our common stock may be purchased over an offering period of three or five years at 85% of the market value of the common shares on the first day of the offering period. Expense recognized under these plans for the years ended December 31, 2017, 2016 and 2015 was $0.4 million, $0.3 million and $0.4 million, respectively. Argo Group International Holdings, Ltd. Deferred Compensation Plan for Non-Employee Directors Until December 16, 2013, the non-employee members of our Board of Directors were entitled to participate in the Argo Group International Holdings, Ltd. Deferred Compensation Plan for Non-Employee Directors (“Directors Plan”), a non-funded and non-qualified deferred compensation plan. Under the Directors Plan, non-employee directors could elect each year to defer payment of 0%, 50% or 100% of their cash compensation payable during the next calendar year. The Directors Plan was amended effective December 16, 2013, and certain amounts were paid out prior to the end of 2013. As of December 31, 2017 the Directors Plan was terminated and cash paid to directors totaled $5.1 million. Compensation expense for December 31, 2017 under the Directors Plan was negligible. Under the Directors Plan, we recorded compensation expense of $0.9 million for both of the years ended December 31, 2016 and 2015.

Underwriting, Acquisition and I

Underwriting, Acquisition and Insurance Expenses	12 Months Ended
	Dec. 31, 2017
Underwriting Acquisition And Insurance Expenses [Abstract]
Underwriting, Acquisition and Insurance Expenses	14. Underwriting, Acquisition and Insurance Expenses Underwriting, acquisition and insurance expenses were as follows: For the Years Ended December 31, (in millions) 2017 2016 2015 Commissions $ 255.9 $ 234.0 $ 232.2 General expenses 359.1 292.1 295.6 Premium taxes, boards and bureaus 32.3 25.2 9.8 647.3 551.3 537.6 Net deferral of policy acquisition costs (11.9 ) (4.3 ) (0.9 ) Total underwriting, acquisition and insurance expenses $ 635.4 $ 547.0 $ 536.7 The increase in general expenses for the year ended December 31, 2017 as compared to 2016 and 2015 was driven by expenses attributable to the operations of Maybrooke, as well as increased personnel costs associated with investments in strategic growth areas, information technology (including digital initiatives), marketing, and occupancy costs.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	15. Income Taxes We are incorporated under the laws of Bermuda and, under current Bermuda law, are not obligated to pay any taxes in Bermuda based upon income or capital gains. We have received an undertaking from the Supervisor of Insurance in Bermuda pursuant to the provisions of the Exempted Undertakings Tax Protection Act, 2011, which exempts us from any Bermuda taxes computed on profits, income or any capital asset, gain or appreciation or any tax in the nature of estate duty or inheritance tax, at least until the year 2035. We do not consider ourselves to be engaged in a trade or business in the United States or the United Kingdom and, accordingly, do not expect to be subject to direct United States or United Kingdom income taxation. We have subsidiaries based in the United Kingdom that are subject to the tax laws of that country. Under current law, these subsidiaries are taxed at the applicable corporate tax rates. Eight of the United Kingdom subsidiaries are deemed to be engaged in business in the United States, and therefore, are subject to United States corporate tax in respect of a proportion of their United States underwriting business only. Relief is available against the United Kingdom tax liabilities in respect of overseas taxes paid that arise from the underwriting business. Our United Kingdom subsidiaries file separate United Kingdom income tax returns. We have subsidiaries based in the United States that are subject to United States tax laws. Under current law, these subsidiaries are taxed at the applicable corporate tax rates. Our United States subsidiaries generally file a consolidated United States federal income tax return. We also have operations in Belgium, Brazil, France, Ireland, Luxembourg, Malta, Spain, and Switzerland, which also are subject to income taxes imposed by the jurisdiction in which they operate. We have operations in Barbados and the United Arab Emirates, which are not subject to income tax under the laws of those countries. On December 22, 2017, U.S. tax legislation referred to as the TCJA was enacted. The effects of changes in tax laws and tax rates are recognized in the period of enactment. Accordingly, we have recorded the impacts of the TCJA in our 2017 consolidated financial statements which, among other changes, primarily includes the remeasurement of our deferred tax assets and liabilities for the reduced US federal tax rate from 35% to 21% beginning on January 1, 2018, and the computation of a provisional amount for the loss reserve discounting modifications. The remeasurement resulted in a reduction of net deferred tax liabilities of $20.2 million, which includes a $20.9 million benefit related to deferred taxes previously recognized in accumulated other comprehensive income. We are still analyzing certain aspects of the TCJA and refining our calculations, which could potentially affect the measurement of these balances or potentially give rise to new deferred tax amounts. Thus, our 2017 consolidated financial statements reflect a reasonably estimated provisional amount based on information available and in accordance with SAB 118. SAB 118 provides guidance on accounting for the effects of the U.S. tax reform where our determinations are incomplete but we are able to determine a reasonable estimate. A final determination is required to be made within a measurement period not to extend beyond one year from the enactment date of the U.S. tax reform. As additional guidance is released, the estimate will be updated as necessary. The following table presents the components of income tax (benefit) provision expense included in the amounts reported in our consolidated financial statements: For the Years Ended December 31, (in millions) 2017 2016 2015 Current income tax (benefit) provision related to: United States $ (0.3 ) $ 30.4 $ 5.1 United Kingdom 7.6 3.9 (1.5 ) Other jurisdictions 0.2 2.0 2.4 Total current income tax provision 7.5 36.3 6.0 Deferred income tax (benefit) provision related to: United States (0.3 ) 3.3 7.8 United Kingdom (17.6 ) (4.4 ) 0.5 Other jurisdictions — — — Total deferred income tax (benefit) provision (17.9 ) (1.1 ) 8.3 Income tax (benefit) provision $ (10.4 ) $ 35.2 $ 14.3 Total income tax benefit for 2017 included a net benefit of 20.2 million to reflect the change in tax laws and tax rates included in TCJA at the date of enactment, resulting primarily from remeasuring our deferred tax assets and liabilities. Our expected income tax provision computed on pre-tax income (loss) at the weighted average tax rate has been calculated as the sum of the pre-tax income (loss) in each jurisdiction multiplied by that jurisdiction’s applicable statutory tax rate. For the years ended December 31, 2017, 2016 and 2015, pre-tax income (loss) attributable to our operations and the operations’ effective tax rates were as follows: (in millions) 2017 2016 2015 Pre-Tax Income (Loss) Effective Tax Rate Pre-Tax Income (Loss) Effective Tax Rate Pre-Tax Income (Loss) Effective Tax Rate Bermuda $ 30.2 0.0 % $ 99.6 0.0 % $ 94.3 0.0 % United States 67.5 (0.9 %) 115.2 29.2 % 64.8 19.9 % United Kingdom (53.8 ) 18.7 % (34.8 ) 1.2 % 21.3 (4.7 %) Belgium 0.1 75.0 % (0.1 ) 23.2 % — (1) 237.0 % Brazil 0.8 0.0 % 0.6 0.0 % (3.2 ) 0.0 % Ireland (2) (0.2 ) 0.0 % (0.2 ) 5.0 % (0.1 ) 5.0 % Luxembourg (5.2 ) 0.0 % — 0.0 % — 0.0 % Malta 0.3 0.0 % 1.6 0.3 % 0.1 0.0 % Switzerland — (1) 21.1 % — (1) 0.0 % 0.1 20.5 % United Arab Emirates 0.2 0.0 % — (1) 0.0 % 0.2 0.0 % Pre-tax income $ 39.9 (26.1 %) $ 181.9 19.4 % $ 177.5 8.1 % (1) Pre-tax income for the respective year was less than $0.1 million. (2) Effective tax rate of 5 percent on intercompany dividends of $40.0 million for the year ended December 31, 2016. Dividends eliminated in consolidation. Our effective tax rate may vary significantly from period to period depending on the jurisdiction generating the pre-tax income (loss) and its corresponding statutory tax rate. The geographic distribution of pre-tax income (loss) can fluctuate significantly between periods given the inherit nature of our business. A reconciliation of the difference between the provision for income taxes and the expected tax provision at the weighted average tax rate is as follows: For the Years Ended December 31, (in millions) 2017 2016 2015 Income tax provision at expected rate $ 12.7 $ 34.1 $ 25.8 Tax effect of: Nontaxable investment income (4.7 ) (5.5 ) (6.5 ) Foreign exchange adjustments 2.1 5.3 (0.1 ) Withholding taxes 0.4 2.4 2.9 Change in valuation allowance (0.9 ) 0.7 (2.6 ) Impact of change in tax rate related to TCJA (20.2 ) — — Other 0.2 (1.8 ) (5.2 ) Income tax provision $ (10.4 ) $ 35.2 $ 14.3 The net deferred tax liability comprises the tax effects of temporary differences related to the following assets and liabilities: December 31, (in millions) 2017 2016 Deferred tax assets: Losses and loss adjustment expense reserve discounting $ 17.2 $ 21.1 Unearned premiums 21.4 29.1 Net operating loss carryforwards 21.9 26.7 Impairment of investment values 4.0 8.6 Accrued bonus 2.6 7.4 Stock option expense 2.0 8.1 United Kingdom underwriting results 3.8 — Other 20.0 32.5 Deferred tax assets, gross 92.9 133.5 Deferred tax liabilities: Unrealized gains on equity securities (30.5 ) (39.1 ) Unrealized gains on fixed maturities and other investment securities (5.0 ) (3.9 ) Deferred acquisition costs (16.9 ) (22.2 ) TCJA reserve transitional liability (5.5 ) — United Kingdom underwriting results — (5.4 ) Deferred gain on like-kind exchange (8.0 ) (13.3 ) Depreciable fixed assets (14.5 ) (21.0 ) Unrealized gains on limited partnership interests (14.6 ) (17.6 ) Other (9.1 ) (11.6 ) Deferred tax liabilities, gross (104.1 ) (134.1 ) Deferred tax (liabilities) assets, net before valuation allowance $ (11.2 ) $ (0.6 ) Valuation allowance (20.1 ) (23.5 ) Deferred tax liabilities, net $ (31.3 ) $ (24.1 ) Net deferred tax (liabilities) assets - Other jurisdictions $ 1.2 $ (6.6 ) Net deferred tax liabilities - United States (32.5 ) (17.5 ) Deferred tax liabilities, net $ (31.3 ) $ (24.1 ) Our gross deferred tax assets (liabilities) are supported by taxes paid in previous periods, reversal of taxable temporary differences and recognition of future taxable income. Management regularly evaluates the recoverability of the deferred tax assets and makes any necessary adjustments to them based upon any changes in management’s expectations of future taxable income. Realization of deferred tax assets is dependent upon our generation of future taxable income sufficient to recover tax benefits that cannot be recovered from taxes paid in the carryback period, generally for our US property and casualty insurers two years for net operating losses and for all our US subsidiaries three years for capital losses. If a company determines that any of its deferred tax assets will not result in future tax benefits, a valuation allowance must be established for the portion of these assets that are not expected to be realized. The net change in valuation allowance for deferred tax assets was a decrease of $3.4 million in 2017 relating to the items discussed below. Based upon a review of our available evidence, both positive and negative discussed above, our management concluded that it is more-likely-than-not that the other deferred tax assets will be realized. Management has determined that a valuation allowance is required for a portion of the tax-effected net operating loss carryforward included as part of the United States consolidated group of $7.2 million generated from PXRE Corporation and for the tax effected net operating loss carryforward of $0.6 million from ARIS. The valuation allowances have been established pursuant to Internal Revenue Code Section 382 limits regarding the application of net operating loss carryforwards following an ownership change. The loss carryforwards available per year for both of these items are $2.8 million, as required by Internal Revenue Code Section 382. Furthermore, due to cumulative losses incurred since inception, management has concluded that a valuation allowance is required for the full amount of the tax-effected net operating losses generated by our Brazil and Malta entities as well as our Luxembourg and US affiliates acquired in the Maybrooke transaction. Accordingly, a valuation allowance is required as of December 31, 2017 of which $7.7 million relates to Brazil operations, and $1.1 million relates to Malta operations. During the year ended December 31, 2017, we acquired affiliates of Maybrooke Holdings SA in which a valuation allowance has also been established in the amount of $3.5 million. For tax return purposes, as of December 31, 2017, we had net operating loss (NOL) carryforwards in Brazil, Luxembourg, Malta and the United States. The amount and timing of realizing the benefits of NOL carryforwards depend on future taxable income and limitation imposed by tax laws. Only a portion of the United States NOL carryforwards have been recognized as mentioned above in the consolidated financial statements and are included in net deferred tax liabilities. The NOL amounts by jurisdiction and year of expiration are as follows: (in millions) December 31, 2017 Expiration Net operating loss carryforwards by jurisdiction Brazil $ 17.0 Indefinite Luxembourg 7.8 Indefinite Luxembourg 5.2 2034 Malta 3.0 Indefinite United States 51.5 2025 - 2036 For any uncertain tax positions not meeting the “more-likely-than-not” recognition threshold, accounting standards require recognition, measurement and disclosure in a company’s financial statements. We had no material unrecognized tax benefits as of December 31, 2017, 2016 and 2015. Our United States subsidiaries are no longer subject to U.S. federal and state income tax examinations by tax authorities for years before 2014. Our United Kingdom subsidiaries are no longer subject to United Kingdom income tax examinations by Her Majesty’s Revenue and Customs for years before 2014.

Pension Benefits and Savings Pl

Pension Benefits and Savings Plans	12 Months Ended
	Dec. 31, 2017
Compensation And Retirement Disclosure [Abstract]
Pension Benefits and Savings Plans	16. Pension Benefits and Savings Plans Argo Group US sponsors a qualified defined benefit plan and non-qualified unfunded supplemental defined benefit plans, all of which were curtailed effective February 2004. As of December 31, 2017 and 2016, the qualified pension plan was underfunded by $2.7 million and $3.7 million, respectively. The non-qualified pension plans were unfunded by $2.2 million $2.2 million at December 31, 2017 and 2016, respectively. Underfunded and unfunded amounts are included in “Other liabilities” in our Consolidated Balance Sheets. Based on the current funding status of the pension plan, effects of the curtailment and expected changes in pension plan asset values and pension obligations, we do not believe any significant funding of the pension plan will be required during the year ending December 31, 2018. Net periodic benefit costs were $0.2 million, $0.1 million and $0.1 million for the years ended December 31, 2017, 2016 and 2015, respectively. Substantially all of our employees are either eligible or mandated by applicable laws to participate in employee savings plans. Under these plans, a percentage of the employee’s pay may be or is mandated based on applicable laws to be contributed to various savings alternatives. The plans also call for our contributions under several formulae. Charges to income related to our contributions were $7.0 million, $6.0 million and $6.3 million in 2017, 2016 and 2015, respectively.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	17. Commitments and Contingencies Argo Group’s subsidiaries are parties to legal actions incidental to their business. Based on the opinion of counsel, management believes that the resolution of these matters will not materially affect our financial condition or results of operations. We have contractual commitments to invest up to $129.9 million related to our limited partnership investments at December 31, 2017. These commitments will be funded as required by the partnership agreements which can be called to be fulfilled at any time, not to exceed thirteen years.

Leases

Leases	12 Months Ended
	Dec. 31, 2017
Leases [Abstract]
Leases	18. Leases We lease office space and equipment under lease agreements that expire at various intervals and are subject to renewal options at market rates prevailing at the time of renewal. At December 31, 2017, the future minimum payments under non-cancelable operating leases are as follows: (in millions) Amount Due 2018 $ 16.2 2019 13.5 2020 10.9 2021 9.5 2022 8.2 Thereafter 52.8 Total $ 111.1 We incurred lease expense of $21.0 million, $12.2 million and $14.2 million for the years ended December 31, 2017, 2016 and 2015, respectively.

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Segment Information	19. Segment Information We are primarily engaged in underwriting property and casualty insurance and reinsurance. We have two ongoing reporting segments: U.S. Operations and International Operations. Additionally, we have a Run-off Lines segment for certain products that we no longer underwrite. See Note 1, “Basis of Presentation,” for information on the changes to our reporting segments that were effective beginning in the first quarter of 2017. We consider many factors, including the nature of each segment’s insurance and reinsurance products, production sources, distribution strategies and the regulatory environment, in determining how to aggregate reporting segments. Transactions between segments are reported in the segment that initiated the transaction. In evaluating the operating performance of our segments, we focus on core underwriting and investing results before the consideration of realized gains or losses from the sales of investments. Realized investment gains are reported as a component of the Corporate and Other segment, as decisions regarding the acquisition and disposal of securities reside with the corporate investment function and are not under the control of the individual business segments. Identifiable assets by segment are those assets used in the operation of each segment. Revenue and income (loss) before income taxes for each segment were as follows: For the Years Ended December 31, (in millions) 2017 2016 2015 Revenue: Earned premiums U.S. Operations $ 936.6 $ 849.5 $ 815.4 International Operations 635.8 560.9 556.1 Run-off Lines (0.1 ) 0.4 0.4 Total earned premiums 1,572.3 1,410.8 1,371.9 Net investment income U.S. Operations 87.2 71.9 52.2 International Operations 32.7 28.7 20.3 Run-off Lines 9.3 11.3 8.1 Corporate and Other 10.8 3.2 8.0 Total net investment income 140.0 115.1 88.6 Fee and other income 22.5 24.5 22.2 Net realized investment and other gains 39.3 26.1 24.1 Total revenue $ 1,774.1 $ 1,576.5 $ 1,506.8 For the Years Ended December 31, (in millions) 2017 2016 2015 Income (loss) before income taxes U.S. Operations $ 169.4 $ 174.4 $ 124.4 International Operations (86.7 ) 51.6 64.4 Run-off Lines (17.9 ) (15.2 ) (7.4 ) Total segment income before taxes 64.8 210.8 181.4 Corporate and Other (64.2 ) (55.0 ) (28.0 ) Net realized investment and other gains 39.3 26.1 24.1 Total income before income taxes $ 39.9 $ 181.9 $ 177.5 The table below presents earned premiums by geographic location. For this disclosure, we determine geographic location by the country of domicile of our subsidiaries that underwrite the business and not by the location of insureds or reinsureds from whom the business was generated. For the Years Ended December 31, (in millions) 2017 2016 2015 Bermuda $ 92.1 $ 112.6 $ 103.4 Brazil 49.0 39.2 43.1 Malta 11.1 2.1 1.9 United Kingdom 489.3 406.9 407.4 United States 930.8 850.0 816.1 Total earned premiums $ 1,572.3 $ 1,410.8 $ 1,371.9 The following table represents identifiable assets: December 31, (in millions) 2017 2016 U.S. Operations $ 4,298.4 $ 3,961.2 International Operations 3,553.8 2,356.9 Run-off Lines 449.6 537.0 Corporate and Other 462.2 349.9 Total $ 8,764.0 $ 7,205.0 Included in total assets at December 31, 2017 and 2016 are $731.8 million and $630.4 million, respectively, in assets associated with trade capital providers. The following table represents goodwill and intangible assets, net of accumulated amortization as of December 31: Goodwill Intangible Assets, Net of Accumulated Amortization (in millions) 2017 2016 2017 2016 U.S. Operations $ 123.5 $ 123.5 $ 1.5 $ 3.7 International Operations 37.9 28.7 95.3 64.0 Total $ 161.4 $ 152.2 $ 96.8 $ 67.7

Statutory Accounting Principles

Statutory Accounting Principles	12 Months Ended
	Dec. 31, 2017
Statutory Accounting Principles [Abstract]
Statutory Accounting Principles	20. Statutory Accounting Principles Financial Information The statutory capital and surplus for our principal operating subsidiaries was as follows: Statutory capital and surplus (1) December 31, (in millions) 2017 2016 Bermuda $ 1,694.4 $ 1,480.1 United Kingdom (2) 330.3 216.6 United States 992.8 886.7 (1) Such amounts include ownership interests in affiliate insurance and reinsurance subsidiaries. (2) Capital on deposit with Lloyd’s in U.S. Dollars The statutory net income (loss) for our principal operating subsidiaries was as follows: Statutory net income (loss) (1) For the Years Ended December 31, (in millions) 2017 2016 2015 Bermuda $ 37.4 $ 158.2 $ 182.2 United Kingdom (2) (95.5 ) 1.8 34.8 United States 57.0 99.0 94.4 (1) Such amounts include ownership interests in affiliate insurance and reinsurance subsidiaries. (2) In U.S. Dollars Dividends As an insurance and reinsurance holding company, we are largely dependent on dividends and other permitted payments from our insurance and reinsurance subsidiaries to pay cash dividends to our shareholders, for debt service and for our operating expenses. The ability of our insurance and reinsurance subsidiaries to pay dividends to us is subject to certain restrictions imposed by the jurisdictions of domicile that regulate our insurance and reinsurance subsidiaries and each jurisdiction has calculations for the amount of dividends that an insurance and reinsurance company can pay without the approval of the insurance regulator. The payment of dividends to our shareholders is governed by the Bermuda Companies Act of 1981, as amended, which permits the payment of dividends so long as (i) we are not, or would not be after the payment, unable to pay our liabilities as they become due and (ii) the realizable value of our assets is in excess of our liabilities after taking such payment into account. In light of these restrictions, we have no material restrictions on dividend payments that may be made to our shareholders at December 31, 2017. Argo Re is the direct subsidiary of Argo Group, and therefore, has direct dividend paying capabilities to the parent. Argo Re is generally prohibited from declaring or paying, in any financial year, dividends of more than 25% of its total statutory capital and surplus (as shown on its previous financial year’s statutory balance sheet) unless it files (at least seven days before payment of such dividends) with the Bermuda Monetary Authority (“BMA”) an affidavit signed by at least two directors (one of whom must be a Bermuda resident director if any of the insurer’s directors are resident in Bermuda) and the principal representative stating that it will continue to meet its solvency margin and minimum liquidity ratio. Argo Re may not reduce its total statutory capital by 15% or more, as set out in its previous year’s financial statements, unless it has received the prior approval of the BMA. Based on these regulatory restrictions, the maximum amount available for payment of dividends to Argo Group by Argo Re during 2018 without prior regulatory approval is $382.4 million. Argo Re did not pay dividend to Argo Group in 2017. In 2016 and 2015, Argo Re paid cash dividends to Argo Group of $41.0 million. The proceeds of the dividends were used to repay intercompany balances related primarily to dividend and interest payments and other corporate expenses. Our U.S. insurance subsidiaries file financial statements prepared in accordance with statutory accounting principles prescribed or permitted by insurance regulatory authorities of the state in which they are domiciled. The differences between statutory-based financial statements and financial statements prepared in accordance with GAAP vary between jurisdictions. The principal differences are that for statutory-based financial statements, deferred policy acquisition costs are not recognized, a portion of the deferred federal income tax asset is non-admitted, bonds are generally carried at amortized cost, certain assets are non-admitted and charged directly to surplus, a collectability allowance related to reinsurance recoverables is charged directly to surplus and outstanding losses and unearned premium are presented net of reinsurance. As an intermediate insurance holding company, Argo Group US is largely dependent on dividends and other permitted payments from its insurance subsidiaries to service its debt, fund operating expenses and pay dividends to Argo Ireland. Various state insurance laws restrict the amount that may be transferred to Argo Group US from its subsidiaries in the form of dividends without prior approval of regulatory authorities. In addition, that portion of the insurance subsidiaries’ net equity that results from the difference between statutory insurance principles and GAAP would not be available for dividends. Argo Group US did not receive dividends from its subsidiaries in 2017. In December 2016, Argo Group US received an ordinary dividend in the amount of $18.1 million in cash from Rockwood. In December 2016, Argo Group US received an ordinary dividend of $41.6 million from Argonaut Insurance Company. In March 2016, Argo Group US received an ordinary dividend of $35.0 million, in the form of $19.9 million in cash and $15.1 million in securities, from Colony. Argo Group US did not receive dividends from its subsidiaries in 2015. Argonaut Insurance Company is a direct subsidiary of Argo Group US and is regulated by the Illinois Division of Insurance. During 2018, Argonaut Insurance Company may be permitted to pay dividends of up to $87.3 million without approval from the Illinois Division of Insurance. Rockwood, a direct subsidiary of Argo Group US, is regulated by the Pennsylvania Department of Insurance. Rockwood may be permitted to pay dividends of up to $33.2 million without approval from the Pennsylvania Department of Insurance during 2018. Each department of insurance may require prior approval for the payment of all dividends, based on business and regulatory conditions of the insurance companies. During 2016 we realigned our internal ownership structure so that Colony became a direct subsidiary of Argonaut Insurance Company. Prior to 2016, Colony had been a direct subsidiary of Argo Group US. Argo Underwriting Agency Ltd. (“AUA”) and Ariel Corporate Member Ltd. (“ACML”) are our wholly-owned subsidiaries through which we conduct the operations of Syndicates 1200 and 1910, respectively. Dividend payments from AUA and ACML to their immediate parents are not restricted by regulatory authority. Dividend payments will be subject to the earnings, operations, financial condition, capital and general business requirements of AUA and ACML. Certain assets of our subsidiaries are pledged to regulatory agencies, serve as collateral for letters of credit or are assigned as the assets of the trade capital providers of our Lloyd’s syndicate, and therefore, are not available funds that may be paid up as dividends to Argo Group. See Note 3, “Investments” and Note 19, “Segment Information” for further discussion.

Insurance Assessments

Insurance Assessments	12 Months Ended
	Dec. 31, 2017
Insurance [Abstract]
Insurance Assessments	21. Insurance Assessments We are required to participate in statutorily created insolvency guarantee and weather-related loss protection associations in all states in the U.S. where we are authorized to transact business. These associations were formed for the purpose of paying the claims of insolvent companies. We are assessed a pro-rata share of such claims based upon our premium writings, subject to a maximum annual assessment per line of insurance. Certain of these assessments can be recovered through premium tax offsets or policy surcharges. We do not believe that assessments on current insolvencies will have a material impact on our financial condition or results of operations. We have accrued assessments of $4.8 million and $4.7 million

Transactions with Related Parti

Transactions with Related Parties	12 Months Ended
	Dec. 31, 2017
Related Party Transactions [Abstract]
Transactions with Related Parties	22. Transactions with Related Parties In 2013, our Surety unit received a submission through its established broker network to issue approximately $13 million of surety bonds on behalf of Kinetica Partners, LLC (“Kinetica”) in connection with a Gulf of Mexico pipeline project. Mr. Gary Woods, Chairman of our Board of Directors, is also the Chairman of the Board of Directors of Kinetica, and beneficially owns 10% of Kinetica through a family trust. The submission was underwritten, priced and bound in the ordinary course of business by the Surety unit. The terms and conditions of the surety bonds that were issued and the premium charged to Kinetica for issuance of the bonds, were consistent with those routinely applied and charged for similarly situated risks bound for unrelated third-parties. As of December 31, 2017, the surety bonds were still outstanding. Per the Surety unit’s standard requirements in connection with the issuance of surety bonds, Kinetica and Mr. Woods, in his personal capacity, among others, executed our Surety unit’s standard form of indemnity agreement holding our Surety unit harmless against any and all losses and expenses incurred resulting from the issuance of the surety bonds.

Unaudited Quarterly Financial D

Unaudited Quarterly Financial Data	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Unaudited Quarterly Financial Data	23. Unaudited Quarterly Financial Data The following tables represent unaudited quarterly financial data for the years ended December 31, 2017 and 2016. In the opinion of management, all adjustments necessary to present fairly the results of operations for such periods have been made. Total revenue, net income before income taxes and net income include realized gains or losses from the sale of investments and other. We cannot anticipate when or if similar gains or losses may occur in the future. Since financial results rely heavily on estimates, caution should be used in drawing specific conclusions from quarterly consolidated results. (in millions, except per share amounts) 1st Quarter 2nd 3rd Quarter 4th Quarter Year 2017 Total revenue $ 428.1 $ 451.0 $ 439.2 $ 455.8 $ 1,774.1 Net income (loss) before income taxes 42.7 50.8 (65.9 ) 12.3 39.9 Net income (loss) 36.7 46.0 (61.3 ) 28.9 50.3 Net income (loss) per common share : Basic* $ 1.22 $ 1.52 $ (2.04 ) $ 0.98 $ 1.68 Diluted* $ 1.19 $ 1.48 $ (2.04 ) $ 0.95 $ 1.64 Comprehensive income (loss) $ 50.6 $ 64.5 $ (44.4 ) $ 28.5 $ 99.2 (in millions, except per share amounts) 1st Quarter 2nd 3rd Quarter 4th Quarter Year 2016 Total revenue $ 370.1 $ 384.3 $ 416.7 $ 405.4 $ 1,576.5 Net income before income taxes 33.1 39.6 62.2 47.0 181.9 Net income 27.7 30.9 55.2 32.9 146.7 Net income per common share : Basic* $ 0.91 $ 1.03 $ 1.84 $ 1.10 $ 4.86 Diluted* $ 0.89 $ 1.00 $ 1.80 $ 1.07 $ 4.75 Comprehensive income $ 56.5 $ 59.7 $ 57.3 $ 9.4 $ 182.9 * Basic and diluted net income per common share are computed independently for each quarter and full year based on the respective average number of common shares outstanding; therefore, the sum of the quarterly net income per common share data may not equal the net income per common share for the year.

Senior Unsecured Fixed Rate Not

Senior Unsecured Fixed Rate Notes	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Senior Unsecured Fixed Rate Notes	24. Senior Unsecured Fixed Rate Notes In September 2012, Argo Group (the “Parent Guarantor”), through its subsidiary Argo Group US (the “Subsidiary Issuer”), issued $143,750,000 aggregate principal amount of the Subsidiary Issuer’s 6.5% Senior Notes due September 15, 2042 (the “Notes”). The Notes are unsecured and unsubordinated obligations of the Subsidiary Issuer and rank equally in right of payment with all of the Subsidiary Issuer’s other unsecured and unsubordinated debt. The Notes are guaranteed on a full and unconditional senior unsecured basis by the Parent Guarantor. The Notes may be redeemed, for cash, in whole or in part, on or after September 15, 2017, at the Subsidiary Issuer’s option, at any time and from time to time, prior to maturity at a redemption price equal to 100% of the principal amount of the Notes to be redeemed, plus accrued but unpaid interest on the principal amount being redeemed to, but not including, the redemption date. In accordance with ASU 2015-03, “Simplifying the Presentation of Debt Issuance Costs” (Topic 835), we present the unamortized debt issuance costs in the balance sheet as a direct deduction from the carrying value of the debt liability. At December 31, 2017 and 2016, the Notes consisted of the following: (in millions) December 31, 2017 December 31, 2016 Senior unsecured fixed rate notes Principal $ 143.8 $ 143.8 Less: unamortized debt issuance costs (4.2 ) (4.3 ) Senior unsecured fixed rate notes, less unamortized debt issuance costs $ 139.6 $ 139.5 In accordance with Article 10 of SEC Regulation S-X, we have elected to present condensed consolidating financial information in lieu of separate financial statements for the Subsidiary Issuer. The following tables present condensed consolidating financial information at December 31, 2017 and 2016 and for the three years ended December 31, 2017, 2016 and 2015 of the Parent Guarantor and the Subsidiary Issuer. The Subsidiary Issuer is an indirect wholly-owned subsidiary of the Parent Guarantor. Investments in subsidiaries are accounted for by the Parent Guarantor under the equity method for purposes of the supplemental consolidating presentation. Earnings of subsidiaries are reflected in the Parent Guarantor’s investment accounts and earnings. The Parent Guarantor fully and unconditionally guarantees certain of the debt of the Subsidiary Issuer. Condensed consolidating financial information of the Subsidiary Issuer is presented on a consolidated basis and consists principally of the net assets, results of operations and cash flows of operating insurance company subsidiaries. CONDENSED CONSOLIDATING BALANCE SHEET DECEMBER 31, 2017 (in millions) Argo Group International Holdings, Ltd (Parent Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Assets Investments $ 0.4 $ 3,079.0 $ 1,663.5 $ — $ 4,742.9 Cash 0.9 47.8 127.9 — 176.6 Accrued investment income — 18.0 5.5 — 23.5 Premiums receivable — 216.5 382.1 — 598.6 Reinsurance recoverables — 1,487.3 606.0 — 2,093.3 Goodwill and other intangible assets, net 43.2 124.9 90.1 — 258.2 Current income taxes receivable, net — 2.4 (1.0 ) — 1.4 Deferred acquisition costs, net — 80.7 79.7 — 160.4 Ceded unearned premiums — 198.5 201.0 — 399.5 Other assets 9.2 171.5 128.9 — 309.6 Intercompany note receivable — 50.9 (50.9 ) — — Investments in subsidiaries 1,940.0 — — (1,940.0 ) — Total assets $ 1,993.7 $ 5,477.5 $ 3,232.8 $ (1,940.0 ) $ 8,764.0 Liabilities and Shareholders' Equity Reserves for losses and loss adjustment expenses $ — $ 2,483.9 $ 1,717.1 $ — $ 4,201.0 Unearned premiums — 704.0 503.7 — 1,207.7 Funds held and ceded reinsurance payable, net — 799.4 (22.7 ) — 776.7 Long-term debt 153.4 284.5 142.8 — 580.7 Deferred tax liabilities, net — 32.5 (1.2 ) — 31.3 Accrued underwriting expenses and other liabilities 8.9 95.0 43.0 — 146.9 Due to affiliates 11.7 (0.4 ) 0.4 (11.7 ) — Total liabilities 174.0 4,398.9 2,383.1 (11.7 ) 6,944.3 Total shareholders' equity 1,819.7 1,078.6 849.7 (1,928.3 ) 1,819.7 Total liabilities and shareholders' equity $ 1,993.7 $ 5,477.5 $ 3,232.8 $ (1,940.0 ) $ 8,764.0 (1) Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations. (2) Includes all Argo Group parent company eliminations. CONDENSED CONSOLIDATING BALANCE SHEET DECEMBER 31, 2016 (in millions) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Assets Investments $ 1.9 $ 2,833.5 $ 1,484.9 $ — $ 4,320.3 Cash — 53.7 32.3 — 86.0 Accrued investment income — 16.0 4.7 — 20.7 Premiums receivable — 204.9 258.9 — 463.8 Reinsurance recoverables — 1,348.4 37.2 — 1,385.6 Goodwill and other intangible assets, net — 127.1 92.8 — 219.9 Deferred acquisition costs, net — 63.5 75.6 — 139.1 Ceded unearned premiums — 168.9 133.9 — 302.8 Other assets 9.0 168.7 89.1 — 266.8 Intercompany note receivable — 50.2 (50.2 ) — — Investments in subsidiaries 1,834.4 — — (1,834.4 ) — Total assets $ 1,845.3 $ 5,034.9 $ 2,159.2 $ (1,834.4 ) $ 7,205.0 Liabilities and Shareholders' Equity Reserves for losses and loss adjustment expenses $ — $ 2,322.4 $ 1,028.4 $ — $ 3,350.8 Unearned premiums — 580.0 390.0 — 970.0 Funds held and ceded reinsurance payable, net — 750.2 (206.5 ) — 543.7 Long-term debt 28.4 284.4 54.8 — 367.6 Current income taxes payable, net — 8.5 (0.4 ) — 8.1 Deferred tax liabilities, net — 17.6 6.5 — 24.1 Accrued underwriting expenses and other liabilities 13.7 92.0 42.3 — 148.0 Due to affiliates 10.5 1.8 (1.8 ) (10.5 ) — Total liabilities 52.6 4,056.9 1,313.3 (10.5 ) 5,412.3 Total shareholders' equity 1,792.7 978.0 845.9 (1,823.9 ) 1,792.7 Total liabilities and shareholders' equity $ 1,845.3 $ 5,034.9 $ 2,159.2 $ (1,834.4 ) $ 7,205.0 (1) Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations. (2) Includes all Argo Group parent company eliminations. CONDENSED CONSOLIDATING STATEMENT OF INCOME FOR THE YEAR ENDED DECEMBER 31, 2017 (in millions) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Premiums and other revenue: Earned premiums $ — $ 555.9 $ 1,016.4 $ — $ 1,572.3 Net investment (expense) income (4.5 ) 87.5 57.0 — 140.0 Fee and other income — 18.8 3.7 — 22.5 Net realized investment and other gains (loss) 0.4 40.8 (1.9 ) — 39.3 Total revenue (4.1 ) 703.0 1,075.2 — 1,774.1 Expenses: Losses and loss adjustment expenses — 337.9 712.3 — 1,050.2 Underwriting, acquisition and insurance expenses 14.3 266.6 354.5 — 635.4 Interest expense 4.3 17.2 6.2 — 27.7 Fee and other expense — 12.4 2.2 — 14.6 Foreign currency exchange loss 0.1 0.1 6.1 — 6.3 Total expenses 18.7 634.2 1,081.3 — 1,734.2 (Loss) Income before income taxes (22.8 ) 68.8 (6.1 ) — 39.9 Provision for income taxes — (0.6 ) (9.8 ) — (10.4 ) Net (loss) income before equity in earnings of subsidiaries (22.8 ) 69.4 3.7 — 50.3 Equity in undistributed earnings of subsidiaries 73.1 — — (73.1 ) — Net income $ 50.3 $ 69.4 $ 3.7 $ (73.1 ) $ 50.3 (1) Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations. (2) Includes all Argo Group parent company eliminations. CONDENSED CONSOLIDATING STATEMENT OF INCOME FOR THE YEAR ENDED DECEMBER 31, 2016 (in millions) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Premiums and other revenue: Earned premiums $ — $ 497.3 $ 913.5 $ — $ 1,410.8 Net investment income 37.2 78.9 40.0 (41.0 ) 115.1 Fee and other income — 21.4 3.1 — 24.5 Net realized investment and other gains 0.6 50.0 (24.5 ) — 26.1 Total revenue 37.8 647.6 932.1 (41.0 ) 1,576.5 Expenses: Losses and loss adjustment expenses — 290.4 519.7 — 810.1 Underwriting, acquisition and insurance expenses 12.8 204.4 329.8 — 547.0 Interest expense 1.4 15.8 2.4 — 19.6 Fee and other expense — 21.7 0.7 — 22.4 Foreign currency exchange loss (gains) — 0.2 (4.7 ) — (4.5 ) Total expenses 14.2 532.5 847.9 — 1,394.6 Income before income taxes 23.6 115.1 84.2 (41.0 ) 181.9 Provision for income taxes — 33.6 1.6 — 35.2 Net income before equity in earnings of subsidiaries 23.6 81.5 82.6 (41.0 ) 146.7 Equity in undistributed earnings of subsidiaries 123.1 — — (123.1 ) — Net income $ 146.7 $ 81.5 $ 82.6 $ (164.1 ) $ 146.7 (1) Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations. (2) Includes all Argo Group parent company eliminations. CONDENSED CONSOLIDATING STATEMENT OF INCOME FOR THE YEAR ENDED DECEMBER 31, 2015 (in millions) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Premiums and other revenue: Earned premiums $ — $ 497.3 $ 874.6 $ — $ 1,371.9 Net investment income 40.1 59.0 30.5 (41.0 ) 88.6 Fee and other income — 19.0 3.2 — 22.2 Net realized investment and other gains — 31.8 (7.7 ) — 24.1 Total revenue 40.1 607.1 900.6 (41.0 ) 1,506.8 Expenses: Losses and loss adjustment expenses — 304.2 461.9 — 766.1 Underwriting, acquisition and insurance expenses 19.0 198.4 319.3 — 536.7 Interest expense 1.5 15.3 2.2 — 19.0 Fee and other expense — 23.3 2.5 — 25.8 Foreign currency exchange (gain) loss — 1.0 (19.3 ) — (18.3 ) Total expenses 20.5 542.2 766.6 — 1,329.3 Income before income taxes 19.6 64.9 134.0 (41.0 ) 177.5 Provision for income taxes — 12.9 1.4 — 14.3 Net income before equity in earnings of subsidiaries 19.6 52.0 132.6 (41.0 ) 163.2 Equity in undistributed earnings of subsidiaries 143.6 — — (143.6 ) — Net income $ 163.2 $ 52.0 $ 132.6 $ (184.6 ) $ 163.2 (1) Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations. (2) Includes all Argo Group parent company eliminations. CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS FOR YEAR ENDED DECEMBER 31, 2017 (in millions) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Net cash flows from operating activities $ 20.8 $ 149.5 $ (5.3 ) $ — $ 165.0 Cash flows from investing activities: Proceeds from sales of investments — 809.5 920.4 — 1,729.9 Maturities and mandatory calls of fixed maturity investments — 483.1 195.2 — 678.3 Purchases of investments — (1,495.6 ) (1,165.2 ) — (2,660.8 ) Change in short-term investments and foreign regulatory deposits 1.5 67.1 230.9 — 299.5 Settlements of foreign currency exchange forward contracts 0.9 (8.2 ) 4.4 — (2.9 ) Acquisition of subsidiaries, net of cash (235.3 ) — 130.1 — (105.2 ) Issuance of intercompany note, net — — (120.0 ) 120.0 — Purchases of fixed assets and other, net (0.1 ) (26.2 ) (33.8 ) — (60.1 ) Cash provided by (used in) investing activities (233.0 ) (170.3 ) 162.0 120.0 (121.3 ) Cash flows from financing activities: Additional long-term borrowings 125.0 — — — 125.0 Borrowing under the intercompany note 120.0 60.0 (60.0 ) (120.0 ) — Activity under stock incentive plans 1.4 — — — 1.4 Repurchase of Company's common shares (0.1 ) (45.1 ) — — (45.2 ) Payment of cash dividend to common shareholders (33.2 ) — — — (33.2 ) Cash provided by (used in) financing activities 213.1 14.9 (60.0 ) (120.0 ) 48.0 Effect of exchange rate changes on cash — — (1.1 ) — (1.1 ) Change in cash 0.9 (5.9 ) 95.6 — 90.6 Cash, beginning of year — 53.7 32.3 — 86.0 Cash, end of year $ 0.9 $ 47.8 $ 127.9 $ — $ 176.6 (1) Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations. (2) Includes all Argo Group parent company eliminations. CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS FOR YEAR ENDED DECEMBER 31, 2016 (in millions) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Net cash flows from operating activities $ 26.5 $ 72.3 $ 83.2 $ — $ 182.0 Cash flows from investing activities: Proceeds from sales of investments — 1,035.9 407.6 — 1,443.5 Maturities and mandatory calls of fixed maturity investments — 543.2 459.5 — 1,002.7 Purchases of investments — (1,450.0 ) (930.5 ) — (2,380.5 ) Change in short-term investments and foreign regulatory deposits (0.9 ) (138.1 ) (56.2 ) — (195.2 ) Settlements of foreign currency exchange forward contracts — — (5.4 ) — (5.4 ) Purchases of fixed assets and other, net — (11.3 ) 1.1 — (10.2 ) Cash used in investing activities (0.9 ) (20.3 ) (123.9 ) $ — (145.1 ) Cash flows from financing activities: Activity under stock incentive plans 1.0 — — — 1.0 Repurchase of Company's common shares — (47.1 ) — — (47.1 ) Payment of cash dividend to common shareholders (26.6 ) — — — (26.6 ) Intercompany cash dividend — (40.0 ) 40.0 — — Cash provided by (used in) financing activities (25.6 ) (87.1 ) 40.0 — (72.7 ) Effect of exchange rate changes on cash — — 0.1 — 0.1 Change in cash — (35.1 ) (0.6 ) — (35.7 ) Cash, beginning of year — 88.8 32.9 — 121.7 Cash, end of year $ — $ 53.7 $ 32.3 $ — $ 86.0 (1) Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations. (2) Includes all Argo Group parent company eliminations. CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS FOR YEAR ENDED DECEMBER 31, 2015 (in millions) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Net cash flows from operating activities $ 32.7 $ 117.0 $ 133.5 $ — $ 283.2 Cash flows from investing activities: Proceeds from sales of investments — 631.1 336.8 — 967.9 Maturities and mandatory calls of fixed maturity investments — 681.8 162.1 — 843.9 Purchases of investments — (1,384.5 ) (649.6 ) — (2,034.1 ) Change in short-term investments and foreign regulatory deposits 0.9 14.9 33.8 — 49.6 Settlements of foreign currency exchange forward contracts 1.5 — (11.6 ) — (10.1 ) Issuance of intercompany note, net — 7.5 (7.5 ) — — Redemption of PXRE Capital Trust V — 18.0 (18.0 ) — — Purchases of fixed assets and other, net 3.8 (16.6 ) 2.0 — (10.8 ) Cash provided by (used in) investing activities 6.2 (47.8 ) (152.0 ) — (193.6 ) Cash flows from financing activities: Redemption of PXRE Capital Trust V (18.0 ) — 18.0 — — Activity under stock incentive plans 1.8 — — — 1.8 Repurchase of Company's common shares — (29.7 ) — — (29.7 ) Payment of cash dividend to common shareholders (22.7 ) — — — (22.7 ) Cash used in financing activities (38.9 ) (29.7 ) 18.0 — (50.6 ) Effect of exchange rate changes on cash — — 1.7 — 1.7 Change in cash — 39.5 1.2 — 40.7 Cash, beginning of year — 49.3 31.7 — 81.0 Cash, end of year $ — $ 88.8 $ 32.9 $ — $ 121.7 (1) Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations. (2) Includes all Argo Group parent company eliminations.

Subsequent Event

Subsequent Event	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	25. Subsequent Events 15% Stock Dividend On February 20, 2018, our Board of Directors declared a 15% stock dividend payable on , to shareholders of record at the close of business . Based upon information available to us as of the dividend declaration date, we estimate approximately million shares will be issued upon payment of the stock dividend. The share numbers and per share amounts disclosed in the Consolidated Financial Statements and Notes to the Consolidated Financial Statements have not been retroactively adjusted to give effect to the stock dividend. Presented below are pro forma basic and diluted net income per common share after giving effect for the stock dividend: For the Years Ended December 31, 2017 2016 2015 Pro forma net income per common share: Basic $ 1.46 $ 4.23 $ 4.61 Diluted $ 1.42 $ 4.13 $ 4.52 The pro forma impact of the stock dividend on the Consolidated Balance Sheet at December 31, 2017 is an increase in common stock of $4.4 million, an increase in additional paid-in capital of $255.7 million and a decrease to retained earnings of $260.1 million.

Schedule II Condensed Financial

Schedule II Condensed Financial Information of Registrant	12 Months Ended
	Dec. 31, 2017
Condensed Financial Information Of Parent Company Only Disclosure [Abstract]
Schedule II Condensed Financial Information of Registrant	ARGO GROUP INTERNATIONAL HOLDINGS, LTD. SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT (in millions) BALANCE SHEETS December 31, 2017 2016 Assets Short-term investments 0.4 1.9 Investment in subsidiaries 1,940.0 1,834.4 Cash 0.9 — Goodwill and other intangible assets, net 43.2 — Other assets 9.2 9.0 Total assets $ 1,993.7 $ 1,845.3 Liabilities and Shareholders' Equity Junior subordinated debentures $ 28.4 $ 28.4 Other indebtedness 125.0 — Accrued underwriting expenses and other liabilities 8.9 13.7 Due to subsidiaries 11.7 10.5 Total liabilities 174.0 52.6 Shareholders' equity 1,819.7 1,792.7 Total liabilities and shareholders' equity $ 1,993.7 $ 1,845.3 STATEMENTS OF INCOME For the Years Ended December 31, 2017 2016 2015 Revenue: Net investment (expense) income (1) $ (4.5 ) $ 37.2 $ 40.1 Net realized investment and other gains 0.4 0.6 — Total revenue (4.1 ) 37.8 40.1 Expenses: Interest expense 4.3 1.4 1.5 Other expenses 14.3 12.8 19.0 Foreign currency exchange loss 0.1 — — Total expenses 18.7 14.2 20.5 Net income before equity in earnings of subsidiaries (2) (22.8 ) 23.6 19.6 Equity in undistributed earnings of subsidiaries 73.1 123.1 143.6 Net income $ 50.3 $ 146.7 $ 163.2 (1) For the years ended December 31, 2016 and 2015, net investment income includes intercompany dividends of $41.0 million. (2) Argo Group International Holdings, Ltd. is not subject to taxation. ARGO GROUP INTERNATIONAL HOLDINGS, LTD. SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT (in millions) STATEMENTS OF CASH FLOWS For the Years Ended December 31, 2017 2016 2015 Cash flows from operating activities: Net income $ 50.3 $ 146.7 $ 163.2 Adjustments to reconcile net income to net cash provided by (used in) operating activities: Amortization and depreciation 1.3 0.2 0.2 Share-based payments expense 3.3 4.2 8.2 Net realized investment and other gains (0.4 ) (0.6 ) — Undistributed earnings of subsidiaries (73.1 ) (123.1 ) (143.6 ) Change in: Prepaid assets (0.3 ) (0.2 ) 0.2 Accrued underwriting expenses (2.3 ) 0.9 (2.4 ) Due to subsidiaries 48.1 (0.6 ) 12.5 Other, net (6.1 ) (1.0 ) (5.6 ) Cash provided by operating activities 20.8 26.5 32.7 Cash flows from investing activities: Change in short-term investments 1.5 (0.9 ) 0.9 Settlements of foreign currency exchange forward contracts 0.9 — 1.5 Acquisition of subsidiaries, net of cash (235.3 ) — — Purchases of fixed assets and other, net (0.1 ) — 3.8 Cash (used in) provided by investing activities (233.0 ) (0.9 ) 6.2 Cash flows from financing activities: Additional borrowings 125.0 — — Borrowings under intercompany note payable, net 120.0 — — Activity under stock incentive plans 1.4 1.0 1.8 Redemption of PXRE Capital Trust V — — (18.0 ) Repurchase of Company's common shares (0.1 ) — — Payment of cash dividend to common shareholders (33.2 ) (26.6 ) (22.7 ) Cash provided by (used in) financing activities 213.1 (25.6 ) (38.9 ) Change in cash 0.9 — — Cash, beginning of year — — — Cash, end of year $ 0.9 $ — $ —

Schedule III Supplemental Insur

Schedule III Supplemental Insurance Information	12 Months Ended
	Dec. 31, 2017
Supplementary Insurance Information [Abstract]
Schedule III Supplemental Insurance Information	ARGO GROUP INTERNATIONAL HOLDINGS, LTD. SCHEDULE III SUPPLEMENTAL INSURANCE INFORMATION FOR THE YEARS ENDED DECEMBER 31, 2017, 2016 AND 2015 (in millions) Segment DAC (a) Reserves for Losses and Loss Adjustment Expenses (b) UPR (c) Premium Revenue (d) Net Investment Income (l) Loss & LAE (e) Amortization (Deferral) DAC (f) (2) Other Operating Expenses (3) Net Premiums Written (g) Year Ended December 31, 2017 U.S. Operations 80.8 2,196.1 695.1 936.6 87.2 528.1 (17.3 ) 336.4 1,031.8 International Operations 79.6 1,723.0 512.6 635.8 32.7 504.8 5.4 236.8 621.7 Run-off Lines — 281.9 — (0.1 ) 9.3 17.3 — 8.3 — Corporate and Other — — — — 10.8 — — 65.8 — Total $ 160.4 $ 4,201.0 $ 1,207.7 $ 1,572.3 $ 140.0 $ 1,050.2 $ (11.9 ) $ 647.3 $ 1,653.5 Year Ended December 31, 2016 U.S. Operations 63.5 2,028.4 575.4 849.5 71.9 467.5 (5.1 ) 275.6 883.5 International Operations 75.6 1,031.5 394.6 560.9 28.7 324.0 0.8 210.3 556.4 Run-off Lines — 290.9 — 0.4 11.3 18.6 — 6.9 0.3 Corporate and Other — — — — 3.2 — — 58.5 — Total $ 139.1 $ 3,350.8 $ 970.0 $ 1,410.8 $ 115.1 $ 810.1 $ (4.3 ) $ 551.3 $ 1,440.2 Year Ended December 31, 2015 U.S. Operations 58.4 1,888.6 501.2 815.4 52.2 471.1 1.0 258.4 838.5 International Operations 74.0 928.7 385.5 556.1 20.3 286.4 (1.9 ) 222.3 563.2 Run-off Lines — 306.3 — 0.4 8.1 8.6 — 5.9 0.4 Corporate and Other — — — — 8.0 — — 51.0 — Total $ 132.4 $ 3,123.6 $ 886.7 $ 1,371.9 $ 88.6 $ 766.1 $ (0.9 ) $ 537.6 $ 1,402.1 (a) Deferred Policy Acquisition Cost (b) Future Policy Benefits, Losses, Claims and Loss Expenses (c) Unearned Premiums (d) Premium Revenue, net (premiums earned) (e) Benefits, Claims, Losses and Settlement Expenses (f) Amortization (Deferral) of Deferred Policy Acquisition Costs (g) Premiums Written, net (1) Net Investment Income allocated based upon each segment’s share of investable funds. (2) The amortization (deferral) of DAC will not equal the change in the balance sheet. See Note 1, “Business and Significant Accounting Policies” for further discussion. (3) Other Insurance Expenses allocated based on specific identification, where possible, and related activities.

Schedule V Valuation and Qualif

Schedule V Valuation and Qualifying Accounts	12 Months Ended
	Dec. 31, 2017
Valuation And Qualifying Accounts [Abstract]
Schedule V Valuation and Qualifying Accounts	ARGO GROUP INTERNATIONAL HOLDINGS, LTD. SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS (in millions) Balance at Beginning of Year Charged Cost and Expense Capital Loss Carryforward Net Operating Loss Carryforward Charged to Other Accounts Deductions Balance at End Year Ended December 31, 2017 Deducted from assets: Valuation allowance for deferred tax asset $ 23.5 $ (6.2 ) $ — $ 2.7 $ 0.1 $ — $ 20.1 Year Ended December 31, 2016 Deducted from assets: Valuation allowance for deferred tax asset $ 22.8 $ (1.0 ) $ — $ — $ 1.7 $ — $ 23.5 Year Ended December 31, 2015 Deducted from assets: Valuation allowance for deferred tax asset $ 25.4 $ (1.0 ) $ — $ — $ (1.6 ) $ — $ 22.8

Schedule VI Supplemental Inform

Schedule VI Supplemental Information for Property-Casualty Insurance Companies	12 Months Ended
	Dec. 31, 2017
Supplemental Information For Property Casualty Insurance Underwriters [Abstract]
Schedule VI Supplemental Information for Property-Casualty Insurance Companies	ARGO GROUP INTERNATIONAL HOLDINGS, LTD. SCHEDULE VI SUPPLEMENTAL INFORMATION FOR PROPERTY-CASUALTY INSURANCE COMPANIES (in millions) For the Years Ended December 31, 2017 2016 2015 Deferred acquisition costs $ 160.4 $ 139.1 $ 132.4 Reserves for losses and loss adjustment expenses $ 4,201.0 $ 3,350.8 $ 3,123.6 Unamortized discount in reserves for losses $ 17.6 $ 19.4 $ 21.5 Unearned premiums $ 1,207.7 $ 970.0 $ 886.7 Premiums earned $ 1,572.3 $ 1,410.8 $ 1,371.9 Net investment income $ 140.0 $ 115.1 $ 88.6 Losses and loss adjustment expenses incurred: Current year $ 1,058.4 $ 843.4 $ 798.5 Prior years (8.2 ) (33.3 ) (32.4 ) Losses and loss adjustment expenses incurred $ 1,050.2 $ 810.1 $ 766.1 Deferral of policy acquisition costs (1) $ (11.9 ) $ (4.3 ) $ (0.9 ) Paid losses and loss adjustment expenses, net of reinsurance $ 889.4 $ 716.5 $ 733.5 Gross premiums written $ 2,697.2 $ 2,164.8 $ 2,012.1 (1) The amortization (deferral) of policy acquisition costs will not equal the change in the balance sheet. For further discussion, see Note 1, “Business and Significant Accounting Policies.”

Business and Significant Acco38

Business and Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Business	Business Argo Group International Holdings, Ltd. (“Argo Group,” “we” or the “Company”) is an international underwriter of specialty insurance and reinsurance products in the property and casualty market. Argo Group US, Inc. (“Argo Group US”) is a subsidiary of Argo Financial Holding (Ireland) UC (“Argo Ireland”). Argo Underwriting Agency Limited (“Syndicate 1200”) is a subsidiary of Argo International Holdings, Ltd. Argo Re, Ltd. (“Argo Re”), a Bermuda based company, is the parent of both Argo Ireland and Argo International Holdings, Ltd. Argo Re is directly owned by Argo Group. Effective February 6, 2017, we acquired all of the issued and outstanding capital stock of Maybrooke Holdings, S.A. (“Maybrooke”), a holding company, and its subsidiaries, which operates under the name “Ariel Re.” As of December 31, 2017, Ariel Reinsurance, Ltd. was dissolved and merged into Argo Re, unifying the two reinsurance platforms as one functional unit. See Note 2, “Acquisition of Maybrooke” for additional detail related to the acquisition. During the first quarter of 2017, we evaluated and modified the presentation of our reportable segments to better reflect our new operating framework and management structure. Under this model, Argo Group’s chief operating decision maker – Mark E. Watson III, President and Chief Executive Officer – evaluates performance and allocates resources based on the review of the U.S. Operations and the International Operations. The business unit that produces the risk and not the location of the underlying exposure is the primary characteristic in distinguishing operating and reportable segments. For example, a U.S. property exposure underwritten through our Syndicate platform would be included in International Operations. Consistent with prior periods, the Run-off Lines and Corporate segments include all other activity of Argo Group and are included in our consolidated financial results. Segment results for the years ended December 31, 2016 and 2015 have been reclassified to conform to the current presentation. The two segments are further described below. U.S. Operations is comprised of the former Excess and Surplus Lines and Commercial Specialty reporting segments. Excess and Surplus Lines products are underwritten by Colony Insurance Company (“Colony”). Commercial Specialty represents our admitted specialty businesses and consists of the following operations: Argo Insurance, Rockwood Casualty Insurance Company (“Rockwood”), Argo Pro, Argo Surety, Trident Insurance Services, Alteris and ARIS Title Insurance Corporation (“ARIS”). International Operations is comprised of the former Syndicate 1200 and International Specialty reporting segments, and the recently acquired Ariel Re business. International Specialty products are provided by our Bermuda operations, which include Argo Re and Argo Insurance – Casualty and Professional Lines and Argo Seguros Brasil S.A. based in Brazil. Syndicate 1200 products are underwritten by Argo Underwriting Agency Limited based in London, on behalf of one underwriting syndicate under the Lloyd’s of London (“Lloyd’s”) global franchise. Ariel Re products are primarily underwritten by Ariel Corporate Member Limited in London, on behalf of Syndicate 1910 under Lloyd’s. Our Run-off Lines
Basis of Presentation and Use of Estimates	Basis of Presentation and Use of Estimates The consolidated financial statements of Argo Group and its subsidiaries have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”). The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenues and expenses during the reporting period. The major estimates reflected in our consolidated financial statements include, but are not limited to, reserves for losses and loss adjustment expenses; reinsurance recoverables, including the reinsurance recoverables allowance for doubtful accounts; estimates of written and earned premiums; reinsurance premium receivable; fair value of investments and assessment of potential impairment; valuation of goodwill and other intangibles and our deferred tax asset valuation allowance. Actual results could differ from those estimates. Specifically, estimates for reserves for losses and loss adjustment expenses are based upon past claim experience modified for current trends as well as prevailing economic, legal and social conditions. Although management believes that amounts included in the accompanying consolidated financial statements are reasonable, such estimates may be more or less than the amounts ultimately paid when the claims are settled. The estimates are continually reviewed and any changes are reflected in current operating results. Further, the nature of loss exposures involves significant variability due to the nature of the long-tailed payments on certain claims. As such, losses and loss adjustment expenses could vary significantly from the recorded amounts. The consolidated financial statements include the accounts and operations of Argo Group and its subsidiaries. All material intercompany accounts and transactions have been eliminated. Certain amounts in prior years’ financial statements have been reclassified to conform to the current presentation. Amounts related to trade capital providers, who are third-party capital participants that provide underwriting capital to the Syndicate 1200 segment, are included in the balance sheet. Trade capital providers participate on a quota share basis, assuming 100% of their contractual participation in the underwriting syndicate results and with such results settled on a year of account basis. We have evaluated our investment in our twelve statutory trusts (collectively, the “Trusts”) and two charitable foundations (collectively, the “Foundations”) under the Financial Accounting Standards Board’s (“FASB’s”) provisions for consolidation of variable interest entities under Accounting Standards Codification (“ASC”) Topic 810-10, “Consolidation,” as amended. We determined that the Trusts and Foundations are variable interest entities due to the fact that the Trusts and Foundations do not have sufficient equity to finance their activities without additional subordinate financial support from other parties. We do not have any power to direct the activities that impact the Trusts’ or Foundations’ economic performance. We are not entitled to receive a majority of the residual returns of the Trusts and U.S. charitable foundations. Additionally, we are not responsible for absorbing the majority of the expected losses of the Trusts or U.S. charitable foundations; therefore, we are not the primary beneficiary and, accordingly, the Trusts and U.S. charitable foundations are not included in our consolidated financial statements. The expenses and donations of the charitable foundations in Bermuda are paid by Argo Group and have been included in the consolidated results. We have used a series of special purpose reinsurance companies to provide reinsurance coverage through a series of transactions, including insurance-linked securities. Under the provisions of ASC Topic 810-10, these reinsurance companies are variable interest entities. However, we do not have a variable interest in these entities, and therefore are not required to consolidate them in our consolidated financial statements.
Stock Dividend	Stock Dividends On February 20, 2018, our Board of Directors declared a 15% stock dividend, payable on March 21, 2018, to shareholders of record at the close of business on March 7, 2018. The share numbers and per share amounts disclosed in the Consolidated Financial Statements and Notes to the Consolidated Financial Statements have not been retroactively adjusted to give effect to the stock dividend. For additional details regarding the stock dividend, see Note 25, “Subsequent Event.” On May 3, 2016, our Board of Directors declared a 10% stock dividend, payable on June 15, 2016, to shareholders of record at the close of business on June 1, 2016. As a result of the stock dividend, 2,735,542 additional shares were issued. Cash was paid in lieu of fractional shares of our common shares. All references to share and per share amounts in this document and related disclosures have been adjusted to reflect the stock dividend for all periods presented. On February 17, 2015, our Board of Directors declared a 10% stock dividend, payable on March 16, 2015, to shareholders of record at the close of business on March 2, 2015. As a result of the stock dividend, 2,554,506 additional shares were issued. Cash was paid in lieu of fractional shares of our common shares. All references to share and per share amounts in this document and related disclosures have been adjusted to reflect the stock dividend for all periods presented.
Cash	Cash Cash consists of cash deposited in banks, generally in concentration and operating accounts. Interest-bearing cash accounts are classified as short-term investments.
Investments	Investments Investments in fixed maturities at December 31, 2017 and 2016 include bonds and structured securities. Equity securities include common stocks, preferred stocks and mutual funds. Other investments consist of foreign regulatory deposits, hedge funds, private equity funds, private equity direct investments, and voluntary pools. Short-term investments consist of money market funds, certificates of deposit, bonds, sovereign debt and interest-bearing cash accounts. Investments maturing in less than one year are classified as short-term investments in our consolidated financial statements. The amortized cost of fixed maturity securities is adjusted for amortization of premiums and accretion of discounts. This amortization or accretion is included in “Net investment income” in our Consolidated Statements of Income. For the structured securities portion of the fixed maturity securities portfolio, we recognize income using a constant effective yield based on anticipated prepayments and the estimated economic life of the securities. Premium or discount on high investment grade securities (rated AA or higher) is amortized into income using the retrospective method. Premium or discount on lower grade securities (rated less than AA) is amortized into income using the prospective method. Our investments in fixed maturities and equity securities are considered available-for-sale and are carried at fair value. Changes in the fair value of investments classified as available-for-sale are not recognized in income during the period, but rather are recognized as a separate component of shareholders’ equity until realized. Fair value of these investments is estimated using prices obtained from third-party pricing services, where available. For securities where we were unable to obtain fair values from a pricing service or broker, fair values were estimated using information obtained from investment advisors. We performed several processes to ascertain the reasonableness of these investment values by i) obtaining and reviewing internal control reports for our service providers that obtain fair values from third-party pricing services, ii) discussing with our investment managers their process for reviewing and validating pricing obtained from outside services and obtaining values for all securities from our investment managers and iii) comparing the security pricing received from the investment managers with the prices used in the consolidated financial statements and obtaining additional information for variances that exceeded a certain threshold. As of December 31, 2017, investments we hold for which we did not receive a fair value from a pricing service or broker accounted for less than 1% of our investment portfolio. The actual value at which these securities could actually be sold or settled with a willing buyer or seller may differ from our estimated fair values depending on a number of factors including, but not limited to, current and future economic conditions, quantity sold or settled, presence of an active market and availability of a willing buyer or seller. The cost of securities sold is based on the specific identification method. Changes in the value of other investments consisting of hedge funds, private equity funds, private equity direct investments and voluntary pools are principally recognized to income during the period using the equity method of accounting. Our foreign regulatory deposits are assets held in trust in jurisdictions where there is a legal and regulatory requirement to maintain funds locally in order to protect policyholders. Lloyd’s is the appointed investment manager for the funds. The underlying assets are invested in government securities, agency securities and corporate bonds whose values are obtained from Lloyd’s. Foreign currency future contracts held by us are valued by our counterparties using market driven foreign currency exchange rates. We regularly evaluate our investments for other-than-temporary impairment. For fixed maturity securities, the evaluation for a credit loss is generally based on the present value of expected cash flows of the security as compared to the amortized book value. For structured securities, frequency and severity of loss inputs are used in projecting future cash flows of the securities. Loss frequency is measured as the credit default rate, which includes such factors as loan-to-value ratios and credit scores of borrowers. Loss severity includes such factors as trends in real estate values and proceeds at foreclosure. We also recognize other-than-temporary losses on our fixed maturity securities that we intend to sell. All investment balances include amounts relating to trade capital providers. The results of operations and other comprehensive income exclude amounts relating to trade capital providers. Trade capital providers’ participation in the syndicate results are included in reinsurance recoverable for ceded losses and reinsurance payable for ceded premiums.
Receivables	Receivables Premiums receivable, representing amounts due from insureds, are presented net of an allowance for doubtful accounts. The allowances for doubtful accounts were $0.6 million and $2.7 million at December 31, 2017 and 2016, respectively. Premiums receivable include amounts relating to the trade capital providers’ quota share. Reinsurance recoverables represent amounts of paid losses and loss adjustment expenses, case reserves and incurred but not reported (“IBNR”) amounts ceded to reinsurers under reinsurance treaties. Reinsurance recoverables also reflect amounts that are due from trade capital providers. Reinsurance recoverables are presented in our Consolidated Balance Sheets net of an allowance for doubtful accounts of $2.1 million and $2.1 million at December 31, 2017 and 2016, respectively (see Note 4, “Reinsurance” for related disclosures). An estimate of amounts that are likely to be charged off is established as an allowance for doubtful accounts as of the balance sheet date. Our estimate includes specific insured and reinsurance balances that are considered probable to be charged off after all collection efforts have ceased and in accordance with historical write-off trends based on aging categories. Premiums receivable and reinsurance recoverables on paid losses written off, net of recoveries against the allowance for doubtful accounts or directly to the income statement are as follows: For the Years Ended December 31, (in millions) 2017 2016 2015 Premiums receivable $ 1.5 $ 1.1 $ 1.0 Reinsurance recoverables — — 0.2 Net written off $ 1.5 $ 1.1 $ 1.2 Recoveries occur when subsequent collection or litigation results in the receipt of amounts previously written off. Amounts recovered are applied against the bad debt expense account.
Earned Premiums	Earned Premiums Premium revenue is generally recognized ratably over the policy period. Premiums that have yet to be earned are reported as “Unearned premiums” in our Consolidated Balance Sheets. Unearned premium balances include cessions to reinsurers including trade capital providers, while the earned premium recognized in our Consolidated Statements of Income excludes amounts relating to trade capital providers. The trade capital providers’ quota share amount is included in “Ceded reinsurance payable, net”. Assumed reinstatement premiums that reinstate coverage are written and earned at the time the associated loss event occurs. The original premium is earned over the remaining exposure period of the contract. Reinstatement premiums are estimated based upon contract terms for reported losses and estimated for incurred but not reported losses.
Retrospectively Rated Policies	Retrospectively Rated Policies We have written a number of workers compensation, property and other liability policies that are retrospectively rated. Under this type of policy, the policyholder or coverholder may be entitled, subsequent to coverage expiration, to a refund or may owe additional premiums based on the amount of losses incurred under the policy. The retrospective premium adjustments on certain policies are limited to a minimum or maximum premium adjustment, which is calculated as a percentage of the standard amount of premium charged during the life of the policy. Accrued retrospectively rated premiums have been determined based on estimated ultimate loss experience of the individual policyholder accounts. The estimated liability for return of premiums under retrospectively rated policies is included in “Unearned premiums” in our Consolidated Balance Sheets and was $7.9 million and $5.7 million at December 31, 2017 and 2016, respectively. The estimated amount included in premiums receivables for additional premiums due under retrospectively rated policies was $0.1 million and $0.1 million at December 31, 2017 and 2016, respectively.
Deferred Acquisition Costs	Deferred Acquisition Costs Policy acquisition costs, which include commissions, premium taxes, fees and certain other costs of underwriting policies, are deferred, when such class of policies are profitable, and amortized over the same period in which the related premiums are earned. To qualify for capitalization, the policy acquisition cost must be directly related to the successful acquisition of an insurance contract. Anticipated investment income is considered in determining whether the deferred acquisition costs are recoverable and whether a premium deficiency exists. We continually review the methods of making such estimates and establishing the deferred costs with any adjustments made in the accounting period in which the adjustment arose. The 2017 and 2016 net amortization of policy acquisition costs will not equal the change in our Consolidated Balance Sheets as the trade capital providers’ share is not reflected in our Consolidated Statements of Income and differences arise from foreign currency exchange rates applied to deferred acquisition costs which are treated as a nonmonetary asset.
Reserves for Losses and Loss Adjustment Expenses	Reserves for Losses and Loss Adjustment Expenses Liabilities for unpaid losses and loss adjustment expenses include the accumulation of individual case estimates for claims reported as well as estimates of IBNR claims and estimates of claim settlement expenses. Reinsurance recoverables on unpaid claims and claim expenses represent estimates of the portion of such liabilities that will be recoverable from reinsurers. Amounts recoverable from reinsurers are recognized as assets at the same time and in a manner consistent with the unpaid claims liabilities associated with the reinsurance policy.
Reinsurance	Reinsurance In the normal course of business, our insurance and reinsurance subsidiaries cede risks above certain retention levels to other insurance companies. Reinsurance recoverables include claims we paid and estimates of unpaid losses and loss adjustment expenses that are subject to reimbursement under reinsurance and retrocessional contracts. The method for determining reinsurance recoverables for unpaid losses and loss adjustment expenses involves reviewing actuarial estimates of gross unpaid losses and loss adjustment expenses to determine our ability to cede unpaid losses and loss adjustment expenses under our existing reinsurance contracts. This method is continually reviewed and updated and any resulting adjustments are reflected in earnings in the period identified. Reinsurance premiums, commissions and expense reimbursements are accounted for on a basis consistent with those used in accounting for the original policies issued and the term of the reinsurance contracts. Amounts recoverable from reinsurers for losses and loss adjustment expenses for which our insurance and reinsurance subsidiaries have not been relieved of their legal obligations to the policyholder are reported as assets.
Goodwill and Intangible Assets	Goodwill and Intangible Assets Goodwill and intangible assets are allocated to the segment in which the results of operations for the acquired company are reported (see Note 19, “Segment Information” for further discussion). Intangible assets with a finite life are amortized over the estimated useful life of the asset. Goodwill and intangible assets with an indefinite useful life are not amortized. Goodwill and intangible assets are tested for impairment on an annual basis or more frequently if events or changes in circumstances indicate that the carrying amount may not be recoverable. For goodwill, we may perform a qualitative test to determine whether it is more-likely-than-not that the fair value of a reporting unit is less than its carrying amount. A quantitative impairment test was performed in 2017. The first step of the quantitative test is to identify if a potential impairment exists by comparing the fair value of a reporting unit with its carrying amount, including goodwill (“Step 1”). If the fair value of a reporting unit exceeds its carrying value amount, goodwill of the reporting unit is not considered to have a potential impairment and the second step is not necessary. However, if the carrying amount of the reporting unit exceeds its fair value, the second step (“Step 2”) is performed to determine if goodwill is impaired and to measure the amount of impairment loss to recognize, if any. Step 2 compares the implied fair value of goodwill with the carrying amount of goodwill. If the implied value of goodwill is less than the carrying amount of goodwill, it is written down to its fair value with a corresponding expense reflected in the Consolidated Statements of Income. The implied goodwill is calculated based on a hypothetical purchase price allocation, similar to the requirements in the accounting guidance for business combinations, whereby the implied fair value of the reporting unit is allocated to the fair value of the assets and liabilities of the reporting unit. We perform our goodwill impairment test on the first day of the fourth quarter of each year, or October 1 of each year. In performing Step 1 of the impairment test, we estimated the fair value of reporting units using an average of three valuation methods: a comparable company analysis, a precedent transaction analysis and a discounted cash flow analysis. All three methods require management to make various judgments and assumptions. The discounted cash flow analysis included projections of earned premiums, loss ratios, expense growth and discount rates for each reporting unit. Assumptions about such future cash flows are based on our budgets, business plans, economic projections, anticipated future cash flows and market data. Finally, the comparable company analysis and precedent transaction analysis required judgment in selecting comparable companies and comparable transactions for use in the calculations. In all instances, future changes in these judgments and assumptions could cause impairment of goodwill. For the years ended December 31, 2017, 2016 and 2015, all of our reporting units passed Step 1 of the goodwill impairment analysis as the fair value of each reporting unit were in excess of their carrying values. Therefore, Step 2 of the goodwill impairment analysis was not required. Any future decline in the fair value of these reporting units could result in the carrying value of the reporting unit being in excess of fair value, triggering Step 2 of the impairment testing model, which could result in an impairment of goodwill. As noted above, we have elected to make the first day of the fourth quarter the annual impairment assessment date for goodwill and indefinite-lived intangible assets. A quantitative impairment analysis subsequent to this date has not been performed as we have concluded that no additional indicators of impairment have arisen through December 31, 2017. The following table presents our intangible assets and accumulated amortization at December 31: December 31, 2017 December 31, 2016 (in millions) Gross Carrying Amount Accumulated Amortization Gross Carrying Amount Accumulated Amortization Lloyd's capacity $ 89.0 n/a $ 60.5 n/a Distribution network 50.2 44.1 45.5 38.5 Additional Lloyd's capacity 4.8 4.8 4.8 4.8 Other 3.3 1.6 1.4 1.2 $ 147.3 $ 50.5 $ 112.2 $ 44.5 Included in the table above is the $35.1 million of intangible assets acquired from the acquisition of Maybrooke (see Note 2, “Acquisition of Maybrooke”). The weighted average useful life by category at December 31, 2017 was 8.9 years for the distribution network, 5.0 years for the additional Lloyd’s capacity and 6.4 years for other. The weighted average useful life for all categories was 8.5 years at December 31, 2017. During the years ended December 31, 2017, 2016 and 2015, amortization expense was $5.9 million, $5.5 million and $7.5 million, respectively, and is included in “Underwriting, acquisition and insurance expenses” in our Consolidated Statements of Income . The estimated amortization expense for the years ended December 31, 2018, 2019, 2020, 2021 and 2022 is $3.5 million, $1.7 million, $1.3 million, $1.3 million and $0.1 million, respectively.
Property and Equipment	Property and Equipment Property and equipment used in operations, including certain costs incurred to develop or obtain computer software for internal use, are capitalized and carried at cost less accumulated depreciation and are reported in “Other assets” in our Consolidated Balance Sheets. Depreciation is calculated using a straight-line method over the estimated useful lives of the assets, generally three to thirty nine years. The accumulated depreciation for property and equipment was $124.1 million and $101.6 million at December 31, 2017 and 2016, respectively. The net book value of our property and equipment at December 31, 2017 and 2016 was $151.0 million and $142.2 million, respectively. The depreciation expense at December 31, 2017, 2016 and 2015 was $24.1 million, $20.9 million and $17.5 million, respectively.
Derivative Instruments	Derivative Instruments We enter into short-term, currency spot and forward contracts to mitigate foreign exchange rate exposure in our non-U.S. Dollar denominated fixed maturity investments. The forward contracts used are typically thirty to ninety days and are renewed as long as the non-U.S. Dollar denominated fixed maturity investments are held in our portfolio. Forward contracts are designated as hedges for accounting purposes. We also enter into foreign currency exchange forward contracts to manage currency exposure on losses related to global catastrophe events. These foreign currency forward contracts are carried at fair value in our Consolidated Balance Sheets in “Other liabilities” and “Other assets” at December 31, 2017 and 2016, respectively. The realized and unrealized gains and losses are included in “Net realized investment and other gains” in our Consolidated Statements of Income.
Share-Based Payments	Share-Based Payments Compensation expense for share-based payments is recognized based on the measurement-date fair value for awards that will settle in shares. Awards that are expected to be settled in cash are accounted for as liability awards, resulting in the fair value of the award being measured at each reporting date until the award is exercised, forfeited or expires unexercised. Compensation expense for awards that are settled in equity are recognized on a straight line pro rata basis over the vesting period, adjusted for expected forfeitures. See Note 13, “Share-based Compensation” for related disclosures.
Foreign Currency Exchange Gain (Loss)	Foreign Currency Exchange Gain (Loss) The U.S. Dollar is the functional currency of all but two of our foreign operations. Monetary assets and liabilities in foreign operations that are denominated in foreign currencies are revalued at the exchange rates in effect at the balance sheet date. The resulting gains and losses from changes in the foreign exchange rates are reflected in net income. Revenues and expenses denominated in foreign currencies are translated at the prevailing exchange rate during the period with the resulting foreign exchange gains and losses included in net income for the period. In the case of our foreign currency denominated available-for-sale investments, the change in exchange rates between the local currency and our functional currency at each balance sheet date represents an unrealized appreciation or depreciation in value of these securities and is included as a component of accumulated other comprehensive gain. Translation gains and losses related to our operations in Brazil and Malta are recorded as a component of shareholders’ equity in our Consolidated Balance Sheets. At December 31, 2017 and 2016, the foreign currency translation adjustments were a loss of $19.0 million and $17.6 million, respectively.
Income Taxes	Income Taxes On December 22, 2017, the Tax Cuts and Jobs Act (“TCJA”) was enacted. Among many changes resulting from TCJA, the new law (i) reduces the corporate tax rate to 21% effective January 1, 2018, (ii) eliminates the corporate alternative minimum tax for tax years beginning after December 31, 2017, (iii) allows businesses to immediately expense, for tax purposes, the cost of new investments in certain qualified depreciable assets, (iv) modifies the computation of loss reserve discounting for tax purposes, (v) modifies the recognition of income rules by requiring the recognition of income for certain items no later than the tax year in which an item is taken into account as income on an applicable financial statement and (vi) significantly modifies the United States international tax system. Deferred tax assets and liabilities are recognized for the estimated future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates in effect for the year in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in net income in the period in which the change is enacted. We recognize potential accrued interest and penalties within our global operations in “Interest expense” and “Underwriting, acquisition and insurance expenses,” respectively, in our Consolidated Statements of Income related to unrecognized tax benefits.
Supplemental Cash Flow Information	Supplemental Cash Flow Information Income taxes paid. We paid income taxes of $16.5 million, $16.6 million and $10.9 million in 2017, 2016 and 2015, respectively. Income taxes recovered. We recovered income taxes of $2.5 million, $0.5 million and $11.7 million in 2017, 2016 and 2015, respectively. Interest paid as follows: For the Years Ended December 31, (in millions) 2017 2016 2015 Senior unsecured fixed rate notes $ 9.3 $ 9.3 $ 9.3 Junior subordinated debentures 12.6 7.7 7.0 Other indebtedness 4.9 2.3 2.4 Revolving credit facility 0.3 — — Total interest paid $ 27.1 $ 19.3 $ 18.7
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements On February 14, 2018, the Financial Accounting Standards Board (“FASB”) On December 22, 2017, the SEC staff issued Staff Accounting Bulletin No. 118 (SAB 118) which provides guidance on accounting for the effects of the TCJA. We have adopted this guidance within our 2017 consolidated financial statements. In May 2017, the FASB issued ASU 2017-09, “Compensation – Stock Compensation” (Topic 718): Scope of Modification Accounting. ASU 2017-09 clarifies when changes to the terms or conditions of a share-based payment award must be accounted for as modifications. The guidance requires entities to apply the modification accounting guidance if the value, vesting conditions or classification of the award changes. In addition to all the disclosures about modifications that are required today, the entities are required to affirmatively disclose when compensation expense has not changed. The ASU will be applied prospectively and is effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2017. We have adopted this ASU effective January 1, 2018. The adoption of this ASU will not have a material impact on our financial results and disclosures. In January 2017, the FASB issued ASU 2017-04, “Intangibles – Goodwill and Other” (Topic 350). ASU 2017-04 eliminates the requirement to calculate the implied fair value of goodwill that is done in Step 2 of the current goodwill impairment test to measure a goodwill impairment loss. Instead, entities will record an impairment loss based on the excess of a reporting unit’s carrying amount over its fair value. The guidance will be applied prospectively and is effective for annual and interim impairment tests performed in periods beginning after December 15, 2019. We do not anticipate that this ASU will have a material impact on our financial results or disclosures. In January 2017, the FASB issued ASU 2017-01, “Business Combinations” (Topic 805). ASU 2017-01 clarifies the definition of a business to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The guidance specifies the minimum inputs and processes required to meet the definition of a business. The guidance is effective for annual periods beginning after December 15, 2017, and interim periods within the year of adoption. We have adopted this ASU effective January 1, 2018. The adoption of this ASU will not have a material impact on our financial results and disclosures. In October 2016, the FASB issued ASU 2016-16, “Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory.” This ASU 2016-16 simplifies the income tax accounting of intra-entity transfers of an asset other than inventory by requiring an entity to recognize the income tax effect when the transfer occurs. The guidance is effective for annual reporting periods beginning after December 15, 2017, including interim reporting periods within those annual reporting periods and early adoption is permitted. We early adopted this ASU in 2017, and the adoption resulted in intercompany transfers being tax effected. In August 2016, the FASB issued ASU 2016-15, “Classification of Certain Cash Receipts and Cash Payments (Topic 230)”. ASU 2016-15 will reduce diversity in practice on how eight specific cash receipts and payments are classified on the statement of cash flows. The ASU will be effective for fiscal years beginning after December 15, 2017, including interim periods within the year of adoption. This ASU will have an impact on how we present the distributions received from equity method investees in our statement of cash flows. We have adopted this ASU effective January 1, 2018. We have elected to adopt the cumulative earnings approach to classify distributions received from equity method investees, which we will adopt retrospectively. This ASU will have no net effect on our consolidated statements of cash flows, but will have an immaterial impact on the classification of specific cash receipts and payments within the statement. In June 2016, the FASB issued ASU 2016-13, “Measurement of Credit Losses on Financial Instruments” (Topic 326). ASU 2016-13 requires organizations to estimate credit losses on certain types of financial instruments, including receivables and available-for-sale debt securities, by introducing an approach based on expected losses. The expected loss approach will require entities to incorporate considerations of historical information, current information and reasonable and supportable forecasts. The guidance is effective for fiscal years beginning after December 15, 2019, including interim periods within the year of adoption. The guidance requires a modified retrospective transition method and early adoption is permitted. We are currently evaluating the impact that the adoption of the ASU will have on our financial results and disclosures, but do not anticipate that any such potential impact would be material. In March 2016, the FASB issued ASU 2016-09, “Improvements to Employee Share-Based Payment Accounting” (Topic 718). ASU 2016-09 simplifies the accounting for share-based payment award transactions including income tax consequences, classification of awards as either equity or liabilities, classification on the statement of cash flows, and accounting for forfeitures. The ASU is effective for fiscal years beginning after December 15, 2016, and interim periods within those fiscal years. We have adopted this ASU as of January 1, 2017, and for presentation purposes, the incremental tax windfall or shortfall associated with these events has been classified as a cash inflow from operating activity as compared with a financing activity, as previously required. The impact to our financial statements was not material. Additionally, we have elected to continue estimating forfeitures based on historical patterns and will true-up the expenses upon vesting. In February 2016, the FASB issued ASU 2016-02, “Leases” (Topic 842). ASU 2016-02 requires organizations that lease assets to recognize on the balance sheet the assets and liabilities for the rights and obligations created by those leases. Additionally, the ASU modifies current guidance for lessors' accounting. The ASU is effective for interim and annual reporting periods beginning on or after January 1, 2019, with early adoption permitted. We do not anticipate that this ASU will have a material impact on our results of operations, but we anticipate an increase to the value of our assets and liabilities related to leases, with no material impact to equity. In January 2016, the FASB issued ASU 2016-01, “Recognition and Measurement of Financial Assets and Financial Liabilities” (Subtopic 825-10). ASU 2016-01 will require equity investments that are not consolidated or accounted for under the equity method of accounting to be measured at fair value with changes in fair value recognized in net income. This ASU will also require us to assess the ability to realize our deferred tax assets related to an available-for-sale debt security in combination with our other deferred tax assets. The ASU will be effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. We have adopted this ASU effective January 1, 2018. We have determined that the change in the fair value of the noted investments will be included in “Net realized investment and other gains” in our consolidated statements of income. We anticipate the standard will increase the volatility of the results reported in our consolidated statements of income, resulting from the remeasurement of our equity investments. In May 2014, the FASB issued ASU 2014-09, “Revenue from Contracts with Customers” (Topic 606), which replaces most existing U.S. GAAP revenue recognition guidance and permits the use of either the retrospective or cumulative effect transition method. In August 2015, “Deferral of the Effective Date” (Topic 606), deferred the effective date of this guidance to interim and annual reporting periods beginning after December 15, 2017. Early adoption is permitted for fiscal years, and interim reporting periods within those years, beginning after December 15, 2016. Subsequently, in 2016, the FASB issued implementation guidance related to ASU 2014-09, including: • ASU 2016-08, “Principal versus Agent Considerations (Reporting Revenue Gross versus Net)” (Topic 606), which is intended to provide further clarification on the application of the principal versus agent implementation; • ASU 2016-10, “Identifying Performance Obligations and Licensing” (Topic 606), which is intended to clarify the guidance for identifying promised goods or services in a contract with a customer; • ASU 2016-11, “Rescission of SEC Guidance Because of Accounting Standards Updates 2014-09 and 2014-16 Pursuant to Staff Announcements at the March 3, 2016 EITF Meeting” (Topic 605 & 815); • ASU 2016-12, “Narrow-Scope Improvements and Practical Expedients” (Topic 606), which amends certain aspects of ASU 2014-09 to address certain implementation issues; and • ASU 2016-20, “Technical Corrections and Improvements” (Topic 606), provides additional guidance for quantitative and qualitative disclosures in certain cases, and makes 12 additional technical corrections and improvements to the new revenue standard. We adopted this ASU effective January 1, 2018, as fee income related to our brokerage operations and management of the third-party capital for our underwriting Syndicates at Lloyd’s are subject to this updated guidance. We have selected the modified retrospective method to adopt this standard, which requires us to recognize the cumulative effect of initially applying the standard as an adjustment to the opening balance of retained earnings at the date of initial application. The adoption of this standard will not have a material impact on our consolidated financial results.

Business and Significant Acco39

Business and Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Receivables	Premiums receivable and reinsurance recoverables on paid losses written off, net of recoveries against the allowance for doubtful accounts or directly to the income statement are as follows: For the Years Ended December 31, (in millions) 2017 2016 2015 Premiums receivable $ 1.5 $ 1.1 $ 1.0 Reinsurance recoverables — — 0.2 Net written off $ 1.5 $ 1.1 $ 1.2
Intangible Assets And Accumulated Amortization	The following table presents our intangible assets and accumulated amortization at December 31: December 31, 2017 December 31, 2016 (in millions) Gross Carrying Amount Accumulated Amortization Gross Carrying Amount Accumulated Amortization Lloyd's capacity $ 89.0 n/a $ 60.5 n/a Distribution network 50.2 44.1 45.5 38.5 Additional Lloyd's capacity 4.8 4.8 4.8 4.8 Other 3.3 1.6 1.4 1.2 $ 147.3 $ 50.5 $ 112.2 $ 44.5
Supplemental Cash Flow Information	Interest paid as follows: For the Years Ended December 31, (in millions) 2017 2016 2015 Senior unsecured fixed rate notes $ 9.3 $ 9.3 $ 9.3 Junior subordinated debentures 12.6 7.7 7.0 Other indebtedness 4.9 2.3 2.4 Revolving credit facility 0.3 — — Total interest paid $ 27.1 $ 19.3 $ 18.7

Acquisition of Maybrooke (Table

Acquisition of Maybrooke (Tables)	12 Months Ended
	Dec. 31, 2017
Business Combinations [Abstract]
Schedule of Fair Value of Assets Acquired and Liabilities Assumed	The following table presents the fair value of the assets acquired and liabilities assumed relating to the acquisition of Maybrooke: (in millions) Assets: Investments $ 340.7 Cash 130.1 Premiums receivable 157.6 Reinsurance balances 172.8 Other assets 35.6 Total assets 836.8 Liabilities: Reserves for losses and loss adjustment expenses 197.0 Unearned premiums 152.5 Ceded reinsurance payable, net 144.2 Junior subordinated debentures 83.6 Accrued underwriting expenses 29.0 Other liabilities 39.5 Total liabilities 645.8 Net assets acquired 191.0 Purchase price 235.3 Excess purchase price $ 44.3
Summary of Acquired Intangible Assets	A summary of the acquired intangible assets is shown below: Fair Value (in millions) Estimated Useful Life Stamp capacity $ 28.5 n/a Distribution network 4.7 5 years Trade name 1.9 5 years Subtotal - intangible assets 35.1 Goodwill 9.2 n/a Total goodwill and intangible assets $ 44.3
Summary of Selected Financial Information and Pro Forma Results	The following selected financial information summarizes the results of Maybrooke from the date of acquisition that have been included in our Consolidated Statement of Income: (in millions) For the Year Ended December 31, 2017 Revenues $ 127.6 Net loss $ (13.1 ) For the Years Ended December 31, (in millions, except per share data) 2017 2016 Pro forma revenues $ 1,787.9 $ 1,793.1 Pro forma net income 58.7 167.9 Pro forma net income per share - basic 1.96 5.57 Pro forma net income per share - diluted 1.91 5.44

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments	The amortized cost, gross unrealized gains, gross unrealized losses and fair value of investments as of December 31 were as follows: December 31, 2017 (in millions) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Fixed maturities U.S. Governments $ 419.9 $ 0.2 $ 5.0 $ 415.1 Foreign Governments 229.0 6.7 2.5 233.2 Obligations of states and political subdivisions 327.7 9.3 1.1 335.9 Corporate bonds 1,514.5 24.4 13.2 1,525.7 Commercial mortgage-backed securities 136.3 0.1 1.5 134.9 Residential mortgage-backed securities 309.3 2.8 2.7 309.4 Asset-backed securities 161.3 0.7 0.8 161.2 Collateralized loan obligations 222.6 5.9 0.5 228.0 Total fixed maturities 3,320.6 50.1 27.3 3,343.4 Equity securities 338.2 154.0 4.8 487.4 Other investments 534.1 9.6 0.1 543.6 Short-term investments 368.5 — — 368.5 Total investments $ 4,561.4 $ 213.7 $ 32.2 $ 4,742.9 December 31, 2016 (in millions) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Fixed maturities U.S. Governments $ 275.1 $ 0.6 $ 4.5 $ 271.2 Foreign Governments 244.2 1.1 8.0 237.3 Obligations of states and political subdivisions 375.7 8.9 1.8 382.8 Corporate bonds 1,316.9 23.3 19.5 1,320.7 Commercial mortgage-backed securities 154.9 0.4 1.6 153.7 Residential mortgage-backed securities 174.8 3.7 1.7 176.8 Asset-backed securities 127.6 0.1 2.1 125.6 Collateralized loan obligations 269.6 3.8 9.1 264.3 Total fixed maturities 2,938.8 41.9 48.3 2,932.4 Equity securities 335.2 117.9 5.7 447.4 Other investments 527.6 7.5 0.1 535.0 Short-term investments 405.5 — — 405.5 Total investments $ 4,207.1 $ 167.3 $ 54.1 $ 4,320.3
Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity	The amortized cost and fair values of fixed maturity investments as of December 31, 2017, by contractual maturity, were as follows: (in millions) Amortized Cost Fair Value Due in one year or less $ 262.3 $ 261.9 Due after one year through five years 1,432.2 1,440.0 Due after five years through ten years 623.3 630.6 Thereafter 173.3 177.4 Structured securities 829.5 833.5 Total $ 3,320.6 $ 3,343.4
Schedule Of Carrying Value Redemption Characteristics And Unfunded Investment Commitments Of Other Invested Assets Portfolio	Details regarding the carrying value, redemption characteristics and unfunded investment commitments of the other invested assets portfolio as of December 31, 2017 and 2016 were as follows: December 31, 2017 (in millions) Carrying Value Unfunded Commitments Investment Type Hedge funds $ 163.6 $ — Private equity 179.2 129.9 Long only funds 196.5 — Other investments 4.3 — Total other invested assets $ 543.6 $ 129.9 December 31, 2016 (in millions) Carrying Value Unfunded Commitments Investment Type Hedge funds $ 180.9 $ — Private equity 179.0 93.4 Long only funds 170.7 — Other investments 4.4 — Total other invested assets $ 535.0 $ 93.4
Schedule of Aging of Unrealized Losses on Company's Investments in Fixed Maturities, Equity Securities and Other Investments	An aging of unrealized losses on our investments in fixed maturities, equity securities, other investments and short-term investments is presented below: December 31, 2017 Less Than One Year One Year or Greater Total (in millions) Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses Fixed maturities U.S. Governments $ 313.7 $ 1.9 $ 83.7 $ 3.1 $ 397.4 $ 5.0 Foreign Governments 175.2 2.0 35.9 0.5 211.1 2.5 Obligations of states and political subdivisions 33.3 0.5 22.4 0.6 55.7 1.1 Corporate bonds 674.1 9.9 77.7 3.3 751.8 13.2 Commercial mortgage-backed securities 58.2 0.4 37.8 1.1 96.0 1.5 Residential mortgage-backed securities 164.4 1.6 52.4 1.1 216.8 2.7 Asset-backed securities 85.4 0.4 31.9 0.4 117.3 0.8 Collateralized loan obligations (2) 34.6 0.5 0.9 — 35.5 0.5 Total fixed maturities 1,538.9 17.2 342.7 10.1 1,881.6 27.3 Equity securities 46.9 4.8 — — 46.9 4.8 Other investments — — 0.3 0.1 0.3 0.1 Total $ 1,585.8 $ 22.0 $ 343.0 $ 10.2 $ 1,928.8 $ 32.2 December 31, 2016 Less Than One Year One Year or Greater Total (in millions) Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses Fixed maturities U.S. Governments $ 183.4 $ 4.5 $ — $ — $ 183.4 $ 4.5 Foreign Governments 201.2 8.0 — — 201.2 8.0 Obligations of states and political subdivisions 72.6 1.7 1.8 0.1 74.4 1.8 Corporate bonds 490.5 17.7 50.6 1.8 541.1 19.5 Commercial mortgage-backed securities 70.6 1.5 7.1 0.1 77.7 1.6 Residential mortgage-backed securities (2) 87.5 1.7 4.4 — 91.9 1.7 Asset-backed securities 69.7 1.4 8.2 0.7 77.9 2.1 Collateralized loan obligations 122.5 8.6 16.9 0.5 139.4 9.1 Total fixed maturities 1,298.0 45.1 89.0 3.2 1,387.0 48.3 Equity securities 62.1 5.7 — — 62.1 5.7 Other investments 0.3 0.1 — — 0.3 0.1 Short-term investments (1) 4.8 — — — 4.8 — Total $ 1,365.2 $ 50.9 $ 89.0 $ 3.2 $ 1,454.2 $ 54.1 (1) Unrealized losses less than one year are less than $0.1 million. (2) Unrealized losses one year or greater are less than $0.1 million.
Schedule of Recognized Other-than-temporary Losses on Fixed Maturities and Equity Portfolios	We recognized other-than-temporary losses on our fixed maturities and equity portfolios as follows: For the Years Ended December 31, (in millions) 2017 2016 2015 Other-than-temporary impairment: Obligations of states and political subdivisions $ (0.1 ) $ — $ — Corporate bonds (0.7 ) (1.7 ) (2.2 ) Equity securities (1.7 ) (8.5 ) (9.7 ) Other-than-temporary impairment losses $ (2.5 ) $ (10.2 ) $ (11.9 )
Schedule of Investment Income and Expenses	Investment income and expenses were as follows: For the Years Ended December 31, (in millions) 2017 2016 2015 Investment income: Interest on fixed maturities $ 97.1 $ 88.9 $ 78.2 Dividends on equity securities 13.9 15.6 16.3 Income on alternative investments 49.5 29.9 10.0 Income on short-term and other investments 7.7 0.4 0.2 Investment income 168.2 134.8 104.7 Investment expenses (28.2 ) (19.7 ) (16.1 ) Net investment income $ 140.0 $ 115.1 $ 88.6
Schedule of Company's Gross Realized Investment Gains (Losses)	The following table presents our gross realized investment gains (losses) and other: For the Years Ended December 31, (in millions) 2017 2016 2015 Realized gains Fixed maturities $ 25.7 $ 21.5 $ 12.3 Equity securities 51.3 57.5 40.5 Other investments 25.7 47.5 37.6 Short-term investments 0.7 0.5 1.2 Other assets — 1.2 — Gain on sale of real estate holdings — — 0.3 Gross realized investment and other gains 103.4 128.2 91.9 Realized losses Fixed maturities (20.0 ) (35.9 ) (23.5 ) Equity securities (5.2 ) (9.7 ) (6.6 ) Other investments (36.2 ) (46.0 ) (24.0 ) Short-term investments (0.2 ) (0.3 ) (1.8 ) Other-than-temporary impairment losses on fixed maturities (0.8 ) (1.7 ) (2.2 ) Other-than-temporary impairment losses on equity securities (1.7 ) (8.5 ) (9.7 ) Gross realized investment and other losses (64.1 ) (102.1 ) (67.8 ) Net realized investment and other gains before income taxes 39.3 26.1 24.1 Income tax expense (12.0 ) (11.6 ) (10.0 ) Net realized investment and other gains, net of income taxes $ 27.3 $ 14.5 $ 14.1
Schedule of Changes in Unrealized Appreciation (Depreciation)	Changes in unrealized appreciation (depreciation) related to investments are summarized as follows: For the Years Ended December 31, (in millions) 2017 2016 2015 Change in unrealized gains (losses) Fixed maturities $ 25.4 $ 36.5 $ (65.3 ) Equity securities 36.6 (2.6 ) (64.1 ) Other investments 2.1 0.6 1.5 Short-term investments — 0.4 (0.4 ) Net unrealized investment and other gains (losses) before income taxes 64.1 34.9 (128.3 ) Income tax (expense) benefit (14.6 ) (2.4 ) 37.6 Net unrealized investment and other gains (losses), net of income taxes $ 49.5 $ 32.5 $ (90.7 )
Schedule of Fair Value of Foreign Currency Exchange Forward Contracts	The fair value of our foreign currency exchange forward contracts as of December 31 was as follows: (in millions) December 31, 2017 December 31, 2016 Operational currency exposure $ (0.2 ) $ — Asset manager investment exposure (0.9 ) 0.7 Total return strategy 0.7 3.3 Total $ (0.4 ) $ 4.0
Schedule of Realized Gains and Losses of Investment on Foreign Currency Exchange Forward Contracts	The following table presents our gross investment realized gains and losses on our foreign currency exchange forward contracts: For the Years Ended December 31, (in millions) 2017 2016 2015 Realized gains Global catastrophe (1) $ — $ — $ 0.5 Operational currency exposure 12.4 10.9 26.1 Asset manager investment exposure 1.5 9.0 8.5 Total return strategy 10.4 25.6 1.4 Gross realized investment gains 24.3 45.5 36.5 Realized losses Global catastrophe — — (2.3 ) Operational currency exposure (13.8 ) (18.0 ) (12.9 ) Asset manager investment exposure (11.3 ) (4.5 ) (2.3 ) Total return strategy (7.6 ) (21.0 ) (3.8 ) Gross realized investment losses (32.7 ) (43.5 ) (21.3 ) Net realized investment (losses) gains on foreign currency exchange forward contracts $ (8.4 ) $ 2.0 $ 15.2 (1)
Schedule of Restricted Assets	The following table presents our components of restricted assets at December 31: (in millions) December 31, 2017 December 31, 2016 Securities on deposit for regulatory and other purposes $ 175.3 $ 168.7 Securities pledged as collateral for letters of credit 78.1 35.9 Securities and cash on deposit supporting Lloyd’s business 355.5 161.8 Total restricted investments $ 608.9 $ 366.4
Financial Assets Measured at Fair Value on Recurring Basis	Based on an analysis of the inputs, our financial assets measured at fair value on a recurring basis have been categorized as follows: Fair (in millions) December 31, 2017 Level 1 (a) Level 2 (b) Level 3 (c) Fixed maturities U.S. Governments $ 415.1 $ 410.6 $ 4.5 $ — Foreign Governments 233.2 — 233.2 — Obligations of states and political subdivisions 335.9 — 335.9 — Corporate bonds 1,525.7 — 1,523.8 1.9 Commercial mortgage-backed securities 134.9 — 134.9 — Residential mortgage-backed securities 309.4 — 309.4 — Asset-backed securities 161.2 — 161.2 — Collateralized loan obligations 228.0 — 228.0 — Total fixed maturities 3,343.4 410.6 2,930.9 1.9 Equity securities 487.4 483.0 2.1 2.3 Other investments 108.8 — 108.8 — Short-term investments 368.5 333.7 34.8 — $ 4,308.1 $ 1,227.3 $ 3,076.6 $ 4.2 (a) Quoted prices in active markets for identical assets (b) Significant other observable inputs (c) Significant unobservable inputs Fair Value Measurements at Reporting Date Using (in millions) December 31, 2016 Level 1 (a) Level 2 (b) Level 3 (c) Fixed maturities U.S. Governments $ 271.2 $ 228.0 $ 43.2 $ — Foreign Governments 237.3 — 237.3 — Obligations of states and political subdivisions 382.8 — 382.8 — Corporate bonds 1,320.7 — 1,318.7 2.0 Commercial mortgage-backed securities 153.7 — 153.7 — Residential mortgage-backed securities 176.8 — 176.8 — Asset-backed securities 125.6 — 125.6 — Collateralized loan obligations 264.3 — 264.3 — Total fixed maturities 2,932.4 228.0 2,702.4 2.0 Equity securities 447.4 444.9 2.1 0.4 Other investments 91.5 — 91.5 — Short-term investments 405.5 375.1 30.4 — $ 3,876.8 $ 1,048.0 $ 2,826.4 $ 2.4 (a) Quoted prices in active markets for identical assets (b) Significant other observable inputs (c) Significant unobservable inputs
Schedule of Reconciliation of Beginning and Ending Balances for Investments Categorized as Level 3	A reconciliation of the beginning and ending balances for the investments categorized as Level 3 are as follows: Fair Value Measurements Using Observable Inputs (Level 3) (in millions) Credit Financial Equity Securities Total Beginning balance, January 1, 2017 $ 2.0 $ 0.4 $ 2.4 Transfers into Level 3 — — — Transfers out of Level 3 — — — Total gains or losses (realized/unrealized): Included in net income (loss) — — — Included in other comprehensive income (loss) (0.1 ) 0.2 0.1 Purchases, issuances, sales, and settlements: Purchases — 1.7 1.7 Issuances — — — Sales — — — Settlements — — — Ending balance, December 31, 2017 $ 1.9 $ 2.3 $ 4.2 Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at December 31, 2017 $ — $ — $ — (in millions) Credit Financial Equity Securities Total Beginning balance, January 1, 2016 $ — $ 0.7 $ 0.7 Transfers into Level 3 — — — Transfers out of Level 3 — — — Total gains or losses (realized/unrealized): Included in net income (loss) — — — Included in other comprehensive income (loss) — — — Purchases, issuances, sales, and settlements: Purchases 2.0 — 2.0 Issuances — — — Sales — (0.3 ) (0.3 ) Settlements — — — Ending balance, December 31, 2016 $ 2.0 $ 0.4 $ 2.4 Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at December 31, 2016 $ — $ — $ —

Reinsurance (Tables)

Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2017
Insurance [Abstract]
Schedule of Reinsurance Premiums	Premiums were as follows: For the Years Ended December 31, (in millions) 2017 2016 2015 Direct written premiums $ 2,029.2 $ 1,792.5 $ 1,651.4 Reinsurance ceded to other companies (1,043.7 ) (724.6 ) (610.0 ) Reinsurance assumed from other companies 668.0 372.3 360.7 Net written premiums $ 1,653.5 $ 1,440.2 $ 1,402.1 Direct earned premiums $ 1,912.2 $ 1,722.8 $ 1,602.2 Reinsurance ceded to other companies (1,033.6 ) (675.8 ) (563.7 ) Reinsurance assumed from other companies 693.7 363.8 333.4 Net earned premiums $ 1,572.3 $ 1,410.8 $ 1,371.9 Percentage of reinsurance assumed to net earned premiums 44.1 % 25.8 % 24.3 %

Reserves for Losses and Loss 43

Reserves for Losses and Loss Adjustment Expenses (Tables)	12 Months Ended
	Dec. 31, 2017
Reserves for Losses and Loss Adjustment Expenses	The following table provides a reconciliation of reserves for losses and loss adjustment expenses (“LAE”): For the Twelve Months Ended December 31, (in millions) 2017 2016 2015 Net reserves beginning of the year $ 2,180.2 $ 2,133.3 $ 2,137.1 Net Maybrooke reserves acquired 131.8 — Add: Losses and LAE incurred during current calendar year, net of reinsurance: Current accident year 1,058.4 843.4 798.5 Prior accident years (8.2 ) (33.3 ) (32.4 ) Losses and LAE incurred during calendar year, net of reinsurance 1,050.2 810.1 766.1 Deduct: Losses and LAE payments made during current calendar year, net of reinsurance: Current accident year 289.6 178.9 169.0 Prior accident years 599.8 537.6 564.5 Losses and LAE payments made during current calendar year, net of reinsurance: 889.4 716.5 733.5 Change in participation interest (1) (23.2 ) (36.3 ) (1.2 ) Foreign exchange adjustments 38.4 (10.4 ) (35.2 ) Net reserves - end of year 2,488.0 2,180.2 2,133.3 Add: Reinsurance recoverables on unpaid losses and LAE, end of year 1,713.0 1,170.6 990.3 Gross reserves - end of year $ 4,201.0 $ 3,350.8 $ 3,123.6 (1) Amount represents (decrease) increase in reserves due to change in our Syndicate 1200 participation.
Impact from (Favorable) Unfavorable Development of Prior Accident Years’ Loss and LAE Reserves on Each Reporting Segment	The impact from the (favorable) unfavorable development of prior accident years’ losses and LAE reserves on each reporting segment is presented below: For the Twelve Months Ended December 31, (in millions) 2017 2016 2015 U.S. Operations $ (38.7 ) $ (35.9 ) $ (23.0 ) International Operations 13.2 (16.0 ) (18.0 ) Run-off Lines 17.3 18.6 8.6 Total unfavorable (favorable) prior-year development $ (8.2 ) $ (33.3 ) $ (32.4 )
Summary of Reconciliation of Net Incurred and Paid Development to Liability for Unpaid Losses and LAE in Consolidated Balance Sheets	The reconciliation of the net incurred and paid development tables to the liability for unpaid losses and LAE in our Consolidated Balance Sheets is as follows: (in millions) As of December 31, 2017 Liabilities for unpaid losses and ALAE: US Operations: Excess and Surplus Lines - Liability $ 835.1 Commercial Specialty - Liability 375.0 Commercial Specialty - Professional 121.9 International Operations: International Specialty - Liability 53.5 International Specialty - Property 82.1 Syndicate 1200 - Liability 108.6 Syndicate 1200 - Professional 109.4 Syndicate 1200 - Property 143.7 Syndicate 1200 - Specialty 149.2 Ariel Re - Property 69.3 Run-off Lines 197.5 Other lines 189.3 Total liabilities for unpaid losses and ALAE, net of reinsurance 2,434.6 Reinsurance recoverables on unpaid losses and LAE: US Operations: Excess and Surplus Lines - Liability 293.3 Commercial Specialty - Liability 246.6 Commercial Specialty - Professional 70.2 International Operations: International Specialty - Liability 95.8 International Specialty - Property 67.4 Syndicate 1200 - Liability 77.1 Syndicate 1200 - Professional 78.6 Syndicate 1200 - Property 91.5 Syndicate 1200 - Specialty 88.6 Ariel Re - Property 280.8 Run-off Lines 91.4 Other lines 231.7 Total reinsurance recoverables on unpaid losses and LAE 1,713.0 Unallocated loss adjustment expenses 71.0 Unamortized reserve discount (17.6 ) Gross liability for unpaid losses and LAE $ 4,201.0
Schedule of Supplementary Unaudited Information About Annual Percentage Payout of Incurred Losses and ALAE, Net of Reinsurance	The following table provides supplementary unaudited information about the annual percentage payout of incurred losses and ALAE, net of reinsurance, as of December 31, 2017: Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance (1) Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 Year 7 Year 8 US Operations: Excess and Surplus Lines - Liability 8.3% 18.3% 19.3% 16.4% 12.1% 8.0% 5.4% N/A Commercial Specialty - Liability 15.0% 21.8% 18.5% 16.0% 9.1% 6.1% 4.0% N/A Commercial Specialty - Professional 8.2% 22.5% 27.3% 18.6% 9.4% 5.0% 2.9% N/A International Operations: International Specialty - Liability 11.5% 13.0% 17.6% 14.8% 10.6% 8.4% 6.1% N/A International Specialty - Property 26.6% 30.7% 24.9% 9.2% 4.5% 2.0% 1.0% N/A Syndicate 1200 - Liability 5.0% 10.3% 13.7% 17.0% 17.9% 10.6% 7.3% 5.1% Syndicate 1200 - Professional 4.4% 9.4% 13.6% 15.6% 13.6% 13.9% 8.1% 5.8% Syndicate 1200 - Property 39.8% 25.1% 16.1% 9.9% 3.8% 2.3% 1.2% 0.7% Syndicate 1200 - Specialty 40.4% 28.0% 15.8% 8.8% 3.3% 1.9% 1.0% 0.5% (1) The average annual percentage payout is calculated from a paid losses and ALAE development pattern based on an actuarial analysis of the paid losses and ALAE movements by accident year for each disaggregation category. The paid losses and ALAE development pattern provides the expected percentage of ultimate losses and ALAE to be paid in each year. The pattern considers all accident years included in the claims development tables .
Schedule of Information About Discounted Liabilities For Unpaid Losses and LAE	The following tables provide information about these discounted liabilities for unpaid losses and LAE: Carrying Amount of Reserves for Losses & LAE Aggregate Amount of Discount As of December 31, As of December 31, (in millions, except discount percentages) 2017 2016 2015 2017 2016 2015 US Operations: Commercial Specialty - Liability $ 126.7 $ 121.1 $ 121.6 $ 10.6 $ 10.3 $ 10.3 Run-off Lines 175.5 184.5 193.7 7.0 9.1 11.2 Total $ 302.2 $ 305.6 $ 315.3 $ 17.6 $ 19.4 $ 21.5 Interest Accretion (1) Discount Rate For the Years Ended December 31, As of December 31, 2017 2016 2015 2017 2016 2015 US Operations: Commercial Specialty - Liability $ 1.9 $ 1.9 $ 1.1 2.25% 2.25% 2.25% Run-off Lines 2.1 2.1 3.1 3.50% 3.50% 3.50% Total $ 4.0 $ 4.0 $ 4.2 (1) Interest accretion is recorded in the line item “Losses and loss adjustment expenses” in our Consolidated Statements of Income.
Excess and Surplus Lines [Member] | Liability [Member]
Summary of Information About Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	The following tables provide information about incurred and cumulative paid losses and allocated loss adjustment expenses (“ALAE”), net of reinsurance. The following tables also include IBNR reserves plus expected development on reported claims and the cumulative number of reported claims as of December 31, 2017. Reporting Segment: U.S. Operations Operating Division: Excess and Surplus Lines Line of Business: Liability (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2011 $ 202.9 $ 206.0 $ 205.8 $ 200.0 $ 193.5 $ 192.8 $ 189.0 2012 189.6 196.0 189.7 183.6 184.4 182.1 2013 217.9 222.6 224.3 227.2 220.4 2014 213.0 215.1 213.1 211.9 2015 232.3 237.0 228.6 2016 246.5 250.9 2017 260.6 Total $ 1,543.5 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2011 $ 17.6 $ 53.8 $ 91.0 $ 122.9 $ 146.6 $ 162.4 $ 170.0 2012 17.2 52.8 89.1 120.8 142.4 157.5 2013 17.6 60.2 100.4 135.2 163.7 2014 15.0 52.2 95.9 131.6 2015 16.5 51.9 91.4 2016 17.4 52.8 2017 11.5 $ 778.5 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance 70.1 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 835.1 As of December 31, 2017 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ 189.0 $ 12.4 8,404 2012 182.1 15.1 7,322 2013 220.4 34.9 7,180 2014 211.9 48.0 6,306 2015 228.6 92.3 5,661 2016 250.9 151.4 4,969 2017 260.6 218.4 3,801 (1) Information presented for calendar years prior to 2017 is required supplementary information and is unaudited. (2) The cumulative number of reported claims is measured at a coverage level by occurrence. Reported occurrences that do not result in a liability are included as reported claims.
Commercial Specialty [Member] | Liability [Member]
Summary of Information About Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	Reporting Segment: U.S. Operations Operating Division: Commercial Specialty Line of Business: Liability (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2011 $ 140.6 $ 155.1 $ 159.0 $ 157.5 $ 158.2 $ 154.0 $ 153.7 2012 140.3 146.3 149.7 152.2 151.5 147.7 2013 126.6 133.2 135.1 133.2 131.1 2014 115.5 118.6 116.9 114.5 2015 102.6 106.7 101.6 2016 95.9 99.4 2017 $ 114.2 Total $ 862.2 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2011 $ 23.1 $ 57.5 $ 85.9 $ 111.3 $ 126.1 $ 135.1 $ 139.8 2012 20.1 51.0 80.7 105.3 120.8 127.9 2013 18.9 49.4 73.6 93.6 102.8 2014 17.4 38.4 58.7 75.2 2015 16.4 35.0 48.8 2016 11.1 31.7 2017 16.3 Total $ 542.5 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance 55.3 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 375.0 As of December 31, 2017 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ 153.7 $ 6.8 28,153 2012 147.7 9.6 23,594 2013 131.1 13.4 18,901 2014 114.5 17.3 16,261 2015 101.6 27.4 14,379 2016 99.4 38.1 11,225 2017 114.2 64.8 10,897 (1) Information presented for calendar years prior to 2017 is required supplementary information and is unaudited. (2) The cumulative number of reported claims is measured by individual claimant at a coverage level. Reported occurrences that do not result in a liability are included as reported claims.
Commercial Specialty [Member] | Professional [Member]
Summary of Information About Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	Reporting Segment: U.S. Operations Operating Division: Commercial Specialty Line of Business: Professional (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2011 $ 35.0 $ 35.0 $ 35.0 $ 32.5 $ 28.2 $ 26.9 $ 26.6 2012 27.8 28.3 28.6 25.8 24.0 24.5 2013 20.9 21.5 21.1 19.0 19.8 2014 22.4 22.4 26.0 33.7 2015 29.9 29.5 33.2 2016 44.2 44.8 2017 60.1 Total $ 242.7 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2011 $ 3.2 $ 11.8 $ 17.8 $ 22.0 $ 24.0 $ 25.4 $ 25.7 2012 2.3 8.6 16.9 19.9 21.4 22.6 2013 1.9 6.3 10.9 14.2 17.6 2014 2.3 5.4 15.1 24.1 2015 1.8 8.3 15.6 2016 2.4 11.9 2017 3.5 Total $ 121.0 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance 0.2 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 121.9 As of December 31, 2017 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ 26.6 $ 0.3 818 2012 24.5 0.2 637 2013 19.8 1.1 616 2014 33.7 2.1 1,041 2015 33.2 6.5 1,802 2016 44.8 19.8 3,190 2017 60.1 50.4 3,272 (1) Information presented for calendar years prior to 2017 is required supplementary information and is unaudited. (2) The cumulative number of reported claims is measured by individual claimant at a coverage level. Reported occurrences that do not result in a liability are included as reported claims.
International Specialty [Member] | Liability [Member]
Summary of Information About Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	Reporting Segment: International Operations Operating Division: International Specialty Line of Business: Liability (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2011 $ 6.6 $ 6.6 $ 6.6 $ 4.4 $ 2.2 $ 1.6 $ 1.0 2012 7.4 7.4 7.4 5.6 4.5 1.7 2013 9.1 9.5 9.5 6.5 6.2 2014 10.7 10.9 10.7 7.2 2015 12.6 15.3 25.7 2016 15.2 15.3 2017 18.4 Total $ 75.5 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2011 $ — $ — $ — $ — $ — $ — $ — 2012 — — — 0.1 0.1 0.1 2013 0.4 0.7 0.8 0.9 3.4 2014 0.2 0.7 0.8 1.0 2015 0.3 0.6 16.8 2016 0.4 0.9 2017 0.2 Total $ 22.4 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance 0.4 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 53.5 As of December 31, 2017 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ 1.0 $ 1.0 1,422 2012 1.7 1.6 1,403 2013 6.2 2.7 1,306 2014 7.2 3.1 1,465 2015 25.7 0.7 1,720 2016 15.3 14.1 2,148 2017 18.4 18.0 1,927 (1) Information presented for calendar years prior to 2017 is required supplementary information and is unaudited. (2) The cumulative number of reported claims is measured by individual claimant at a coverage level. Reported occurrences that do not result in a liability are included as reported claims.
International Specialty [Member] | Property [Member]
Summary of Information About Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	Reporting Segment: International Operations Operating Division: International Specialty Line of Business: Property (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2011 $ 118.0 $ 107.4 $ 109.6 $ 107.6 $ 106.9 $ 106.8 $ 107.1 2012 47.4 51.7 50.6 51.8 46.7 46.6 2013 35.5 39.8 39.2 35.1 34.6 2014 28.3 28.5 26.5 25.6 2015 28.4 24.7 23.5 2016 46.2 40.1 2017 54.5 Total $ 332.0 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2011 $ 41.1 $ 67.5 $ 89.3 $ 97.6 $ 100.0 $ 101.3 $ 102.3 2012 12.4 31.3 40.8 49.8 44.2 44.5 2013 5.2 21.7 32.2 32.5 33.2 2014 3.3 14.0 20.6 23.5 2015 4.6 11.9 15.5 2016 14.7 22.8 2017 12.1 Total $ 253.9 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance 4.0 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 82.1 As of December 31, 2017 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ 107.1 $ — 400 2012 46.6 0.8 322 2013 34.6 0.5 357 2014 25.6 1.5 355 2015 23.5 3.7 420 2016 40.1 8.7 647 2017 54.5 4.2 786 (1) Information presented for calendar years prior to 2017 is required supplementary information and is unaudited. (2) The cumulative number of reported claims is measured by individual claimant at a coverage level. Reported occurrences that do not result in a liability are included as reported claims.
Syndicate 1200 [Member] | Liability [Member]
Summary of Information About Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	Reporting Segment: International Operations Operating Division: Syndicate 1200 Line of Business: Liability (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2010 $ 5.9 $ 5.0 $ 4.7 $ 6.4 $ 6.2 $ 6.1 $ 6.0 $ 6.1 2011 8.5 9.0 11.4 11.5 10.8 10.6 11.2 2012 9.1 11.3 15.5 14.9 14.4 15.2 2013 23.7 27.8 27.4 25.6 25.7 2014 38.8 38.0 35.2 34.6 2015 35.9 30.9 31.0 2016 26.6 27.9 2017 25.3 Total $ 177.0 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2010 $ — $ 0.2 $ 0.6 $ 1.1 $ 2.1 $ 2.9 $ 3.9 $ 4.2 2011 0.3 0.8 1.7 3.5 5.7 7.3 8.2 2012 0.4 1.2 2.7 6.0 8.6 10.5 2013 1.6 3.4 7.2 11.8 16.2 2014 2.0 4.8 10.2 14.1 2015 0.8 5.3 7.6 2016 1.9 5.8 2017 1.8 Total $ 68.4 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 108.6 As of December 31, 2017 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims 2010 $ 6.1 $ 0.1 2011 11.2 0.6 2012 15.2 1.5 2013 25.7 3.9 2014 34.6 7.2 2015 31.0 11.1 2016 27.9 14.1 2017 25.3 19.8 (1) Information presented for calendar years prior to 2017 is required supplementary information and is unaudited.
Syndicate 1200 [Member] | Professional [Member]
Summary of Information About Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	Reporting Segment: International Operations Operating Division: Syndicate 1200 Line of Business: Professional (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2010 $ 16.1 $ 13.2 $ 13.1 $ 11.6 $ 10.4 $ 9.7 $ 9.7 $ 9.8 2011 19.9 21.8 19.2 16.1 15.3 15.3 15.8 2012 14.5 14.4 14.7 14.6 14.6 15.7 2013 23.6 23.6 23.7 23.3 23.6 2014 36.4 37.6 37.9 41.4 2015 39.6 39.1 40.7 2016 34.2 28.5 2017 25.5 Total $ 201.0 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2010 $ 0.1 $ 1.0 $ 2.0 $ 3.1 $ 3.9 $ 5.3 $ 6.7 $ 7.0 2011 1.1 2.6 4.3 6.8 8.6 11.0 11.7 2012 0.6 1.9 4.5 6.0 8.5 9.9 2013 1.7 3.9 7.4 12.1 16.0 2014 1.7 6.7 15.3 24.7 2015 2.3 8.6 15.3 2016 2.1 5.9 2017 1.1 Total $ 91.6 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 109.4 As of December 31, 2017 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims 2010 $ 9.8 $ 0.2 2011 15.8 0.9 2012 15.7 1.8 2013 23.6 4.2 2014 41.4 9.0 2015 40.7 15.1 2016 28.5 14.8 2017 25.5 20.1 (1) Information presented for calendar years prior to 2017 is required supplementary information and is unaudited.
Syndicate 1200 [Member] | Property [Member]
Summary of Information About Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	Reporting Segment: International Operations Operating Division: Syndicate 1200 Line of Business: Property (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2010 $ 51.1 $ 59.0 $ 55.3 $ 54.8 $ 52.1 $ 51.8 $ 51.3 $ 51.9 2011 109.8 115.1 109.1 96.2 94.3 93.7 93.6 2012 89.5 89.3 93.5 92.6 91.6 91.3 2013 84.0 79.8 78.8 77.3 76.9 2014 71.2 64.5 65.9 66.1 2015 54.9 66.3 73.2 2016 70.7 86.3 2017 83.9 Total $ 623.2 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2010 $ 1.8 $ 19.8 $ 28.4 $ 34.2 $ 36.2 $ 40.5 $ 42.0 $ 43.0 2011 23.7 47.8 62.6 74.4 80.3 81.9 83.3 2012 29.9 48.2 63.5 74.6 77.0 77.9 2013 44.8 57.2 69.7 73.8 74.2 2014 29.7 51.8 57.8 59.5 2015 23.0 42.9 51.9 2016 39.1 62.3 2017 27.4 Total $ 479.5 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 143.7 As of December 31, 2017 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims 2010 $ 51.9 $ — 2011 93.6 — 2012 91.3 — 2013 76.9 — 2014 66.1 — 2015 73.2 3.7 2016 86.3 8.6 2017 83.9 33.6 (1) Information presented for calendar years prior to 2017 is required supplementary information and is unaudited.
Syndicate 1200 [Member] | Specialty [Member]
Summary of Information About Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	Reporting Segment: International Operations Operating Division: Syndicate 1200 Line of Business: Specialty (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2010 $ 13.1 $ 16.0 $ 13.8 $ 13.3 $ 12.3 $ 12.3 $ 12.2 $ 12.2 2011 39.0 40.7 39.4 34.6 33.8 33.8 33.6 2012 53.1 57.0 61.4 49.9 59.7 59.4 2013 76.7 82.2 83.5 83.1 82.6 2014 93.2 99.6 101.0 101.9 2015 90.5 88.7 94.4 2016 86.6 85.1 2017 80.1 Total $ 549.3 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident For the Years Ended December 31, Year 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 2010 $ 1.9 $ 8.8 $ 9.0 $ 9.4 $ 9.1 $ 9.9 $ 10.1 $ 10.2 2011 11.7 19.8 24.0 27.5 28.9 29.5 29.8 2012 18.2 27.9 39.4 46.3 49.3 50.3 2013 31.4 53.1 69.6 77.0 78.6 2014 38.2 72.3 83.5 88.8 2015 31.4 54.8 65.7 2016 37.9 57.9 2017 18.8 Total $ 400.1 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 149.2 As of December 31, 2017 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims 2010 $ 12.2 $ — 2011 33.6 — 2012 59.4 — 2013 82.6 — 2014 101.9 2.6 2015 94.4 7.3 2016 85.1 13.0 2017 80.1 41.0 (1) Information presented for calendar years prior to 2017 is required supplementary information and is unaudited.
Ariel Re [Member] | Property [Member]
Summary of Information About Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	Reporting Segment: International Operations Operating Division: Ariel Re Line of Business: Property Products (in millions, except number of claims reported) For the Year Ended December 31, 2017 Accident Year Incurred Losses & ALAE, Net of Reinsurance Cumulative Paid Losses & ALAE, Net of Reinsurance Outstanding liabilities for unpaid losses and ALAE prior to 2017, net of reinsurance Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance 2017 $ 108.3 $ 72.5 $ 33.5 $ 69.3 As of December 31, 2017 IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims $ 30.4 91

Run-off Lines (Tables)

Run-off Lines (Tables)	12 Months Ended
	Dec. 31, 2017
Discontinued Operations And Disposal Groups [Abstract]
Gross Reserves for Run-Off Lines	The following table presents our gross reserves for Run-off Lines as of December 31: December 31, (in millions) 2017 2016 Asbestos and Environmental: Reinsurance assumed $ 29.3 $ 28.4 Other 26.6 20.0 Total Asbestos and Environmental 55.9 48.4 Risk management 219.6 236.9 Run-off reinsurance lines 1.8 1.9 Other run-off lines 4.6 3.7 Gross reserves - Run-off Lines $ 281.9 $ 290.9
Total Gross Reserves for Asbestos Exposure	The following table represents the total gross reserves for our asbestos exposure: December 31, (in millions) 2017 2016 2015 Direct written Case reserves $ 2.1 $ 2.8 $ 2.0 Unallocated loss adjustment expense ("ULAE") 0.5 0.5 0.5 Incurred but not reported ("IBNR") 18.8 12.1 8.2 Total direct written reserves 21.4 15.4 10.7 Assumed domestic Case reserves 9.8 10.5 12.2 ULAE 0.8 0.8 0.8 IBNR 13.7 10.5 13.7 Total assumed domestic reserves 24.3 21.8 26.7 Assumed London Case reserves 2.3 3.3 4.0 ULAE — — — IBNR 0.6 1.4 1.4 Total assumed London reserves 2.9 4.7 5.4 Total asbestos reserves $ 48.6 $ 41.9 $ 42.8
Underwriting Losses for Run-Off Lines	The following table presents our underwriting losses for Run-off Lines: For the Years Ended December 31, (in millions) 2017 2016 2015 Asbestos and Environmental: Reinsurance assumed $ (8.7 ) $ (1.3 ) $ (1.0 ) Other (6.7 ) (9.6 ) (3.4 ) Total Asbestos and Environmental (15.4 ) (10.9 ) (4.4 ) Risk management (8.8 ) (13.1 ) (8.2 ) Run-off reinsurance lines (0.1 ) 0.1 2.0 Other run-off lines (1.4 ) (1.2 ) (3.5 ) Total underwriting loss - Run-off Lines $ (25.7 ) $ (25.1 ) $ (14.1 )

Junior Subordinated Debentures

Junior Subordinated Debentures (Tables)	12 Months Ended
	Dec. 31, 2017
Junior Subordinated Debentures [Member]
Schedule of Junior Subordinated Debentures	A summary of our outstanding junior subordinated debentures is presented below: (in millions) Issue Date Trust Preferred Pools Maturity Rate Structure Interest Rate at December 31, 2017 Amount Argo Group 05/15/2003 PXRE Capital Statutory Trust II 05/15/2033 3M LIBOR + 4.10% 5.52% $ 18.1 11/06/2003 PXRE Capital Trust VI 09/30/2033 3M LIBOR + 3.90% 5.59% 10.3 Argo Group US 05/15/2003 Argonaut Group Statutory Trust I 05/15/2033 3M LIBOR + 4.10% 5.52% 15.5 12/16/2003 Argonaut Group Statutory Trust III 01/08/2034 3M LIBOR + 4.10% 5.46% 12.3 04/29/2004 Argonaut Group Statutory Trust IV 04/29/2034 3M LIBOR + 3.85% 5.27% 13.4 05/26/2004 Argonaut Group Statutory Trust V 05/24/2034 3M LIBOR + 3.85% 5.30% 12.3 05/12/2004 Argonaut Group Statutory Trust VI 05/12/2034 3M LIBOR + 3.80% 5.40% 13.4 09/17/2004 Argonaut Group Statutory Trust VII 12/15/2034 3M LIBOR + 3.60% 5.19% 15.5 09/22/2004 Argonaut Group Statutory Trust VIII 09/22/2034 3M LIBOR + 3.55% 5.21% 15.5 10/22/2004 Argonaut Group Statutory Trust IX 12/15/2034 3M LIBOR + 3.60% 5.19% 15.5 09/15/2005 Argonaut Group Statutory Trust X 09/15/2035 3M LIBOR + 3.40% 4.99% 30.9 Total Outstanding $ 172.7 (in millions) Issue Date Trust Preferred Pools Maturity Rate Structure Interest Rate at December 31, 2016 Amount Argo Group 05/15/2003 PXRE Capital Statutory Trust II 05/15/2033 3M LIBOR + 4.10% 5.00% $ 18.1 11/06/2003 PXRE Capital Trust VI 09/30/2033 3M LIBOR + 3.90% 4.90% 10.3 Argo Group US 05/15/2003 Argonaut Group Statutory Trust I 05/15/2033 3M LIBOR + 4.10% 5.00% 15.5 12/16/2003 Argonaut Group Statutory Trust III 01/08/2034 3M LIBOR + 4.10% 4.98% 12.3 04/29/2004 Argonaut Group Statutory Trust IV 04/29/2034 3M LIBOR + 3.85% 4.76% 13.4 05/26/2004 Argonaut Group Statutory Trust V 05/24/2034 3M LIBOR + 3.85% 4.77% 12.3 05/12/2004 Argonaut Group Statutory Trust VI 05/12/2034 3M LIBOR + 3.80% 4.79% 13.4 09/17/2004 Argonaut Group Statutory Trust VII 12/15/2034 3M LIBOR + 3.60% 4.56% 15.5 09/22/2004 Argonaut Group Statutory Trust VIII 09/22/2034 3M LIBOR + 3.55% 4.55% 15.5 10/22/2004 Argonaut Group Statutory Trust IX 12/15/2034 3M LIBOR + 3.60% 4.56% 15.5 09/15/2005 Argonaut Group Statutory Trust X 09/15/2035 3M LIBOR + 3.40% 4.36% 30.9 Total Outstanding $ 172.7
Junior Subordinated Debentures, Maybrooke [Member]
Schedule of Junior Subordinated Debentures	A summary of the terms of the acquired debt outstanding at December 31, 2017 is presented below: (in millions) Issue Date Maturity Rate Structure Interest Rate at December 31, 2017 Principal at December 31, 2017 Carrying Value at December 31, 2017 9/15/2007 9/15/2037 3 month LIBOR + 3.15% 4.74 % $ 91.8 $ 83.9

Other Indebtedness (Tables)

Other Indebtedness (Tables)	12 Months Ended
	Dec. 31, 2017
Schedule of Terms and Principal Amounts of Each Debt Instrument	Our Consolidated Balance Sheets includes various long-term debt instruments under the caption “Other indebtedness,” as detailed in the table below. Information regarding the terms and principal amounts of each of these debt instruments is also provided. (in millions) Debt Type December 31, 2017 December 31, 2016 Floating rate loan stock $ 58.9 $ 54.8 Term loan 125.0 — Other debt 0.6 0.6 Total other indebtedness $ 184.5 $ 55.4
Floating Rate Loan Stock [Member]
Schedule of Junior Subordinated Debentures	A summary of the notes outstanding at December 31, 2017 and 2016 is presented below: (in millions) Issue Date Currency Maturity Rate Structure Interest Rate at December 31, 2017 Amount 12/08/2004 U.S. Dollar 11/15/2034 6 month LIBOR + 4.2% 5.66% $ 6.5 09/06/2005 Euro 08/22/2035 3 month LIBOR + 4.0% 3.67% 14.1 10/31/2006 U.S. Dollar 01/15/2036 6 month LIBOR + 4.0% 5.46% 10.0 10/31/2006 Euro 11/22/2036 3 month LIBOR + 4.0% 3.67% 12.4 06/08/2007 Euro 09/15/2037 3 month LIBOR + 3.9% 3.57% 15.9 $ 58.9 (in millions) Issue Date Currency Maturity Rate Structure Interest Rate at December 31, 2016 Amount 12/08/2004 U.S. Dollar 11/15/2034 6 month LIBOR + 4.2% 5.18% $ 6.5 09/06/2005 Euro 08/22/2035 3 month LIBOR + 4.0% 3.70% 12.8 10/31/2006 U.S. Dollar 01/15/2036 6 month LIBOR + 4.0% 4.98% 10.0 10/31/2006 Euro 11/22/2036 3 month LIBOR + 4.0% 3.70% 11.2 06/08/2007 Euro 09/15/2037 3 month LIBOR + 3.9% 3.58% 14.3 $ 54.8
New Credit Agreement [Member]
Schedule of Floating Rate Loan Stock and Borrowing Under Credit Facility, Notes Outstanding	A summary of the terms of the outstanding balance at December 31, 2017 is presented below: (in millions) Issue Date Maturity Rate Structure Interest Rate at December 31, 2017 Amount 3/3/2017 3/3/2019 2 month LIBOR + 1.5% 2.94% $ 125.0

Disclosures about Fair Value 47

Disclosures about Fair Value of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Summary of Company's Financial Instruments Whose Carrying Amount Did Not Equal Fair Value	A summary of our financial instruments whose carrying value did not equal fair value is shown below: December 31, 2017 December 31, 2016 (in millions) Carrying Amount Fair Value Carrying Amount Fair Value Junior subordinated debentures Trust preferred debentures $ 172.7 $ 172.9 $ 172.7 $ 162.4 Subordinated debentures acquired with Maybrooke 83.9 85.0 — — Senior unsecured fixed rate notes 139.6 141.2 139.5 139.3 Floating rate loan stock 58.9 59.0 54.8 51.5

Shareholders' Equity (Tables)

Shareholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Schedule of Repurchase of Shares	A summary of common shares repurchased for the twelve months ended December 31, 2017 is shown below: Repurchase Type Date Trading Plan Initiated 2017 Purchase Period Number of Shares Repurchased Average Price of Shares Repurchased Total Cost (in millions) Repurchase Authorization Year 10b5-1 Trading Plan 6/14/2017 06/20/2017-08/10/2017 156,570 $ 59.53 $ 9.4 2016 10b5-1 Trading Plan 9/15/2017 09/18/2017-11/07/2017 84,711 $ 60.71 5.1 2016 10b5-1 Trading Plan 12/15/2017 12/27/2017 1,420 $ 60.53 0.1 2016 Open Market N/A 08/11/2017-09/15/2017 421,976 $ 59.75 25.2 2016 Open Market N/A 11/08/2017-11/21/2017 91,575 $ 59.28 5.4 2016 Total 756,252 $ 59.76 $ 45.2
Schedule of Authorized and Unissued Common Shares Reserved	At December 31, 2017, we had the following authorized, unissued common shares reserved for future issuance: Reserve Name Shares Reserved 2014 Long-Term Incentive Plan 3,178,971 2016 Employee Share Purchase Plan 492,082 Total 3,671,053

Accumulated Other Comprehensi49

Accumulated Other Comprehensive Income (Loss) (Tables)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Changes in Accumulated Other Comprehensive Income (Loss)	A summary of changes in accumulated other comprehensive income (loss), net of taxes (where applicable) by component is presented below: (in millions) Foreign Translation Adjustments Unrealized Holding Gains on Securities Defined Benefit Pension Plans Total Balance, January 1, 2016 $ (21.6 ) $ 40.0 $ (6.9 ) $ 11.5 Other comprehensive income (loss) before reclassifications 4.0 42.4 (0.2 ) 46.2 Amounts reclassified from accumulated other comprehensive income (loss) — (10.0 ) — (10.0 ) Net current-period other comprehensive income (loss) 4.0 32.4 (0.2 ) 36.2 Balance, December 31, 2016 (17.6 ) 72.4 (7.1 ) 47.7 Other comprehensive income (loss) before reclassifications (1.4 ) 77.7 0.8 77.1 Amounts reclassified from accumulated other comprehensive income (loss) — (28.2 ) — (28.2 ) Net current-period other comprehensive income (loss) (1.4 ) 49.5 0.8 48.9 Balance, December 31, 2017 $ (19.0 ) $ 121.9 $ (6.3 ) $ 96.6
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	The amounts reclassified from accumulated other comprehensive income (loss) shown in the above table have been included in the following captions in our Consolidated Statements of Income: For the Years Ended December 31, (in millions) 2017 2016 2015 Unrealized gains and losses on securities: Net realized investment gains $ (41.6 ) $ (23.2 ) $ (7.2 ) Provision for income taxes 13.4 13.2 6.3 Net of taxes $ (28.2 ) $ (10.0 ) $ (0.9 )

Net Income Per Common Share (Ta

Net Income Per Common Share (Tables)	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Net Income Per Common Share on Basic and Diluted Basis	The following table presents the calculation of net income per common share on a basic and diluted basis: For the Years Ended December 31, (in millions, except number of shares and per share amounts) 2017 2016 2015 Net income $ 50.3 $ 146.7 $ 163.2 Weighted average common shares outstanding - basic 29,962,524 30,166,440 30,769,089 Effect of dilutive securities: Equity compensation awards 794,710 679,270 616,371 Weighted average common shares outstanding - diluted 30,757,234 30,845,710 31,385,460 Net income per common share: Basic $ 1.68 $ 4.86 $ 5.31 Diluted $ 1.64 $ 4.75 $ 5.20

Share-based Compensation (Table

Share-based Compensation (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Fair Value Assumptions	The following table summarizes the assumptions we used: For the Years Ended December 31, 2017 2016 2015 Risk-free rate of return 1.83% to 2.22% 1.00% to 2.02% 1.44% to 1.81% Expected dividend yields 1.63% to 1.72% 1.62% to 1.70% 1.46% to 1.60% Expected award life (years) 4.48 to 4.49 4.50 to 4.61 4.62 to 4.71 Expected volatility 18.13% to 18.70% 18.73% to 19.70% 20.04% to 22.09%
Summary of Restricted Share Activity	A summary of restricted share activity as of December 31, 2017 and changes during the year then ended is as follows: Shares Weighted-Average Grant Date Fair Value Outstanding at January 1, 2017 702,030 $ 42.69 Granted 235,431 $ 62.85 Vested and issued (183,458 ) $ 41.96 Expired or forfeited (72,334 ) $ 50.53 Outstanding at December 31, 2017 681,669 $ 49.02
Summary of Stock-Settled SARs Activity	A summary of stock-settled SARs activity as of December 31, 2017 and changes during the year then ended is as follows: Shares Weighted-Average Exercise Price Outstanding at January 1, 2017 1,982,695 $ 34.80 Exercised (486,454 ) $ 30.77 Expired or forfeited (186,089 ) $ 40.75 Outstanding at December 31, 2017 1,310,152 $ 35.45 Vested or expected to vest as of end of year 1,236,514 $ 19.80 Exercisable at end of year 891,067 $ 22.80
Summary of Cash-Settled SARs Activity	A summary of cash-settled SARs activity as of December 31, 2017 and changes during the year then ended is as follows: Shares Weighted-Average Exercise Price Outstanding at January 1, 2017 500,486 $ 32.08 Exercised (318,562 ) $ 31.87 Expired or forfeited (14,628 ) $ 23.40 Outstanding at December 31, 2017 167,296 $ 33.23

Underwriting, Acquisition and52

Underwriting, Acquisition and Insurance Expenses (Tables)	12 Months Ended
	Dec. 31, 2017
Underwriting Acquisition And Insurance Expenses [Abstract]
Underwriting, Acquisition and Insurance Expenses	Underwriting, acquisition and insurance expenses were as follows: For the Years Ended December 31, (in millions) 2017 2016 2015 Commissions $ 255.9 $ 234.0 $ 232.2 General expenses 359.1 292.1 295.6 Premium taxes, boards and bureaus 32.3 25.2 9.8 647.3 551.3 537.6 Net deferral of policy acquisition costs (11.9 ) (4.3 ) (0.9 ) Total underwriting, acquisition and insurance expenses $ 635.4 $ 547.0 $ 536.7

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Tax Provision (Benefit) Expense	The following table presents the components of income tax (benefit) provision expense included in the amounts reported in our consolidated financial statements: For the Years Ended December 31, (in millions) 2017 2016 2015 Current income tax (benefit) provision related to: United States $ (0.3 ) $ 30.4 $ 5.1 United Kingdom 7.6 3.9 (1.5 ) Other jurisdictions 0.2 2.0 2.4 Total current income tax provision 7.5 36.3 6.0 Deferred income tax (benefit) provision related to: United States (0.3 ) 3.3 7.8 United Kingdom (17.6 ) (4.4 ) 0.5 Other jurisdictions — — — Total deferred income tax (benefit) provision (17.9 ) (1.1 ) 8.3 Income tax (benefit) provision $ (10.4 ) $ 35.2 $ 14.3
Schedule Of Pre Tax Loss Income And Effective Income Tax Rate Table Text Block	For the years ended December 31, 2017, 2016 and 2015, pre-tax income (loss) attributable to our operations and the operations’ effective tax rates were as follows: (in millions) 2017 2016 2015 Pre-Tax Income (Loss) Effective Tax Rate Pre-Tax Income (Loss) Effective Tax Rate Pre-Tax Income (Loss) Effective Tax Rate Bermuda $ 30.2 0.0 % $ 99.6 0.0 % $ 94.3 0.0 % United States 67.5 (0.9 %) 115.2 29.2 % 64.8 19.9 % United Kingdom (53.8 ) 18.7 % (34.8 ) 1.2 % 21.3 (4.7 %) Belgium 0.1 75.0 % (0.1 ) 23.2 % — (1) 237.0 % Brazil 0.8 0.0 % 0.6 0.0 % (3.2 ) 0.0 % Ireland (2) (0.2 ) 0.0 % (0.2 ) 5.0 % (0.1 ) 5.0 % Luxembourg (5.2 ) 0.0 % — 0.0 % — 0.0 % Malta 0.3 0.0 % 1.6 0.3 % 0.1 0.0 % Switzerland — (1) 21.1 % — (1) 0.0 % 0.1 20.5 % United Arab Emirates 0.2 0.0 % — (1) 0.0 % 0.2 0.0 % Pre-tax income $ 39.9 (26.1 %) $ 181.9 19.4 % $ 177.5 8.1 % (1) Pre-tax income for the respective year was less than $0.1 million. (2) Effective tax rate of 5 percent on intercompany dividends of $40.0 million for the year ended December 31, 2016. Dividends eliminated in consolidation.
Reconciliation of Difference Between Provision for Income Taxes and Expected Tax Provision at Weighted Average Tax Rate	A reconciliation of the difference between the provision for income taxes and the expected tax provision at the weighted average tax rate is as follows: For the Years Ended December 31, (in millions) 2017 2016 2015 Income tax provision at expected rate $ 12.7 $ 34.1 $ 25.8 Tax effect of: Nontaxable investment income (4.7 ) (5.5 ) (6.5 ) Foreign exchange adjustments 2.1 5.3 (0.1 ) Withholding taxes 0.4 2.4 2.9 Change in valuation allowance (0.9 ) 0.7 (2.6 ) Impact of change in tax rate related to TCJA (20.2 ) — — Other 0.2 (1.8 ) (5.2 ) Income tax provision $ (10.4 ) $ 35.2 $ 14.3
Schedule of Net Deferred Tax Liability Comprises Tax Effect Related to Assets and Liabilities	The net deferred tax liability comprises the tax effects of temporary differences related to the following assets and liabilities: December 31, (in millions) 2017 2016 Deferred tax assets: Losses and loss adjustment expense reserve discounting $ 17.2 $ 21.1 Unearned premiums 21.4 29.1 Net operating loss carryforwards 21.9 26.7 Impairment of investment values 4.0 8.6 Accrued bonus 2.6 7.4 Stock option expense 2.0 8.1 United Kingdom underwriting results 3.8 — Other 20.0 32.5 Deferred tax assets, gross 92.9 133.5 Deferred tax liabilities: Unrealized gains on equity securities (30.5 ) (39.1 ) Unrealized gains on fixed maturities and other investment securities (5.0 ) (3.9 ) Deferred acquisition costs (16.9 ) (22.2 ) TCJA reserve transitional liability (5.5 ) — United Kingdom underwriting results — (5.4 ) Deferred gain on like-kind exchange (8.0 ) (13.3 ) Depreciable fixed assets (14.5 ) (21.0 ) Unrealized gains on limited partnership interests (14.6 ) (17.6 ) Other (9.1 ) (11.6 ) Deferred tax liabilities, gross (104.1 ) (134.1 ) Deferred tax (liabilities) assets, net before valuation allowance $ (11.2 ) $ (0.6 ) Valuation allowance (20.1 ) (23.5 ) Deferred tax liabilities, net $ (31.3 ) $ (24.1 ) Net deferred tax (liabilities) assets - Other jurisdictions $ 1.2 $ (6.6 ) Net deferred tax liabilities - United States (32.5 ) (17.5 ) Deferred tax liabilities, net $ (31.3 ) $ (24.1 )
Schedule of Net Operating Loss Carryforwards Amounts by Jurisdiction and Year of Expiration	The NOL amounts by jurisdiction and year of expiration are as follows: (in millions) December 31, 2017 Expiration Net operating loss carryforwards by jurisdiction Brazil $ 17.0 Indefinite Luxembourg 7.8 Indefinite Luxembourg 5.2 2034 Malta 3.0 Indefinite United States 51.5 2025 - 2036

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2017
Leases [Abstract]
Schedule of Future Minimum Payments Under Non-Cancelable Operating Leases	At December 31, 2017, the future minimum payments under non-cancelable operating leases are as follows: (in millions) Amount Due 2018 $ 16.2 2019 13.5 2020 10.9 2021 9.5 2022 8.2 Thereafter 52.8 Total $ 111.1

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Revenue and Income (Loss) Before Income Taxes for Each Segment	Revenue and income (loss) before income taxes for each segment were as follows: For the Years Ended December 31, (in millions) 2017 2016 2015 Revenue: Earned premiums U.S. Operations $ 936.6 $ 849.5 $ 815.4 International Operations 635.8 560.9 556.1 Run-off Lines (0.1 ) 0.4 0.4 Total earned premiums 1,572.3 1,410.8 1,371.9 Net investment income U.S. Operations 87.2 71.9 52.2 International Operations 32.7 28.7 20.3 Run-off Lines 9.3 11.3 8.1 Corporate and Other 10.8 3.2 8.0 Total net investment income 140.0 115.1 88.6 Fee and other income 22.5 24.5 22.2 Net realized investment and other gains 39.3 26.1 24.1 Total revenue $ 1,774.1 $ 1,576.5 $ 1,506.8 For the Years Ended December 31, (in millions) 2017 2016 2015 Income (loss) before income taxes U.S. Operations $ 169.4 $ 174.4 $ 124.4 International Operations (86.7 ) 51.6 64.4 Run-off Lines (17.9 ) (15.2 ) (7.4 ) Total segment income before taxes 64.8 210.8 181.4 Corporate and Other (64.2 ) (55.0 ) (28.0 ) Net realized investment and other gains 39.3 26.1 24.1 Total income before income taxes $ 39.9 $ 181.9 $ 177.5
Schedule of Earned Premiums by Geographic Location	The table below presents earned premiums by geographic location. For this disclosure, we determine geographic location by the country of domicile of our subsidiaries that underwrite the business and not by the location of insureds or reinsureds from whom the business was generated. For the Years Ended December 31, (in millions) 2017 2016 2015 Bermuda $ 92.1 $ 112.6 $ 103.4 Brazil 49.0 39.2 43.1 Malta 11.1 2.1 1.9 United Kingdom 489.3 406.9 407.4 United States 930.8 850.0 816.1 Total earned premiums $ 1,572.3 $ 1,410.8 $ 1,371.9
Identifiable Assets	The following table represents identifiable assets: December 31, (in millions) 2017 2016 U.S. Operations $ 4,298.4 $ 3,961.2 International Operations 3,553.8 2,356.9 Run-off Lines 449.6 537.0 Corporate and Other 462.2 349.9 Total $ 8,764.0 $ 7,205.0
Schedule of Goodwill and Intangible Assets Net of Accumulated Amortization	The following table represents goodwill and intangible assets, net of accumulated amortization as of December 31: Goodwill Intangible Assets, Net of Accumulated Amortization (in millions) 2017 2016 2017 2016 U.S. Operations $ 123.5 $ 123.5 $ 1.5 $ 3.7 International Operations 37.9 28.7 95.3 64.0 Total $ 161.4 $ 152.2 $ 96.8 $ 67.7

Statutory Accounting Principl56

Statutory Accounting Principles (Tables)	12 Months Ended
	Dec. 31, 2017
Statutory Accounting Principles [Abstract]
Statutory Capital and Surplus for Principal Operating Subsidiaries	The statutory capital and surplus for our principal operating subsidiaries was as follows: Statutory capital and surplus (1) December 31, (in millions) 2017 2016 Bermuda $ 1,694.4 $ 1,480.1 United Kingdom (2) 330.3 216.6 United States 992.8 886.7 (1) Such amounts include ownership interests in affiliate insurance and reinsurance subsidiaries. (2) Capital on deposit with Lloyd’s in U.S. Dollars
Statutory Net Income (Loss) for Principal Operating Subsidiaries	The statutory net income (loss) for our principal operating subsidiaries was as follows: Statutory net income (loss) (1) For the Years Ended December 31, (in millions) 2017 2016 2015 Bermuda $ 37.4 $ 158.2 $ 182.2 United Kingdom (2) (95.5 ) 1.8 34.8 United States 57.0 99.0 94.4 (1) Such amounts include ownership interests in affiliate insurance and reinsurance subsidiaries. (2) In U.S. Dollars

Unaudited Quarterly Financial57

Unaudited Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data	(in millions, except per share amounts) 1st Quarter 2nd 3rd Quarter 4th Quarter Year 2017 Total revenue $ 428.1 $ 451.0 $ 439.2 $ 455.8 $ 1,774.1 Net income (loss) before income taxes 42.7 50.8 (65.9 ) 12.3 39.9 Net income (loss) 36.7 46.0 (61.3 ) 28.9 50.3 Net income (loss) per common share : Basic* $ 1.22 $ 1.52 $ (2.04 ) $ 0.98 $ 1.68 Diluted* $ 1.19 $ 1.48 $ (2.04 ) $ 0.95 $ 1.64 Comprehensive income (loss) $ 50.6 $ 64.5 $ (44.4 ) $ 28.5 $ 99.2 (in millions, except per share amounts) 1st Quarter 2nd 3rd Quarter 4th Quarter Year 2016 Total revenue $ 370.1 $ 384.3 $ 416.7 $ 405.4 $ 1,576.5 Net income before income taxes 33.1 39.6 62.2 47.0 181.9 Net income 27.7 30.9 55.2 32.9 146.7 Net income per common share : Basic* $ 0.91 $ 1.03 $ 1.84 $ 1.10 $ 4.86 Diluted* $ 0.89 $ 1.00 $ 1.80 $ 1.07 $ 4.75 Comprehensive income $ 56.5 $ 59.7 $ 57.3 $ 9.4 $ 182.9 * Basic and diluted net income per common share are computed independently for each quarter and full year based on the respective average number of common shares outstanding; therefore, the sum of the quarterly net income per common share data may not equal the net income per common share for the year.

Senior Unsecured Fixed Rate N58

Senior Unsecured Fixed Rate Notes (Tables)	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Schedule Of Unamortized Debt Issuance Costs Deducted From Carrying Value Of Debt Liability	At December 31, 2017 and 2016, the Notes consisted of the following: (in millions) December 31, 2017 December 31, 2016 Senior unsecured fixed rate notes Principal $ 143.8 $ 143.8 Less: unamortized debt issuance costs (4.2 ) (4.3 ) Senior unsecured fixed rate notes, less unamortized debt issuance costs $ 139.6 $ 139.5
Condensed Consolidating Balance Sheet	CONDENSED CONSOLIDATING BALANCE SHEET DECEMBER 31, 2017 (in millions) Argo Group International Holdings, Ltd (Parent Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Assets Investments $ 0.4 $ 3,079.0 $ 1,663.5 $ — $ 4,742.9 Cash 0.9 47.8 127.9 — 176.6 Accrued investment income — 18.0 5.5 — 23.5 Premiums receivable — 216.5 382.1 — 598.6 Reinsurance recoverables — 1,487.3 606.0 — 2,093.3 Goodwill and other intangible assets, net 43.2 124.9 90.1 — 258.2 Current income taxes receivable, net — 2.4 (1.0 ) — 1.4 Deferred acquisition costs, net — 80.7 79.7 — 160.4 Ceded unearned premiums — 198.5 201.0 — 399.5 Other assets 9.2 171.5 128.9 — 309.6 Intercompany note receivable — 50.9 (50.9 ) — — Investments in subsidiaries 1,940.0 — — (1,940.0 ) — Total assets $ 1,993.7 $ 5,477.5 $ 3,232.8 $ (1,940.0 ) $ 8,764.0 Liabilities and Shareholders' Equity Reserves for losses and loss adjustment expenses $ — $ 2,483.9 $ 1,717.1 $ — $ 4,201.0 Unearned premiums — 704.0 503.7 — 1,207.7 Funds held and ceded reinsurance payable, net — 799.4 (22.7 ) — 776.7 Long-term debt 153.4 284.5 142.8 — 580.7 Deferred tax liabilities, net — 32.5 (1.2 ) — 31.3 Accrued underwriting expenses and other liabilities 8.9 95.0 43.0 — 146.9 Due to affiliates 11.7 (0.4 ) 0.4 (11.7 ) — Total liabilities 174.0 4,398.9 2,383.1 (11.7 ) 6,944.3 Total shareholders' equity 1,819.7 1,078.6 849.7 (1,928.3 ) 1,819.7 Total liabilities and shareholders' equity $ 1,993.7 $ 5,477.5 $ 3,232.8 $ (1,940.0 ) $ 8,764.0 (1) Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations. (2) Includes all Argo Group parent company eliminations. CONDENSED CONSOLIDATING BALANCE SHEET DECEMBER 31, 2016 (in millions) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Assets Investments $ 1.9 $ 2,833.5 $ 1,484.9 $ — $ 4,320.3 Cash — 53.7 32.3 — 86.0 Accrued investment income — 16.0 4.7 — 20.7 Premiums receivable — 204.9 258.9 — 463.8 Reinsurance recoverables — 1,348.4 37.2 — 1,385.6 Goodwill and other intangible assets, net — 127.1 92.8 — 219.9 Deferred acquisition costs, net — 63.5 75.6 — 139.1 Ceded unearned premiums — 168.9 133.9 — 302.8 Other assets 9.0 168.7 89.1 — 266.8 Intercompany note receivable — 50.2 (50.2 ) — — Investments in subsidiaries 1,834.4 — — (1,834.4 ) — Total assets $ 1,845.3 $ 5,034.9 $ 2,159.2 $ (1,834.4 ) $ 7,205.0 Liabilities and Shareholders' Equity Reserves for losses and loss adjustment expenses $ — $ 2,322.4 $ 1,028.4 $ — $ 3,350.8 Unearned premiums — 580.0 390.0 — 970.0 Funds held and ceded reinsurance payable, net — 750.2 (206.5 ) — 543.7 Long-term debt 28.4 284.4 54.8 — 367.6 Current income taxes payable, net — 8.5 (0.4 ) — 8.1 Deferred tax liabilities, net — 17.6 6.5 — 24.1 Accrued underwriting expenses and other liabilities 13.7 92.0 42.3 — 148.0 Due to affiliates 10.5 1.8 (1.8 ) (10.5 ) — Total liabilities 52.6 4,056.9 1,313.3 (10.5 ) 5,412.3 Total shareholders' equity 1,792.7 978.0 845.9 (1,823.9 ) 1,792.7 Total liabilities and shareholders' equity $ 1,845.3 $ 5,034.9 $ 2,159.2 $ (1,834.4 ) $ 7,205.0 (1) Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations. (2) Includes all Argo Group parent company eliminations.
Condensed Consolidating Statement of Income	CONDENSED CONSOLIDATING STATEMENT OF INCOME FOR THE YEAR ENDED DECEMBER 31, 2017 (in millions) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Premiums and other revenue: Earned premiums $ — $ 555.9 $ 1,016.4 $ — $ 1,572.3 Net investment (expense) income (4.5 ) 87.5 57.0 — 140.0 Fee and other income — 18.8 3.7 — 22.5 Net realized investment and other gains (loss) 0.4 40.8 (1.9 ) — 39.3 Total revenue (4.1 ) 703.0 1,075.2 — 1,774.1 Expenses: Losses and loss adjustment expenses — 337.9 712.3 — 1,050.2 Underwriting, acquisition and insurance expenses 14.3 266.6 354.5 — 635.4 Interest expense 4.3 17.2 6.2 — 27.7 Fee and other expense — 12.4 2.2 — 14.6 Foreign currency exchange loss 0.1 0.1 6.1 — 6.3 Total expenses 18.7 634.2 1,081.3 — 1,734.2 (Loss) Income before income taxes (22.8 ) 68.8 (6.1 ) — 39.9 Provision for income taxes — (0.6 ) (9.8 ) — (10.4 ) Net (loss) income before equity in earnings of subsidiaries (22.8 ) 69.4 3.7 — 50.3 Equity in undistributed earnings of subsidiaries 73.1 — — (73.1 ) — Net income $ 50.3 $ 69.4 $ 3.7 $ (73.1 ) $ 50.3 (1) Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations. (2) Includes all Argo Group parent company eliminations. CONDENSED CONSOLIDATING STATEMENT OF INCOME FOR THE YEAR ENDED DECEMBER 31, 2016 (in millions) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Premiums and other revenue: Earned premiums $ — $ 497.3 $ 913.5 $ — $ 1,410.8 Net investment income 37.2 78.9 40.0 (41.0 ) 115.1 Fee and other income — 21.4 3.1 — 24.5 Net realized investment and other gains 0.6 50.0 (24.5 ) — 26.1 Total revenue 37.8 647.6 932.1 (41.0 ) 1,576.5 Expenses: Losses and loss adjustment expenses — 290.4 519.7 — 810.1 Underwriting, acquisition and insurance expenses 12.8 204.4 329.8 — 547.0 Interest expense 1.4 15.8 2.4 — 19.6 Fee and other expense — 21.7 0.7 — 22.4 Foreign currency exchange loss (gains) — 0.2 (4.7 ) — (4.5 ) Total expenses 14.2 532.5 847.9 — 1,394.6 Income before income taxes 23.6 115.1 84.2 (41.0 ) 181.9 Provision for income taxes — 33.6 1.6 — 35.2 Net income before equity in earnings of subsidiaries 23.6 81.5 82.6 (41.0 ) 146.7 Equity in undistributed earnings of subsidiaries 123.1 — — (123.1 ) — Net income $ 146.7 $ 81.5 $ 82.6 $ (164.1 ) $ 146.7 (1) Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations. (2) Includes all Argo Group parent company eliminations. CONDENSED CONSOLIDATING STATEMENT OF INCOME FOR THE YEAR ENDED DECEMBER 31, 2015 (in millions) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Premiums and other revenue: Earned premiums $ — $ 497.3 $ 874.6 $ — $ 1,371.9 Net investment income 40.1 59.0 30.5 (41.0 ) 88.6 Fee and other income — 19.0 3.2 — 22.2 Net realized investment and other gains — 31.8 (7.7 ) — 24.1 Total revenue 40.1 607.1 900.6 (41.0 ) 1,506.8 Expenses: Losses and loss adjustment expenses — 304.2 461.9 — 766.1 Underwriting, acquisition and insurance expenses 19.0 198.4 319.3 — 536.7 Interest expense 1.5 15.3 2.2 — 19.0 Fee and other expense — 23.3 2.5 — 25.8 Foreign currency exchange (gain) loss — 1.0 (19.3 ) — (18.3 ) Total expenses 20.5 542.2 766.6 — 1,329.3 Income before income taxes 19.6 64.9 134.0 (41.0 ) 177.5 Provision for income taxes — 12.9 1.4 — 14.3 Net income before equity in earnings of subsidiaries 19.6 52.0 132.6 (41.0 ) 163.2 Equity in undistributed earnings of subsidiaries 143.6 — — (143.6 ) — Net income $ 163.2 $ 52.0 $ 132.6 $ (184.6 ) $ 163.2 (1) Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations. (2) Includes all Argo Group parent company eliminations.
Condensed Consolidating Statement of Cash Flows	CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS FOR YEAR ENDED DECEMBER 31, 2017 (in millions) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Net cash flows from operating activities $ 20.8 $ 149.5 $ (5.3 ) $ — $ 165.0 Cash flows from investing activities: Proceeds from sales of investments — 809.5 920.4 — 1,729.9 Maturities and mandatory calls of fixed maturity investments — 483.1 195.2 — 678.3 Purchases of investments — (1,495.6 ) (1,165.2 ) — (2,660.8 ) Change in short-term investments and foreign regulatory deposits 1.5 67.1 230.9 — 299.5 Settlements of foreign currency exchange forward contracts 0.9 (8.2 ) 4.4 — (2.9 ) Acquisition of subsidiaries, net of cash (235.3 ) — 130.1 — (105.2 ) Issuance of intercompany note, net — — (120.0 ) 120.0 — Purchases of fixed assets and other, net (0.1 ) (26.2 ) (33.8 ) — (60.1 ) Cash provided by (used in) investing activities (233.0 ) (170.3 ) 162.0 120.0 (121.3 ) Cash flows from financing activities: Additional long-term borrowings 125.0 — — — 125.0 Borrowing under the intercompany note 120.0 60.0 (60.0 ) (120.0 ) — Activity under stock incentive plans 1.4 — — — 1.4 Repurchase of Company's common shares (0.1 ) (45.1 ) — — (45.2 ) Payment of cash dividend to common shareholders (33.2 ) — — — (33.2 ) Cash provided by (used in) financing activities 213.1 14.9 (60.0 ) (120.0 ) 48.0 Effect of exchange rate changes on cash — — (1.1 ) — (1.1 ) Change in cash 0.9 (5.9 ) 95.6 — 90.6 Cash, beginning of year — 53.7 32.3 — 86.0 Cash, end of year $ 0.9 $ 47.8 $ 127.9 $ — $ 176.6 (1) Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations. (2) Includes all Argo Group parent company eliminations. CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS FOR YEAR ENDED DECEMBER 31, 2016 (in millions) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Net cash flows from operating activities $ 26.5 $ 72.3 $ 83.2 $ — $ 182.0 Cash flows from investing activities: Proceeds from sales of investments — 1,035.9 407.6 — 1,443.5 Maturities and mandatory calls of fixed maturity investments — 543.2 459.5 — 1,002.7 Purchases of investments — (1,450.0 ) (930.5 ) — (2,380.5 ) Change in short-term investments and foreign regulatory deposits (0.9 ) (138.1 ) (56.2 ) — (195.2 ) Settlements of foreign currency exchange forward contracts — — (5.4 ) — (5.4 ) Purchases of fixed assets and other, net — (11.3 ) 1.1 — (10.2 ) Cash used in investing activities (0.9 ) (20.3 ) (123.9 ) $ — (145.1 ) Cash flows from financing activities: Activity under stock incentive plans 1.0 — — — 1.0 Repurchase of Company's common shares — (47.1 ) — — (47.1 ) Payment of cash dividend to common shareholders (26.6 ) — — — (26.6 ) Intercompany cash dividend — (40.0 ) 40.0 — — Cash provided by (used in) financing activities (25.6 ) (87.1 ) 40.0 — (72.7 ) Effect of exchange rate changes on cash — — 0.1 — 0.1 Change in cash — (35.1 ) (0.6 ) — (35.7 ) Cash, beginning of year — 88.8 32.9 — 121.7 Cash, end of year $ — $ 53.7 $ 32.3 $ — $ 86.0 (1) Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations. (2) Includes all Argo Group parent company eliminations. CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS FOR YEAR ENDED DECEMBER 31, 2015 (in millions) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Net cash flows from operating activities $ 32.7 $ 117.0 $ 133.5 $ — $ 283.2 Cash flows from investing activities: Proceeds from sales of investments — 631.1 336.8 — 967.9 Maturities and mandatory calls of fixed maturity investments — 681.8 162.1 — 843.9 Purchases of investments — (1,384.5 ) (649.6 ) — (2,034.1 ) Change in short-term investments and foreign regulatory deposits 0.9 14.9 33.8 — 49.6 Settlements of foreign currency exchange forward contracts 1.5 — (11.6 ) — (10.1 ) Issuance of intercompany note, net — 7.5 (7.5 ) — — Redemption of PXRE Capital Trust V — 18.0 (18.0 ) — — Purchases of fixed assets and other, net 3.8 (16.6 ) 2.0 — (10.8 ) Cash provided by (used in) investing activities 6.2 (47.8 ) (152.0 ) — (193.6 ) Cash flows from financing activities: Redemption of PXRE Capital Trust V (18.0 ) — 18.0 — — Activity under stock incentive plans 1.8 — — — 1.8 Repurchase of Company's common shares — (29.7 ) — — (29.7 ) Payment of cash dividend to common shareholders (22.7 ) — — — (22.7 ) Cash used in financing activities (38.9 ) (29.7 ) 18.0 — (50.6 ) Effect of exchange rate changes on cash — — 1.7 — 1.7 Change in cash — 39.5 1.2 — 40.7 Cash, beginning of year — 49.3 31.7 — 81.0 Cash, end of year $ — $ 88.8 $ 32.9 $ — $ 121.7 (1) Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations. (2) Includes all Argo Group parent company eliminations.

Subsequent Events (Tables)

Subsequent Events (Tables)	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Pro forma Basic and Diluted Net Income Per Common Share	Presented below are pro forma basic and diluted net income per common share after giving effect for the stock dividend: For the Years Ended December 31, 2017 2016 2015 Pro forma net income per common share: Basic $ 1.46 $ 4.23 $ 4.61 Diluted $ 1.42 $ 4.13 $ 4.52

Business and Significant Acco60

Business and Significant Accounting Policies - Additional Information (Detail) $ in Millions	Feb. 20, 2018shares	Feb. 06, 2017USD ($)	May 03, 2016shares	Feb. 17, 2015shares	Dec. 31, 2018	Dec. 31, 2017USD ($)TrustInstitution	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Business And Significant Accounting Policies [Line Items]
Percentage of contractual participation						100.00%
Number of Statutory Trusts | Trust						12
Number of Charitable Foundations | Institution						2
Stock dividend declared							10.00%	10.00%
Short-term investments, maturity term						1 year
Percentage of investment portfolio in fair value from pricing service or broker						1.00%
Allowance for doubtful accounts						$ 0.6	$ 2.7
Reinsurance recoverables, allowance for doubtful accounts						2.1	2.1
Estimated liability for return of premiums						7.9	5.7
Estimated amount of premiums receivables due						$ 0.1	0.1
Weighted average useful life						8 years 6 months
Amortization expense						$ 5.9	5.5	$ 7.5
Estimated amortization expense for 2018						3.5
Estimated amortization expense for 2019						1.7
Estimated amortization expense for 2020						1.3
Estimated amortization expense for 2021						1.3
Estimated amortization expense for 2022						0.1
Accumulated depreciation for property and equipment						124.1	101.6
Net book value of property and equipment						151	142.2
Depreciation expense						24.1	20.9	17.5
Foreign currency translation adjustment						$ 19	17.6
US federal tax rate						35.00%
Income taxes paid						$ 16.5	16.6	10.9
Income taxes recovered						$ 2.5	$ 0.5	$ 11.7
Scenario Forecast [Member]
Business And Significant Accounting Policies [Line Items]
US federal tax rate					21.00%
Minimum [Member]
Business And Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life						3 years
Maximum [Member]
Business And Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life						39 years
Distribution Network [Member]
Business And Significant Accounting Policies [Line Items]
Weighted average useful life						8 years 10 months 24 days
Additional Lloyd's Capacity [Member]
Business And Significant Accounting Policies [Line Items]
Weighted average useful life						5 years
Other [Member]
Business And Significant Accounting Policies [Line Items]
Weighted average useful life						6 years 4 months 24 days
Stock Dividend [Member]
Business And Significant Accounting Policies [Line Items]
Stock dividend declared			10.00%	10.00%
Dividends declaration date			May 3, 2016	Feb. 17, 2015
Dividends payable date			Jun. 15, 2016	Mar. 16, 2015
Dividend payable, date of record			Jun. 1, 2016	Mar. 2, 2015
Additional stock issued as dividend | shares			2,735,542	2,554,506
Subsequent Event [Member]
Business And Significant Accounting Policies [Line Items]
Dividends declaration date	Feb. 20, 2018
Dividends payable date	Mar. 23, 2018
Dividend payable, date of record	Mar. 7, 2018
Subsequent Event [Member] | Stock Dividend [Member]
Business And Significant Accounting Policies [Line Items]
Stock dividend declared	15.00%
Dividends declaration date	Feb. 20, 2018
Dividends payable date	Mar. 21, 2018
Dividend payable, date of record	Mar. 7, 2018
Additional stock issued as dividend | shares	4,400,000
Maybrooke [Member]
Business And Significant Accounting Policies [Line Items]
Business acquisition effective date						Feb. 6, 2017
Intangible assets acquired		$ 35.1
Amortization expense						$ 1.2
Maybrooke [Member] | Distribution Network [Member]
Business And Significant Accounting Policies [Line Items]
Intangible assets acquired		$ 4.7
Weighted average useful life		5 years

Business and Significant Acco61

Business and Significant Accounting Policies - Summary of Receivables (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Premiums receivable	$ 1.5	$ 1.1	$ 1
Reinsurance recoverables	0	0	0.2
Net written off	$ 1.5	$ 1.1	$ 1.2

Business and Significant Acco62

Business and Significant Accounting Policies - Intangible Assets and Accumulated Amortization (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 147.3	$ 112.2
Accumulated Amortization	50.5	44.5
Lloyd's Capacity [Member]
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	89	60.5
Accumulated Amortization	0	0
Distribution Network [Member]
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	50.2	45.5
Accumulated Amortization	44.1	38.5
Additional Lloyd's Capacity [Member]
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	4.8	4.8
Accumulated Amortization	4.8	4.8
Other [Member]
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	3.3	1.4
Accumulated Amortization	$ 1.6	$ 1.2

Business and Significant Acco63

Business and Significant Accounting Policies - Schedule of Interest Paid (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure - Business and Significant Accounting Policies - Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) [Line Items]
Total interest paid	$ 27.1	$ 19.3	$ 18.7
Revolving credit facility [Member]
Disclosure - Business and Significant Accounting Policies - Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) [Line Items]
Total interest paid	0.3	0	0
Senior Unsecured Fixed Rate Notes [Member]
Disclosure - Business and Significant Accounting Policies - Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) [Line Items]
Total interest paid	9.3	9.3	9.3
Junior Subordinated Debentures [Member]
Disclosure - Business and Significant Accounting Policies - Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) [Line Items]
Total interest paid	12.6	7.7	7
Other Indebtedness [Member]
Disclosure - Business and Significant Accounting Policies - Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) [Line Items]
Total interest paid	$ 4.9	$ 2.3	$ 2.4

Acquisition of Maybrooke - Addi

Acquisition of Maybrooke - Additional Information (Detail) - USD ($) $ in Millions	Feb. 06, 2017	Jan. 31, 2017	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Goodwill			$ 161.4		$ 161.4	$ 152.2
Amortization of Intangible Assets					5.9	5.5	$ 7.5
Junior subordinated debentures principal balance			143.8		143.8	143.8
Underwriting, acquisition and insurance expenses					635.4	547	536.7
Interest expense					27.7	19.6	19
Favorable development from prior accident years					(8.2)	$ (33.3)	$ (32.4)
Junior Subordinated Debentures, Maybrooke [Member]
Business Acquisition [Line Items]
Junior subordinated debentures principal balance			91.8		91.8
Debt instrument, carrying amount			83.9		83.9
Debt’s fair value			83.6		$ 83.6
Maybrooke [Member]
Business Acquisition [Line Items]
Business acquisition effective date					Feb. 6, 2017
Purchase price paid	$ 235.3
Goodwill	9.2
Intangible assets acquired	35.1
Amortization of Intangible Assets					$ 1.2
Debt’s fair value	$ 83.6
Transaction cost from acquisition			$ 0	$ 2.5	2.5
Underwriting, acquisition and insurance expenses				2.2
Favorable development from prior accident years					$ 11.2
Maybrooke [Member] | One Specific Claim [Member]
Business Acquisition [Line Items]
Favorable development from prior accident years		$ 6.2
Maybrooke [Member] | Credit Facility [Member]
Business Acquisition [Line Items]
Interest expense				$ 0.3

Acquisition of Maybrooke - Sche

Acquisition of Maybrooke - Schedule of Fair Value of Assets Acquired and Liabilities Assumed (Detail) - Maybrooke [Member] $ in Millions	Feb. 06, 2017USD ($)
Assets:
Investments	$ 340.7
Cash	130.1
Premiums receivable	157.6
Reinsurance balances	172.8
Other assets	35.6
Total assets	836.8
Liabilities:
Reserves for losses and loss adjustment expenses	197
Unearned premiums	152.5
Ceded reinsurance payable, net	144.2
Junior subordinated debentures	83.6
Accrued underwriting expenses	29
Other liabilities	39.5
Total liabilities	645.8
Net assets acquired	191
Purchase price	235.3
Excess purchase price	$ 44.3

Acquisition of Maybrooke - Summ

Acquisition of Maybrooke - Summary of Acquired Intangible Assets (Detail) - USD ($) $ in Millions	Feb. 06, 2017	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Weighted average useful life		8 years 6 months
Goodwill		$ 161.4	$ 152.2
Distribution Network [Member]
Business Acquisition [Line Items]
Weighted average useful life		8 years 10 months 24 days
Maybrooke [Member]
Business Acquisition [Line Items]
Fair Value	$ 35.1
Goodwill	9.2
Total goodwill and intangible assets	44.3
Maybrooke [Member] | Stamp Capacity [Member]
Business Acquisition [Line Items]
Fair Value	28.5
Maybrooke [Member] | Distribution Network [Member]
Business Acquisition [Line Items]
Fair Value	$ 4.7
Weighted average useful life	5 years
Maybrooke [Member] | Trade Name [Member]
Business Acquisition [Line Items]
Fair Value	$ 1.9
Weighted average useful life	5 years

Acquisition of Maybrooke - Su67

Acquisition of Maybrooke - Summary of Selected Financial Information (Detail) - Maybrooke [Member] $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
Business Acquisition [Line Items]
Revenues	$ 127.6
Net loss	$ (13.1)

Acquisition of Maybrooke - Su68

Acquisition of Maybrooke - Summary of Pro Forma Results (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Business Combinations [Abstract]
Pro forma revenues	$ 1,787.9	$ 1,793.1
Pro forma net income	$ 58.7	$ 167.9
Pro forma net income per share - basic	$ 1.96	$ 5.57
Pro forma net income per share - diluted	$ 1.91	$ 5.44

Investments - Schedule of Amort

Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Invested Assets [Line Items]
Amortized Cost	$ 3,320.6	$ 2,938.8
Fair Value	3,343.4	2,932.4
Amortized Cost	338.2	335.2
Fair Value	487.4	447.4
Invested Assets Amortized Cost	4,561.4	4,207.1
Investments Gross Unrealized Gains	213.7	167.3
Investments Gross Unrealized losses	32.2	54.1
Total investments	4,742.9	4,320.3
Fixed Maturities [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	3,320.6	2,938.8
Gross Unrealized Gains	50.1	41.9
Gross Unrealized Losses	27.3	48.3
Fair Value	3,343.4	2,932.4
Fixed Maturities [Member] | Collateralized Loan Obligations [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	222.6	269.6
Gross Unrealized Gains	5.9	3.8
Gross Unrealized Losses	0.5	9.1
Fair Value	228	264.3
Fixed Maturities [Member] | Residential Mortgage-backed Securities [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	309.3	174.8
Gross Unrealized Gains	2.8	3.7
Gross Unrealized Losses	2.7	1.7
Fair Value	309.4	176.8
Fixed Maturities [Member] | U.S. Governments [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	419.9	275.1
Gross Unrealized Gains	0.2	0.6
Gross Unrealized Losses	5	4.5
Fair Value	415.1	271.2
Fixed Maturities [Member] | Foreign Governments [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	229	244.2
Gross Unrealized Gains	6.7	1.1
Gross Unrealized Losses	2.5	8
Fair Value	233.2	237.3
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	327.7	375.7
Gross Unrealized Gains	9.3	8.9
Gross Unrealized Losses	1.1	1.8
Fair Value	335.9	382.8
Fixed Maturities [Member] | Corporate Bonds [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	1,514.5	1,316.9
Gross Unrealized Gains	24.4	23.3
Gross Unrealized Losses	13.2	19.5
Fair Value	1,525.7	1,320.7
Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	136.3	154.9
Gross Unrealized Gains	0.1	0.4
Gross Unrealized Losses	1.5	1.6
Fair Value	134.9	153.7
Fixed Maturities [Member] | Asset-backed Securities [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	161.3	127.6
Gross Unrealized Gains	0.7	0.1
Gross Unrealized Losses	0.8	2.1
Fair Value	161.2	125.6
Equity Securities [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	338.2	335.2
Gross Unrealized Gains	154	117.9
Gross Unrealized Losses	4.8	5.7
Fair Value	487.4	447.4
Other Investments [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost	534.1	527.6
Investments Gross Unrealized Gains	9.6	7.5
Investments Gross Unrealized losses	0.1	0.1
Total investments	543.6	535
Short-Term Investments [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost	368.5	405.5
Investments Gross Unrealized Gains	0	0
Investments Gross Unrealized losses	0	0
Total investments	$ 368.5	$ 405.5

Investments - Additional Inform

Investments - Additional Information (Detail)	Dec. 31, 2017USD ($)Security	Dec. 31, 2016USD ($)
Schedule of Investments [Line Items]
Fair value of investments, assets managed on behalf of the trade capital providers	$ 130,800,000	$ 131,900,000
Number of securities in an unrealized loss position, total | Security	8,084
Number of securities in an unrealized loss position for less than one year | Security	2,273
Number of securities in an unrealized loss position for a period of one year or greater | Security	357
Transfers between Level 1 and Level 2 securities	$ 0	0
Transfers between Level 2 and Level 1 securities	0	$ 0
Fixed Maturities Term Loan [Member]
Schedule of Investments [Line Items]
Own term loan fixed maturity level 3 value	$ 1,900,000

Investments - Schedule of Amo71

Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) $ in Millions	Dec. 31, 2017USD ($)
Investments Debt And Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 262.3
Amortized Cost, Due after one year through five years	1,432.2
Amortized Cost, Due after five years through ten years	623.3
Amortized Cost, Thereafter	173.3
Amortized Cost, Structured securities	829.5
Amortized Cost, Total	3,320.6
Fair Value, Due in one year or less	261.9
Fair Value, Due after one year through five years	1,440
Fair Value, Due after five years through ten years	630.6
Fair Value, Thereafter	177.4
Fair Value, Structured securities	833.5
Fair Value, Total	$ 3,343.4

Investments - Schedule of Carry

Investments - Schedule of Carrying Value and Unfunded Investment Commitments of Other Invested Assets Portfolio (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Carrying Value	$ 4,742.9	$ 4,320.3
Hedge funds [Member]
Schedule of Investments [Line Items]
Carrying Value	163.6	180.9
Unfunded Commitments	0	0
Private equity [Member]
Schedule of Investments [Line Items]
Carrying Value	179.2	179
Unfunded Commitments	129.9	93.4
Long only funds [Member]
Schedule of Investments [Line Items]
Carrying Value	196.5	170.7
Unfunded Commitments	0	0
Other Investments [Member]
Schedule of Investments [Line Items]
Carrying Value	4.3	4.4
Unfunded Commitments	0	0
Other Invested Assets [Member]
Schedule of Investments [Line Items]
Carrying Value	543.6	535
Unfunded Commitments	$ 129.9	$ 93.4

Investments - Schedule of Aging

Investments - Schedule of Aging of Unrealized Losses on Company's Investments in Fixed Maturities, Equity Securities and Other Investments (Detail) - USD ($) $ in Millions		Dec. 31, 2017		Dec. 31, 2016
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		$ 1,585.8		$ 1,365.2
Unrealized Losses, Less Than One Year		22		50.9
Fair Value, One Year or Greater		343		89
Unrealized Losses, One Year or Greater		10.2		3.2
Fair Value, Total		1,928.8		1,454.2
Unrealized Losses, Total		32.2		54.1
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		1,538.9		1,298
Unrealized Losses, Less Than One Year		17.2		45.1
Fair Value, One Year or Greater		342.7		89
Unrealized Losses, One Year or Greater		10.1		3.2
Fair Value, Total		1,881.6		1,387
Unrealized Losses, Total		27.3		48.3
Fixed Maturities [Member] | Collateralized Loan Obligations [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		34.6	[1]	122.5
Unrealized Losses, Less Than One Year		0.5	[1]	8.6
Fair Value, One Year or Greater		0.9	[1]	16.9
Unrealized Losses, One Year or Greater		0	[1]	0.5
Fair Value, Total		35.5	[1]	139.4
Unrealized Losses, Total		0.5	[1]	9.1
Fixed Maturities [Member] | Residential Mortgage-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		164.4		87.5	[1]
Unrealized Losses, Less Than One Year		1.6		1.7	[1]
Fair Value, One Year or Greater		52.4		4.4	[1]
Unrealized Losses, One Year or Greater		1.1		0	[1]
Fair Value, Total		216.8		91.9	[1]
Unrealized Losses, Total		2.7		1.7	[1]
Fixed Maturities [Member] | U.S. Governments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		313.7		183.4
Unrealized Losses, Less Than One Year		1.9		4.5
Fair Value, One Year or Greater		83.7		0
Unrealized Losses, One Year or Greater		3.1		0
Fair Value, Total		397.4		183.4
Unrealized Losses, Total		5		4.5
Fixed Maturities [Member] | Foreign Governments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		175.2		201.2
Unrealized Losses, Less Than One Year		2		8
Fair Value, One Year or Greater		35.9		0
Unrealized Losses, One Year or Greater		0.5		0
Fair Value, Total		211.1		201.2
Unrealized Losses, Total		2.5		8
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		33.3		72.6
Unrealized Losses, Less Than One Year		0.5		1.7
Fair Value, One Year or Greater		22.4		1.8
Unrealized Losses, One Year or Greater		0.6		0.1
Fair Value, Total		55.7		74.4
Unrealized Losses, Total		1.1		1.8
Fixed Maturities [Member] | Corporate Bonds [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		674.1		490.5
Unrealized Losses, Less Than One Year		9.9		17.7
Fair Value, One Year or Greater		77.7		50.6
Unrealized Losses, One Year or Greater		3.3		1.8
Fair Value, Total		751.8		541.1
Unrealized Losses, Total		13.2		19.5
Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		58.2		70.6
Unrealized Losses, Less Than One Year		0.4		1.5
Fair Value, One Year or Greater		37.8		7.1
Unrealized Losses, One Year or Greater		1.1		0.1
Fair Value, Total		96		77.7
Unrealized Losses, Total		1.5		1.6
Fixed Maturities [Member] | Asset-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		85.4		69.7
Unrealized Losses, Less Than One Year		0.4		1.4
Fair Value, One Year or Greater		31.9		8.2
Unrealized Losses, One Year or Greater		0.4		0.7
Fair Value, Total		117.3		77.9
Unrealized Losses, Total		0.8		2.1
Equity Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		46.9		62.1
Unrealized Losses, Less Than One Year		4.8		5.7
Fair Value, One Year or Greater		0		0
Unrealized Losses, One Year or Greater		0		0
Fair Value, Total		46.9		62.1
Unrealized Losses, Total		4.8		5.7
Other Investments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		0		0.3
Unrealized Losses, Less Than One Year		0		0.1
Fair Value, One Year or Greater		0.3		0
Unrealized Losses, One Year or Greater		0.1		0
Fair Value, Total		0.3		0.3
Unrealized Losses, Total		$ 0.1		0.1
Short-Term Investments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	[2]			4.8
Unrealized Losses, Less Than One Year	[2]			0
Fair Value, One Year or Greater	[2]			0
Unrealized Losses, One Year or Greater	[2]			0
Fair Value, Total	[2]			4.8
Unrealized Losses, Total	[2]			$ 0
[1]	Unrealized losses one year or greater are less than $0.1 million.
[2]	Unrealized losses less than one year are less than $0.1 million.

Investments - Schedule of Agi74

Investments - Schedule of Aging of Unrealized Losses on Company's Investments in Fixed Maturities, Equity Securities and Other Investments (Parenthetical) (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Investments Debt And Equity Securities [Abstract]
Unrealized losses less than one year	$ 0.1	$ 0.1
Unrealized losses one year or greater	$ 0.1	$ 0.1

Investments - Schedule of Recog

Investments - Schedule of Recognized Other-than-temporary Losses on Fixed Maturities and Equity Portfolios (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other-than-temporary impairment:
Other-than-temporary impairment losses	$ (2.5)	$ (10.2)	$ (11.9)
Obligations of States and Political Subdivisions [Member]
Other-than-temporary impairment:
Other-than-temporary impairment losses	(0.1)	0	0
Corporate Bonds [Member]
Other-than-temporary impairment:
Other-than-temporary impairment losses	(0.7)	(1.7)	(2.2)
Equity Securities [Member]
Other-than-temporary impairment:
Other-than-temporary impairment losses	$ (1.7)	$ (8.5)	$ (9.7)

Investments - Schedule of Inves

Investments - Schedule of Investment Income and Expenses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amortized Cost And Fair Value Debt Securities [Abstract]
Interest on fixed maturities	$ 97.1	$ 88.9	$ 78.2
Dividends on equity securities	13.9	15.6	16.3
Income on alternative investments	49.5	29.9	10
Income on short-term and other investments	7.7	0.4	0.2
Investment income	168.2	134.8	104.7
Investment expenses	(28.2)	(19.7)	(16.1)
Net investment income	$ 140	$ 115.1	$ 88.6

Investments - Schedule of Compa

Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Investments [Line Items]
Gross realized investment and other gains	$ 103.4	$ 128.2	$ 91.9
Gross realized investment and other losses	(64.1)	(102.1)	(67.8)
Net realized investment and other gains	39.3	26.1	24.1
Income tax expense	(12)	(11.6)	(10)
Net realized investment and other gains, net of income taxes	27.3	14.5	14.1
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Gross realized investment and other gains	25.7	21.5	12.3
Gross realized investment and other losses	(20)	(35.9)	(23.5)
Equity Securities [Member]
Schedule of Investments [Line Items]
Gross realized investment and other gains	51.3	57.5	40.5
Gross realized investment and other losses	(5.2)	(9.7)	(6.6)
Other Investments [Member]
Schedule of Investments [Line Items]
Gross realized investment and other gains	25.7	47.5	37.6
Gross realized investment and other losses	(36.2)	(46)	(24)
Short-Term Investments [Member]
Schedule of Investments [Line Items]
Gross realized investment and other gains	0.7	0.5	1.2
Gross realized investment and other losses	(0.2)	(0.3)	(1.8)
Other Assets [Member]
Schedule of Investments [Line Items]
Gross realized investment and other gains	0	1.2	0
Real Estate Holdings [Member]
Schedule of Investments [Line Items]
Gross realized investment and other gains	0	0	0.3
Other-Than-Temporary Impairment Losses on Fixed Maturities [Member]
Schedule of Investments [Line Items]
Gross realized investment and other losses	(0.8)	(1.7)	(2.2)
Other-Than-Temporary Impairment Losses on Equity Securities [Member]
Schedule of Investments [Line Items]
Gross realized investment and other losses	$ (1.7)	$ (8.5)	$ (9.7)

Investments - Schedule of Chang

Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Investments [Line Items]
Net unrealized investment and other gains (losses) before income taxes	$ 64.1	$ 34.9	$ (128.3)
Income tax (expense) benefit	(14.6)	(2.4)	37.6
Net unrealized investment and other gains (losses), net of income taxes	49.5	32.5	(90.7)
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Net unrealized investment and other gains (losses) before income taxes	25.4	36.5	(65.3)
Equity Securities [Member]
Schedule of Investments [Line Items]
Net unrealized investment and other gains (losses) before income taxes	36.6	(2.6)	(64.1)
Other Investments [Member]
Schedule of Investments [Line Items]
Net unrealized investment and other gains (losses) before income taxes	2.1	0.6	1.5
Short-Term Investments [Member]
Schedule of Investments [Line Items]
Net unrealized investment and other gains (losses) before income taxes	$ 0	$ 0.4	$ (0.4)

Investments - Schedule of Fair

Investments - Schedule of Fair Value of Foreign Currency Exchange Forward Contracts (Detail) - Foreign Currency Exchange Forward Contracts [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Fair value of foreign currency exchange forward contracts, Liability	$ (0.4)
Fair value of foreign currency exchange forward contracts, Asset		$ 4
Operational Currency Exposure [Member]
Schedule of Investments [Line Items]
Fair value of foreign currency exchange forward contracts, Liability	(0.2)	0
Asset Manager Investment Exposure [Member]
Schedule of Investments [Line Items]
Fair value of foreign currency exchange forward contracts, Liability	(0.9)
Fair value of foreign currency exchange forward contracts, Asset		0.7
Total return strategy [Member]
Schedule of Investments [Line Items]
Fair value of foreign currency exchange forward contracts, Asset	$ 0.7	$ 3.3

Investments - Schedule of Reali

Investments - Schedule of Realized Gains and Losses of Investment on Foreign Currency Exchange Forward Contracts (Detail) - Foreign Currency Exchange Forward Contracts [Member] - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Investments [Line Items]
Realized gains from foreign currency exchange forward contracts		$ 24.3	$ 45.5	$ 36.5
Realized losses from foreign currency exchange forward contracts		(32.7)	(43.5)	(21.3)
Net realized investment (losses) gains on foreign currency exchange forward contracts		(8.4)	2	15.2
Global Catastrophe [Member]
Schedule of Investments [Line Items]
Realized gains from foreign currency exchange forward contracts	[1]	0	0	0.5
Realized losses from foreign currency exchange forward contracts		0	0	(2.3)
Operational Currency Exposure [Member]
Schedule of Investments [Line Items]
Realized gains from foreign currency exchange forward contracts		12.4	10.9	26.1
Realized losses from foreign currency exchange forward contracts		(13.8)	(18)	(12.9)
Asset Manager Investment Exposure [Member]
Schedule of Investments [Line Items]
Realized gains from foreign currency exchange forward contracts		1.5	9	8.5
Realized losses from foreign currency exchange forward contracts		(11.3)	(4.5)	(2.3)
Total return strategy [Member]
Schedule of Investments [Line Items]
Realized gains from foreign currency exchange forward contracts		10.4	25.6	1.4
Realized losses from foreign currency exchange forward contracts		$ (7.6)	$ (21)	$ (3.8)
[1]	Global Catastrophe program ended 2015

Investments - Components of Res

Investments - Components of Restricted Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Securities and cash on deposit supporting Lloyd’s business	$ 355.5	$ 161.8
Total restricted investments	608.9	366.4
Securities Deposits For Regulatory and Other Purposes [Member]
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Securities on deposit for regulatory and other purposes	175.3	168.7
Securities Pledged as Collateral [Member]
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Securities pledged as collateral for letters of credit	$ 78.1	$ 35.9

Investments - Financial Assets

Investments - Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		$ 4,742.9	$ 4,320.3
Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		543.6	535
Short-Term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		368.5	405.5
Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		4,308.1	3,876.8
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[1]	1,227.3	1,048
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[2]	3,076.6	2,826.4
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[3]	4.2	2.4
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		3,343.4	2,932.4
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		228	264.3
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Residential Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		309.4	176.8
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[1]	410.6	228
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[1]	0	0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Residential Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[1]	0	0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[2]	2,930.9	2,702.4
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[2]	228	264.3
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[2]	309.4	176.8
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[3]	1.9	2
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[3]	0	0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[3]	0	0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | U.S. Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		415.1	271.2
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | U.S. Governments [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[1]	410.6	228
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | U.S. Governments [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[2]	4.5	43.2
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | U.S. Governments [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[3]	0	0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Foreign Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		233.2	237.3
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Foreign Governments [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[1]	0	0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Foreign Governments [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[2]	233.2	237.3
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Foreign Governments [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[3]	0	0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		335.9	382.8
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[1]	0	0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[2]	335.9	382.8
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[3]	0	0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Corporate Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		1,525.7	1,320.7
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Corporate Bonds [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[1]	0	0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Corporate Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[2]	1,523.8	1,318.7
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Corporate Bonds [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[3]	1.9	2
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		134.9	153.7
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[1]	0	0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[2]	134.9	153.7
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[3]	0	0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Asset-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		161.2	125.6
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Asset-backed Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[1]	0	0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Asset-backed Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[2]	161.2	125.6
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Asset-backed Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[3]	0	0
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		487.4	447.4
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[1]	483	444.9
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[2]	2.1	2.1
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[3]	2.3	0.4
Fair Value, Measurements, Recurring [Member] | Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		108.8	91.5
Fair Value, Measurements, Recurring [Member] | Other Investments [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[1]	0	0
Fair Value, Measurements, Recurring [Member] | Other Investments [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[2]	108.8	91.5
Fair Value, Measurements, Recurring [Member] | Other Investments [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[3]	0	0
Fair Value, Measurements, Recurring [Member] | Short-Term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		368.5	405.5
Fair Value, Measurements, Recurring [Member] | Short-Term Investments [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[1]	333.7	375.1
Fair Value, Measurements, Recurring [Member] | Short-Term Investments [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[2]	34.8	30.4
Fair Value, Measurements, Recurring [Member] | Short-Term Investments [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[3]	$ 0	$ 0
[1]	Quoted prices in active markets for identical assets
[2]	Significant other observable inputs
[3]	Significant unobservable inputs

Investments - Schedule of Recon

Investments - Schedule of Reconciliation of Beginning and Ending Balances for Investments Categorized as Level 3 (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 2.4	$ 0.7
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Included in net income (loss)	0	0
Included in other comprehensive income (loss)	0.1	0
Purchases	1.7	2
Issuances	0	0
Sales	0	(0.3)
Settlements	0	0
Ending balance	4.2	2.4
Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at end of period	0	0
Credit-Financial [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	2	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Included in net income (loss)	0	0
Included in other comprehensive income (loss)	(0.1)	0
Purchases	0	2
Issuances	0	0
Sales	0	0
Settlements	0	0
Ending balance	1.9	2
Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at end of period	0	0
Equity Securities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	0.4	0.7
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Included in net income (loss)	0	0
Included in other comprehensive income (loss)	0.2	0
Purchases	1.7	0
Issuances	0	0
Sales	0	(0.3)
Settlements	0	0
Ending balance	2.3	0.4
Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at end of period	$ 0	$ 0

Reinsurance - Additional Inform

Reinsurance - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Disclosures [Abstract]
Reinsurance recoverables, allowance for doubtful accounts	$ 2.1	$ 2.1
Amount of collateral under reinsurance agreement	907.7	492.6
Paid loss recoverables in insurance recoverables	380.3	215
Losses and loss adjustment expenses	1,050.2	810.1	$ 766.1
Net of amounts ceded to reinsurers	$ 992.6	$ 419.3	$ 284.6

Reinsurance - Schedule of Reins

Reinsurance - Schedule of Reinsurance Premium (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Premiums Written And Earned [Abstract]
Direct written premiums	$ 2,029.2	$ 1,792.5	$ 1,651.4
Reinsurance ceded to other companies	(1,043.7)	(724.6)	(610)
Reinsurance assumed from other companies	668	372.3	360.7
Net written premiums	1,653.5	1,440.2	1,402.1
Direct earned premiums	1,912.2	1,722.8	1,602.2
Reinsurance ceded to other companies	(1,033.6)	(675.8)	(563.7)
Reinsurance assumed from other companies	693.7	363.8	333.4
Net earned premiums	$ 1,572.3	$ 1,410.8	$ 1,371.9
Percentage of reinsurance assumed to net earned premiums	44.10%	25.80%	24.30%

Reserves for Losses and Loss 86

Reserves for Losses and Loss Adjustment Expenses - Reserves for Losses and Loss Adjustment Expenses (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Abstract]
Net reserves beginning of the year		$ 2,180.2	$ 2,133.3	$ 2,137.1
Net Maybrooke reserves acquired		131.8	0	0
Losses and LAE incurred during current calendar year, net of reinsurance:
Current accident year		1,058.4	843.4	798.5
Prior accident years		(8.2)	(33.3)	(32.4)
Losses and LAE incurred during calendar year, net of reinsurance		1,050.2	810.1	766.1
Current accident year		289.6	178.9	169
Prior accident years		599.8	537.6	564.5
Losses and LAE payments made during current calendar year, net of reinsurance:		889.4	716.5	733.5
Change in participation interest	[1]	(23.2)	(36.3)	(1.2)
Foreign exchange adjustments		38.4	(10.4)	(35.2)
Net reserves - end of year		2,488	2,180.2	2,133.3
Reinsurance recoverables on unpaid losses and LAE, end of year		(1,713)	(1,170.6)	(990.3)
Reserves for losses and loss adjustment expenses		$ 4,201	$ 3,350.8	$ 3,123.6
[1]	Amount represents (decrease) increase in reserves due to change in our Syndicate 1200 participation.

Reserves for Losses and Loss 87

Reserves for Losses and Loss Adjustment Expenses - Impact from (Favorable) Unfavorable Development of Prior Accident Years' Loss and LAE Reserves on Each Reporting Segment (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	$ (8.2)	$ (33.3)	$ (32.4)
U.S. Operations [Member]
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	(38.7)	(35.9)	(23)
International Operations [Member]
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	13.2	(16)	(18)
Run Off Lines [Member]
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	$ 17.3	$ 18.6	$ 8.6

Reserves for Losses and Loss 88

Reserves for Losses and Loss Adjustment Expenses - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017Segment
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Claims remain outstanding period	10 years
Number of reporting segments	2
Run Off Lines [Member]
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Percentage of gross reserves for losses	7.00%

Reserves for Losses and Loss 89

Reserves for Losses and Loss Adjustment Expenses - Summary of Information about Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance, Excess and Surplus Lines, Liability (Detail) $ in Millions		Dec. 31, 2017USD ($)Claim	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	[1]	Dec. 31, 2014USD ($)	[1]	Dec. 31, 2013USD ($)	[1]	Dec. 31, 2012USD ($)	[1]	Dec. 31, 2011USD ($)	[1]
Claims Development [Line Items]
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance		$ 2,434.6
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		1,543.5
Cumulative Paid Losses & ALAE, Net of Reinsurance		778.5
Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance		70.1
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance		835.1
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Accident Year 2011 [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		189	$ 192.8	$ 193.5		$ 200		$ 205.8		$ 206		$ 202.9
Cumulative Paid Losses & ALAE, Net of Reinsurance		170	162.4	146.6		122.9		91		53.8		$ 17.6
IBNR & Expected Development on Reported Claims		$ 12.4
Cumulative Number of Reported Claims | Claim	[2]	8,404
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Accident Year 2012 [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 182.1	184.4	183.6		189.7		196		189.6
Cumulative Paid Losses & ALAE, Net of Reinsurance		157.5	142.4	120.8		89.1		52.8		$ 17.2
IBNR & Expected Development on Reported Claims		$ 15.1
Cumulative Number of Reported Claims | Claim	[2]	7,322
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Accident Year 2013 [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 220.4	227.2	224.3		222.6		217.9
Cumulative Paid Losses & ALAE, Net of Reinsurance		163.7	135.2	100.4		60.2		$ 17.6
IBNR & Expected Development on Reported Claims		$ 34.9
Cumulative Number of Reported Claims | Claim	[2]	7,180
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Accident Year 2014 [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 211.9	213.1	215.1		213
Cumulative Paid Losses & ALAE, Net of Reinsurance		131.6	95.9	52.2		$ 15
IBNR & Expected Development on Reported Claims		$ 48
Cumulative Number of Reported Claims | Claim	[2]	6,306
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Accident Year 2015 [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 228.6	237	232.3
Cumulative Paid Losses & ALAE, Net of Reinsurance		91.4	51.9	$ 16.5
IBNR & Expected Development on Reported Claims		$ 92.3
Cumulative Number of Reported Claims | Claim	[2]	5,661
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Accident Year 2016 [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 250.9	246.5
Cumulative Paid Losses & ALAE, Net of Reinsurance		52.8	$ 17.4
IBNR & Expected Development on Reported Claims		$ 151.4
Cumulative Number of Reported Claims | Claim	[2]	4,969
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Accident Year 2017 [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 260.6
Cumulative Paid Losses & ALAE, Net of Reinsurance		11.5
IBNR & Expected Development on Reported Claims		$ 218.4
Cumulative Number of Reported Claims | Claim	[2]	3,801
[1]	Information presented for calendar years prior to 2017 is required supplementary information and is unaudited.
[2]	The cumulative number of reported claims is measured at a coverage level by occurrence. Reported occurrences that do not result in a liability are included as reported claims.

Reserves for Losses and Loss 90

Reserves for Losses and Loss Adjustment Expenses - Summary of Information about Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance, Commercial Specialty, Liability (Detail) $ in Millions		Dec. 31, 2017USD ($)Claim	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	[1]	Dec. 31, 2014USD ($)	[1]	Dec. 31, 2013USD ($)	[1]	Dec. 31, 2012USD ($)	[1]	Dec. 31, 2011USD ($)	[1]
Claims Development [Line Items]
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance		$ 2,434.6
U.S. Operations [Member] | Commercial Specialty [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		862.2
Cumulative Paid Losses & ALAE, Net of Reinsurance		542.5
Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance		55.3
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance		375
U.S. Operations [Member] | Commercial Specialty [Member] | Liability [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		153.7	$ 154	$ 158.2		$ 157.5		$ 159		$ 155.1		$ 140.6
Cumulative Paid Losses & ALAE, Net of Reinsurance		139.8	135.1	126.1		111.3		85.9		57.5		$ 23.1
IBNR & Expected Development on Reported Claims		$ 6.8
Cumulative Number of Reported Claims | Claim	[2]	28,153
U.S. Operations [Member] | Commercial Specialty [Member] | Liability [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 147.7	151.5	152.2		149.7		146.3		140.3
Cumulative Paid Losses & ALAE, Net of Reinsurance		127.9	120.8	105.3		80.7		51		$ 20.1
IBNR & Expected Development on Reported Claims		$ 9.6
Cumulative Number of Reported Claims | Claim	[2]	23,594
U.S. Operations [Member] | Commercial Specialty [Member] | Liability [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 131.1	133.2	135.1		133.2		126.6
Cumulative Paid Losses & ALAE, Net of Reinsurance		102.8	93.6	73.6		49.4		$ 18.9
IBNR & Expected Development on Reported Claims		$ 13.4
Cumulative Number of Reported Claims | Claim	[2]	18,901
U.S. Operations [Member] | Commercial Specialty [Member] | Liability [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 114.5	116.9	118.6		115.5
Cumulative Paid Losses & ALAE, Net of Reinsurance		75.2	58.7	38.4		$ 17.4
IBNR & Expected Development on Reported Claims		$ 17.3
Cumulative Number of Reported Claims | Claim	[2]	16,261
U.S. Operations [Member] | Commercial Specialty [Member] | Liability [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 101.6	106.7	102.6
Cumulative Paid Losses & ALAE, Net of Reinsurance		48.8	35	$ 16.4
IBNR & Expected Development on Reported Claims		$ 27.4
Cumulative Number of Reported Claims | Claim	[2]	14,379
U.S. Operations [Member] | Commercial Specialty [Member] | Liability [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 99.4	95.9
Cumulative Paid Losses & ALAE, Net of Reinsurance		31.7	$ 11.1
IBNR & Expected Development on Reported Claims		$ 38.1
Cumulative Number of Reported Claims | Claim	[2]	11,225
U.S. Operations [Member] | Commercial Specialty [Member] | Liability [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 114.2
Cumulative Paid Losses & ALAE, Net of Reinsurance		16.3
IBNR & Expected Development on Reported Claims		$ 64.8
Cumulative Number of Reported Claims | Claim	[2]	10,897
[1]	Information presented for calendar years prior to 2017 is required supplementary information and is unaudited.
[2]	The cumulative number of reported claims is measured by individual claimant at a coverage level. Reported occurrences that do not result in a liability are included as reported claims.

Reserves for Losses and Loss 91

Reserves for Losses and Loss Adjustment Expenses - Summary of Information about Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance, Commercial Specialty, Professional (Detail) $ in Millions		Dec. 31, 2017USD ($)Claim	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	[1]	Dec. 31, 2014USD ($)	[1]	Dec. 31, 2013USD ($)	[1]	Dec. 31, 2012USD ($)	[1]	Dec. 31, 2011USD ($)	[1]
Claims Development [Line Items]
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance		$ 2,434.6
U.S. Operations [Member] | Commercial Specialty [Member] | Professional [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		242.7
Cumulative Paid Losses & ALAE, Net of Reinsurance		121
Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance		0.2
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance		121.9
U.S. Operations [Member] | Commercial Specialty [Member] | Accident Year 2011 [Member] | Professional [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		26.6	$ 26.9	$ 28.2		$ 32.5		$ 35		$ 35		$ 35
Cumulative Paid Losses & ALAE, Net of Reinsurance		25.7	25.4	24		22		17.8		11.8		$ 3.2
IBNR & Expected Development on Reported Claims		$ 0.3
Cumulative Number of Reported Claims | Claim	[2]	818
U.S. Operations [Member] | Commercial Specialty [Member] | Accident Year 2012 [Member] | Professional [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 24.5	24	25.8		28.6		28.3		27.8
Cumulative Paid Losses & ALAE, Net of Reinsurance		22.6	21.4	19.9		16.9		8.6		$ 2.3
IBNR & Expected Development on Reported Claims		$ 0.2
Cumulative Number of Reported Claims | Claim	[2]	637
U.S. Operations [Member] | Commercial Specialty [Member] | Accident Year 2013 [Member] | Professional [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 19.8	19	21.1		21.5		20.9
Cumulative Paid Losses & ALAE, Net of Reinsurance		17.6	14.2	10.9		6.3		$ 1.9
IBNR & Expected Development on Reported Claims		$ 1.1
Cumulative Number of Reported Claims | Claim	[2]	616
U.S. Operations [Member] | Commercial Specialty [Member] | Accident Year 2014 [Member] | Professional [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 33.7	26	22.4		22.4
Cumulative Paid Losses & ALAE, Net of Reinsurance		24.1	15.1	5.4		$ 2.3
IBNR & Expected Development on Reported Claims		$ 2.1
Cumulative Number of Reported Claims | Claim	[2]	1,041
U.S. Operations [Member] | Commercial Specialty [Member] | Accident Year 2015 [Member] | Professional [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 33.2	29.5	29.9
Cumulative Paid Losses & ALAE, Net of Reinsurance		15.6	8.3	$ 1.8
IBNR & Expected Development on Reported Claims		$ 6.5
Cumulative Number of Reported Claims | Claim	[2]	1,802
U.S. Operations [Member] | Commercial Specialty [Member] | Accident Year 2016 [Member] | Professional [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 44.8	44.2
Cumulative Paid Losses & ALAE, Net of Reinsurance		11.9	$ 2.4
IBNR & Expected Development on Reported Claims		$ 19.8
Cumulative Number of Reported Claims | Claim	[2]	3,190
U.S. Operations [Member] | Commercial Specialty [Member] | Accident Year 2017 [Member] | Professional [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 60.1
Cumulative Paid Losses & ALAE, Net of Reinsurance		3.5
IBNR & Expected Development on Reported Claims		$ 50.4
Cumulative Number of Reported Claims | Claim	[2]	3,272
[1]	Information presented for calendar years prior to 2017 is required supplementary information and is unaudited.
[2]	The cumulative number of reported claims is measured by individual claimant at a coverage level. Reported occurrences that do not result in a liability are included as reported claims.

Reserves for Losses and Loss 92

Reserves for Losses and Loss Adjustment Expenses - Summary of Information about Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance, International Specialty, Liability (Detail) $ in Millions		Dec. 31, 2017USD ($)Claim	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	[1]	Dec. 31, 2014USD ($)	[1]	Dec. 31, 2013USD ($)	[1]	Dec. 31, 2012USD ($)	[1]	Dec. 31, 2011USD ($)	[1]
Claims Development [Line Items]
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance		$ 2,434.6
International Operations [Member] | International Specialty [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		75.5
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance		53.5
Cumulative Paid Losses & ALAE, Net of Reinsurance		22.4
Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance		0.4
International Operations [Member] | International Specialty [Member] | Accident Year 2011 [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		1	$ 1.6	$ 2.2		$ 4.4		$ 6.6		$ 6.6		$ 6.6
Cumulative Paid Losses & ALAE, Net of Reinsurance			0	0		0		0		0		$ 0
IBNR & Expected Development on Reported Claims		$ 1
Cumulative Number of Reported Claims | Claim	[2]	1,422
International Operations [Member] | International Specialty [Member] | Accident Year 2012 [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 1.7	4.5	5.6		7.4		7.4		7.4
Cumulative Paid Losses & ALAE, Net of Reinsurance		0.1	0.1	0.1		0		0		$ 0
IBNR & Expected Development on Reported Claims		$ 1.6
Cumulative Number of Reported Claims | Claim	[2]	1,403
International Operations [Member] | International Specialty [Member] | Accident Year 2013 [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 6.2	6.5	9.5		9.5		9.1
Cumulative Paid Losses & ALAE, Net of Reinsurance		3.4	0.9	0.8		0.7		$ 0.4
IBNR & Expected Development on Reported Claims		$ 2.7
Cumulative Number of Reported Claims | Claim	[2]	1,306
International Operations [Member] | International Specialty [Member] | Accident Year 2014 [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 7.2	10.7	10.9		10.7
Cumulative Paid Losses & ALAE, Net of Reinsurance		1	0.8	0.7		$ 0.2
IBNR & Expected Development on Reported Claims		$ 3.1
Cumulative Number of Reported Claims | Claim	[2]	1,465
International Operations [Member] | International Specialty [Member] | Accident Year 2015 [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 25.7	15.3	12.6
Cumulative Paid Losses & ALAE, Net of Reinsurance		16.8	0.6	$ 0.3
IBNR & Expected Development on Reported Claims		$ 0.7
Cumulative Number of Reported Claims | Claim	[2]	1,720
International Operations [Member] | International Specialty [Member] | Accident Year 2016 [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 15.3	15.2
Cumulative Paid Losses & ALAE, Net of Reinsurance		0.9	$ 0.4
IBNR & Expected Development on Reported Claims		$ 14.1
Cumulative Number of Reported Claims | Claim	[2]	2,148
International Operations [Member] | International Specialty [Member] | Accident Year 2017 [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 18.4
Cumulative Paid Losses & ALAE, Net of Reinsurance		0.2
IBNR & Expected Development on Reported Claims		$ 18
Cumulative Number of Reported Claims | Claim	[2]	1,927
[1]	Information presented for calendar years prior to 2017 is required supplementary information and is unaudited.
[2]	The cumulative number of reported claims is measured by individual claimant at a coverage level. Reported occurrences that do not result in a liability are included as reported claims.

Reserves for Losses and Loss 93

Reserves for Losses and Loss Adjustment Expenses - Summary of Information about Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance, International Specialty, Property (Detail) $ in Millions		Dec. 31, 2017USD ($)Claim	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	[1]	Dec. 31, 2014USD ($)	[1]	Dec. 31, 2013USD ($)	[1]	Dec. 31, 2012USD ($)	[1]	Dec. 31, 2011USD ($)	[1]
Claims Development [Line Items]
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance		$ 2,434.6
International Operations [Member] | Property [Member]
Claims Development [Line Items]
IBNR & Expected Development on Reported Claims		$ 30.4
Cumulative Number of Reported Claims | Claim		91
International Operations [Member] | Accident Year 2017 [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 108.3
Cumulative Paid Losses & ALAE, Net of Reinsurance		72.5
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance		69.3
IBNR & Expected Development on Reported Claims		33.5
International Operations [Member] | International Specialty [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		332
Cumulative Paid Losses & ALAE, Net of Reinsurance		253.9
Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance		4
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance		82.1
International Operations [Member] | International Specialty [Member] | Accident Year 2011 [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		107.1	$ 106.8	$ 106.9		$ 107.6		$ 109.6		$ 107.4		$ 118
Cumulative Paid Losses & ALAE, Net of Reinsurance		102.3	101.3	100		97.6		89.3		67.5		$ 41.1
IBNR & Expected Development on Reported Claims		$ 0
Cumulative Number of Reported Claims | Claim	[2]	400
International Operations [Member] | International Specialty [Member] | Accident Year 2012 [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 46.6	46.7	51.8		50.6		51.7		47.4
Cumulative Paid Losses & ALAE, Net of Reinsurance		44.5	44.2	49.8		40.8		31.3		$ 12.4
IBNR & Expected Development on Reported Claims		$ 0.8
Cumulative Number of Reported Claims | Claim	[2]	322
International Operations [Member] | International Specialty [Member] | Accident Year 2013 [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 34.6	35.1	39.2		39.8		35.5
Cumulative Paid Losses & ALAE, Net of Reinsurance		33.2	32.5	32.2		21.7		$ 5.2
IBNR & Expected Development on Reported Claims		$ 0.5
Cumulative Number of Reported Claims | Claim	[2]	357
International Operations [Member] | International Specialty [Member] | Accident Year 2014 [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 25.6	26.5	28.5		28.3
Cumulative Paid Losses & ALAE, Net of Reinsurance		23.5	20.6	14		$ 3.3
IBNR & Expected Development on Reported Claims		$ 1.5
Cumulative Number of Reported Claims | Claim	[2]	355
International Operations [Member] | International Specialty [Member] | Accident Year 2015 [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 23.5	24.7	28.4
Cumulative Paid Losses & ALAE, Net of Reinsurance		15.5	11.9	$ 4.6
IBNR & Expected Development on Reported Claims		$ 3.7
Cumulative Number of Reported Claims | Claim	[2]	420
International Operations [Member] | International Specialty [Member] | Accident Year 2016 [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 40.1	46.2
Cumulative Paid Losses & ALAE, Net of Reinsurance		22.8	$ 14.7
IBNR & Expected Development on Reported Claims		$ 8.7
Cumulative Number of Reported Claims | Claim	[2]	647
International Operations [Member] | International Specialty [Member] | Accident Year 2017 [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 54.5
Cumulative Paid Losses & ALAE, Net of Reinsurance		12.1
IBNR & Expected Development on Reported Claims		$ 4.2
Cumulative Number of Reported Claims | Claim	[2]	786
[1]	Information presented for calendar years prior to 2017 is required supplementary information and is unaudited.
[2]	The cumulative number of reported claims is measured by individual claimant at a coverage level. Reported occurrences that do not result in a liability are included as reported claims.

Reserves for Losses and Loss 94

Reserves for Losses and Loss Adjustment Expenses - Summary of Information about Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance, Syndicate 1200, Liability (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	[1]	Dec. 31, 2014	[1]	Dec. 31, 2013	[1]	Dec. 31, 2012	[1]	Dec. 31, 2011	[1]	Dec. 31, 2010	[1]
Claims Development [Line Items]
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	$ 2,434.6
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member]
Claims Development [Line Items]
Cumulative Paid Losses & ALAE, Net of Reinsurance	68.4
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	108.6
Incurred Losses & ALAE, Net of Reinsurance	177
International Operations [Member] | Syndicate 1200 [Member] | Accident Year 2010 [Member] | Liability [Member]
Claims Development [Line Items]
Cumulative Paid Losses & ALAE, Net of Reinsurance	4.2	$ 3.9	$ 2.9		$ 2.1		$ 1.1		$ 0.6		$ 0.2		$ 0
Incurred Losses & ALAE, Net of Reinsurance	6.1	6	6.1		6.2		6.4		4.7		5		$ 5.9
IBNR & Expected Development on Reported Claims	0.1
International Operations [Member] | Syndicate 1200 [Member] | Accident Year 2011 [Member] | Liability [Member]
Claims Development [Line Items]
Cumulative Paid Losses & ALAE, Net of Reinsurance	8.2	7.3	5.7		3.5		1.7		0.8		0.3
Incurred Losses & ALAE, Net of Reinsurance	11.2	10.6	10.8		11.5		11.4		9		$ 8.5
IBNR & Expected Development on Reported Claims	0.6
International Operations [Member] | Syndicate 1200 [Member] | Accident Year 2012 [Member] | Liability [Member]
Claims Development [Line Items]
Cumulative Paid Losses & ALAE, Net of Reinsurance	10.5	8.6	6		2.7		1.2		0.4
Incurred Losses & ALAE, Net of Reinsurance	15.2	14.4	14.9		15.5		11.3		$ 9.1
IBNR & Expected Development on Reported Claims	1.5
International Operations [Member] | Syndicate 1200 [Member] | Accident Year 2013 [Member] | Liability [Member]
Claims Development [Line Items]
Cumulative Paid Losses & ALAE, Net of Reinsurance	16.2	11.8	7.2		3.4		1.6
Incurred Losses & ALAE, Net of Reinsurance	25.7	25.6	27.4		27.8		$ 23.7
IBNR & Expected Development on Reported Claims	3.9
International Operations [Member] | Syndicate 1200 [Member] | Accident Year 2014 [Member] | Liability [Member]
Claims Development [Line Items]
Cumulative Paid Losses & ALAE, Net of Reinsurance	14.1	10.2	4.8		2
Incurred Losses & ALAE, Net of Reinsurance	34.6	35.2	38		$ 38.8
IBNR & Expected Development on Reported Claims	7.2
International Operations [Member] | Syndicate 1200 [Member] | Accident Year 2015 [Member] | Liability [Member]
Claims Development [Line Items]
Cumulative Paid Losses & ALAE, Net of Reinsurance	7.6	5.3	0.8
Incurred Losses & ALAE, Net of Reinsurance	31	30.9	$ 35.9
IBNR & Expected Development on Reported Claims	11.1
International Operations [Member] | Syndicate 1200 [Member] | Accident Year 2016 [Member] | Liability [Member]
Claims Development [Line Items]
Cumulative Paid Losses & ALAE, Net of Reinsurance	5.8	1.9
Incurred Losses & ALAE, Net of Reinsurance	27.9	$ 26.6
IBNR & Expected Development on Reported Claims	14.1
International Operations [Member] | Syndicate 1200 [Member] | Accident Year 2017 [Member] | Liability [Member]
Claims Development [Line Items]
Cumulative Paid Losses & ALAE, Net of Reinsurance	1.8
Incurred Losses & ALAE, Net of Reinsurance	25.3
IBNR & Expected Development on Reported Claims	$ 19.8
[1]	Information presented for calendar years prior to 2017 is required supplementary information and is unaudited.

Reserves for Losses and Loss 95

Reserves for Losses and Loss Adjustment Expenses - Summary of Information about Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance, Syndicate 1200, Professional (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	[1]	Dec. 31, 2014	[1]	Dec. 31, 2013	[1]	Dec. 31, 2012	[1]	Dec. 31, 2011	[1]	Dec. 31, 2010	[1]
Claims Development [Line Items]
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	$ 2,434.6
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	201
Cumulative Paid Losses & ALAE, Net of Reinsurance	91.6
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	109.4
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2010 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	9.8	$ 9.7	$ 9.7		$ 10.4		$ 11.6		$ 13.1		$ 13.2		$ 16.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	7	6.7	5.3		3.9		3.1		2		1		$ 0.1
IBNR & Expected Development on Reported Claims	0.2
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	15.8	15.3	15.3		16.1		19.2		21.8		19.9
Cumulative Paid Losses & ALAE, Net of Reinsurance	11.7	11	8.6		6.8		4.3		2.6		$ 1.1
IBNR & Expected Development on Reported Claims	0.9
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	15.7	14.6	14.6		14.7		14.4		14.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	9.9	8.5	6		4.5		1.9		$ 0.6
IBNR & Expected Development on Reported Claims	1.8
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	23.6	23.3	23.7		23.6		23.6
Cumulative Paid Losses & ALAE, Net of Reinsurance	16	12.1	7.4		3.9		$ 1.7
IBNR & Expected Development on Reported Claims	4.2
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	41.4	37.9	37.6		36.4
Cumulative Paid Losses & ALAE, Net of Reinsurance	24.7	15.3	6.7		$ 1.7
IBNR & Expected Development on Reported Claims	9
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	40.7	39.1	39.6
Cumulative Paid Losses & ALAE, Net of Reinsurance	15.3	8.6	$ 2.3
IBNR & Expected Development on Reported Claims	15.1
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	28.5	34.2
Cumulative Paid Losses & ALAE, Net of Reinsurance	5.9	2.1
IBNR & Expected Development on Reported Claims	14.8
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	25.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	1.1	$ 0
IBNR & Expected Development on Reported Claims	$ 20.1
[1]	Information presented for calendar years prior to 2017 is required supplementary information and is unaudited.

Reserves for Losses and Loss 96

Reserves for Losses and Loss Adjustment Expenses - Summary of Information about Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance, Syndicate 1200, Property (Detail) $ in Millions	Dec. 31, 2017USD ($)Claim	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	[1]	Dec. 31, 2014USD ($)	[1]	Dec. 31, 2013USD ($)	[1]	Dec. 31, 2012USD ($)	[1]	Dec. 31, 2011USD ($)	[1]	Dec. 31, 2010USD ($)	[1]
Claims Development [Line Items]
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	$ 2,434.6
International Operations [Member] | Property [Member]
Claims Development [Line Items]
IBNR & Expected Development on Reported Claims	$ 30.4
Cumulative Number of Reported Claims | Claim	91
International Operations [Member] | Accident Year 2017 [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 108.3
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	69.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	72.5
IBNR & Expected Development on Reported Claims	33.5
International Operations [Member] | Syndicate 1200 [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	623.2
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	143.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	479.5
International Operations [Member] | Syndicate 1200 [Member] | Accident Year 2010 [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	51.9	$ 51.3	$ 51.8		$ 52.1		$ 54.8		$ 55.3		$ 59		$ 51.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	43	42	40.5		36.2		34.2		28.4		19.8		$ 1.8
IBNR & Expected Development on Reported Claims	0
International Operations [Member] | Syndicate 1200 [Member] | Accident Year 2011 [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	93.6	93.7	94.3		96.2		109.1		115.1		109.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	83.3	81.9	80.3		74.4		62.6		47.8		$ 23.7
IBNR & Expected Development on Reported Claims	0
International Operations [Member] | Syndicate 1200 [Member] | Accident Year 2012 [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	91.3	91.6	92.6		93.5		89.3		89.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	77.9	77	74.6		63.5		48.2		$ 29.9
IBNR & Expected Development on Reported Claims	0
International Operations [Member] | Syndicate 1200 [Member] | Accident Year 2013 [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	76.9	77.3	78.8		79.8		84
Cumulative Paid Losses & ALAE, Net of Reinsurance	74.2	73.8	69.7		57.2		$ 44.8
IBNR & Expected Development on Reported Claims	0
International Operations [Member] | Syndicate 1200 [Member] | Accident Year 2014 [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	66.1	65.9	64.5		71.2
Cumulative Paid Losses & ALAE, Net of Reinsurance	59.5	57.8	51.8		$ 29.7
IBNR & Expected Development on Reported Claims	0
International Operations [Member] | Syndicate 1200 [Member] | Accident Year 2015 [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	73.2	66.3	54.9
Cumulative Paid Losses & ALAE, Net of Reinsurance	51.9	42.9	$ 23
IBNR & Expected Development on Reported Claims	3.7
International Operations [Member] | Syndicate 1200 [Member] | Accident Year 2016 [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	86.3	70.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	62.3	$ 39.1
IBNR & Expected Development on Reported Claims	8.6
International Operations [Member] | Syndicate 1200 [Member] | Accident Year 2017 [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	83.9
Cumulative Paid Losses & ALAE, Net of Reinsurance	27.4
IBNR & Expected Development on Reported Claims	$ 33.6
[1]	Information presented for calendar years prior to 2017 is required supplementary information and is unaudited.

Reserves for Losses and Loss 97

Reserves for Losses and Loss Adjustment Expenses - Summary of Information about Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance, Syndicate 1200, Specialty (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	[1]	Dec. 31, 2014	[1]	Dec. 31, 2013	[1]	Dec. 31, 2012	[1]	Dec. 31, 2011	[1]	Dec. 31, 2010	[1]
Claims Development [Line Items]
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	$ 2,434.6
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	549.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	400.1
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	149.2
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2010 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	12.2	$ 12.2	$ 12.3		$ 12.3		$ 13.3		$ 13.8		$ 16		$ 13.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	10.2	10.1	9.9		9.1		9.4		9		8.8		$ 1.9
IBNR & Expected Development on Reported Claims	0
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	33.6	33.8	33.8		34.6		39.4		40.7		39
Cumulative Paid Losses & ALAE, Net of Reinsurance	29.8	29.5	28.9		27.5		24		19.8		$ 11.7
IBNR & Expected Development on Reported Claims	0
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	59.4	59.7	49.9		61.4		57		53.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	50.3	49.3	46.3		39.4		27.9		$ 18.2
IBNR & Expected Development on Reported Claims	0
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	82.6	83.1	83.5		82.2		76.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	78.6	77	69.6		53.1		$ 31.4
IBNR & Expected Development on Reported Claims	0
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	101.9	101	99.6		93.2
Cumulative Paid Losses & ALAE, Net of Reinsurance	88.8	83.5	72.3		$ 38.2
IBNR & Expected Development on Reported Claims	2.6
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	94.4	88.7	90.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	65.7	54.8	$ 31.4
IBNR & Expected Development on Reported Claims	7.3
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	85.1	86.6
Cumulative Paid Losses & ALAE, Net of Reinsurance	57.9	$ 37.9
IBNR & Expected Development on Reported Claims	13
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	80.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	18.8
IBNR & Expected Development on Reported Claims	$ 41
[1]	Information presented for calendar years prior to 2017 is required supplementary information and is unaudited.

Reserves for Losses and Loss 98

Reserves for Losses and Loss Adjustment Expenses - Summary of Information about Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance, Ariel Re, Property Products (Detail) $ in Millions	Dec. 31, 2017USD ($)Claim
Claims Development [Line Items]
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	$ 2,434.6
International Operations [Member] | Property [Member]
Claims Development [Line Items]
IBNR & Expected Development on Reported Claims	$ 30.4
Cumulative Number of Reported Claims | Claim	91
International Operations [Member] | Property [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 108.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	72.5
IBNR & Expected Development on Reported Claims	33.5
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	$ 69.3

Reserves for Losses and Loss 99

Reserves for Losses and Loss Adjustment Expenses - Summary of Reconciliation of Net Incurred and Paid Development to Liability for Unpaid Losses and LAE in Consolidated Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	$ 2,434.6
Total reinsurance recoverables on unpaid losses and LAE	1,713
Unallocated loss adjustment expenses	71
Unamortized reserve discount	(17.6)	$ (19.4)	$ (21.5)
Gross liability for unpaid losses and LAE	4,201	3,350.8	3,123.6
Other Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	189.3
Total reinsurance recoverables on unpaid losses and LAE	231.7
Ariel Re [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	69.3
Total reinsurance recoverables on unpaid losses and LAE	280.8
Excess and Surplus Lines [Member] | Liability [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	835.1
Total reinsurance recoverables on unpaid losses and LAE	293.3
Commercial Specialty [Member] | Liability [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	375
Total reinsurance recoverables on unpaid losses and LAE	246.6
Unamortized reserve discount	(10.6)	(10.3)	(10.3)
Commercial Specialty [Member] | Professional [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	121.9
Total reinsurance recoverables on unpaid losses and LAE	70.2
International Specialty [Member] | Liability [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	53.5
Total reinsurance recoverables on unpaid losses and LAE	95.8
International Specialty [Member] | Property [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	82.1
Total reinsurance recoverables on unpaid losses and LAE	67.4
Syndicate 1200 [Member] | Liability [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	108.6
Total reinsurance recoverables on unpaid losses and LAE	77.1
Syndicate 1200 [Member] | Professional [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	109.4
Total reinsurance recoverables on unpaid losses and LAE	78.6
Syndicate 1200 [Member] | Property [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	143.7
Total reinsurance recoverables on unpaid losses and LAE	91.5
Syndicate 1200 [Member] | Specialty [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	149.2
Total reinsurance recoverables on unpaid losses and LAE	88.6
Run Off Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	197.5
Total reinsurance recoverables on unpaid losses and LAE	91.4
Unamortized reserve discount	$ (7)	$ (9.1)	$ (11.2)

Reserves for Losses and Loss100

Reserves for Losses and Loss Adjustment Expenses - Schedule of Supplementary Unaudited Information About Annual Percentage Payout of Incurred Losses and ALAE, Net of Reinsurance (Detail)	Dec. 31, 2017	[1]
Excess and Surplus Lines [Member] | Liability [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 1	8.30%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 2	18.30%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 3	19.30%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 4	16.40%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 5	12.10%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 6	8.00%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 7	5.40%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 8	0.00%
Commercial Specialty [Member] | Liability [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 1	15.00%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 2	21.80%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 3	18.50%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 4	16.00%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 5	9.10%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 6	6.10%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 7	4.00%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 8	0.00%
Commercial Specialty [Member] | Professional [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 1	8.20%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 2	22.50%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 3	27.30%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 4	18.60%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 5	9.40%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 6	5.00%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 7	2.90%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 8	0.00%
International Specialty [Member] | Liability [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 1	11.50%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 2	13.00%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 3	17.60%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 4	14.80%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 5	10.60%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 6	8.40%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 7	6.10%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 8	0.00%
International Specialty [Member] | Property [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 1	26.60%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 2	30.70%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 3	24.90%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 4	9.20%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 5	4.50%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 6	2.00%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 7	1.00%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 8	0.00%
Syndicate 1200 [Member] | Liability [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 1	5.00%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 2	10.30%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 3	13.70%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 4	17.00%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 5	17.90%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 6	10.60%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 7	7.30%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 8	5.10%
Syndicate 1200 [Member] | Professional [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 1	4.40%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 2	9.40%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 3	13.60%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 4	15.60%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 5	13.60%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 6	13.90%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 7	8.10%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 8	5.80%
Syndicate 1200 [Member] | Property [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 1	39.80%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 2	25.10%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 3	16.10%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 4	9.90%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 5	3.80%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 6	2.30%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 7	1.20%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 8	0.70%
Syndicate 1200 [Member] | Specialty [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 1	40.40%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 2	28.00%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 3	15.80%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 4	8.80%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 5	3.30%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 6	1.90%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 7	1.00%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance,Year 8	0.50%
[1]	The average annual percentage payout is calculated from a paid losses and ALAE development pattern based on an actuarial analysis of the paid losses and ALAE movements by accident year for each disaggregation category. The paid losses and ALAE development pattern provides the expected percentage of ultimate losses and ALAE to be paid in each year. The pattern considers all accident years included in the claims development tables.

Reserves for Losses and Loss101

Reserves for Losses and Loss Adjustment Expenses - Schedule of Information About Discounted Liabilities For Unpaid Losses and LAE (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shortduration Insurance Contracts Discounted Liabilities [Line Items]
Carrying Amount of Reserves for Losses & LAE		$ 302.2	$ 305.6	$ 315.3
Aggregate Amount of Discount		17.6	19.4	21.5
Interest Accretion	[1]	$ 4	$ 4	$ 4.2
Discount Rate		0.00%	0.00%	0.00%
Commercial Specialty [Member] | Liability [Member]
Shortduration Insurance Contracts Discounted Liabilities [Line Items]
Carrying Amount of Reserves for Losses & LAE		$ 126.7	$ 121.1	$ 121.6
Aggregate Amount of Discount		10.6	10.3	10.3
Interest Accretion	[1]	$ 1.9	$ 1.9	$ 1.1
Discount Rate		2.25%	2.25%	2.25%
Run Off Lines [Member]
Shortduration Insurance Contracts Discounted Liabilities [Line Items]
Carrying Amount of Reserves for Losses & LAE		$ 175.5	$ 184.5	$ 193.7
Aggregate Amount of Discount		7	9.1	11.2
Interest Accretion	[1]	$ 2.1	$ 2.1	$ 3.1
Discount Rate		3.50%	3.50%	3.50%
[1]	Interest accretion is recorded in the line item “Losses and loss adjustment expenses” in our Consolidated Statements of Income.

Run-Off Lines - Gross Reserves

Run-Off Lines - Gross Reserves for Run-Off Lines (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	$ 55.9	$ 48.4
Gross reserves - Run-off Lines	281.9	290.9
Reinsurance Assumed [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	29.3	28.4
Other [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	26.6	20
Risk Management [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Gross reserves - Run-off Lines	219.6	236.9
Run-Off Reinsurance Lines [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Gross reserves - Run-off Lines	1.8	1.9
Other Run-Off Lines [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Gross reserves - Run-off Lines	$ 4.6	$ 3.7

Run-Off Lines - Total Gross Res

Run-Off Lines - Total Gross Reserves for Asbestos Exposure (Detail) - Asbestos and Environmental Claims [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total direct written reserves	$ 21.4	$ 15.4	$ 10.7
Total asbestos reserves	48.6	41.9	42.8
Case Reserves [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total direct written reserves	2.1	2.8	2
ULAE [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total direct written reserves	0.5	0.5	0.5
IBNR [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total direct written reserves	18.8	12.1	8.2
Assumed domestic [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total assumed reserves	24.3	21.8	26.7
Assumed domestic [Member] | Case Reserves [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total assumed reserves	9.8	10.5	12.2
Assumed domestic [Member] | ULAE [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total assumed reserves	0.8	0.8	0.8
Assumed domestic [Member] | IBNR [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total assumed reserves	13.7	10.5	13.7
Assumed London [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total assumed reserves	2.9	4.7	5.4
Assumed London [Member] | Case Reserves [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total assumed reserves	2.3	3.3	4
Assumed London [Member] | ULAE [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total assumed reserves	0	0	0
Assumed London [Member] | IBNR [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total assumed reserves	$ 0.6	$ 1.4	$ 1.4

Run-Off Lines - Underwriting Lo

Run-Off Lines - Underwriting Losses for Run-Off Lines (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	$ (15.4)	$ (10.9)	$ (4.4)
Total underwriting loss - Run-off Lines	(25.7)	(25.1)	(14.1)
Reinsurance Assumed [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	(8.7)	(1.3)	(1)
Other [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	(6.7)	(9.6)	(3.4)
Risk Management [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total underwriting loss - Run-off Lines	(8.8)	(13.1)	(8.2)
Run-Off Reinsurance Lines [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total underwriting loss - Run-off Lines	(0.1)	0.1	2
Other Run-Off Lines [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total underwriting loss - Run-off Lines	$ (1.4)	$ (1.2)	$ (3.5)

Junior Subordinated Debentur105

Junior Subordinated Debentures - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Junior subordinated debentures	$ 256.6	$ 172.7
Junior Subordinated Debentures [Member]
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage	100.00%	100.00%
Junior Subordinated Debentures, Maybrooke [Member]
Debt Instrument [Line Items]
Junior subordinated debentures	$ 91.8
Unsecured Debt	$ 83.9

Junior Subordinated Debentur106

Junior Subordinated Debentures - Schedule of Junior Subordinated Debentures (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Junior subordinated debentures, Amount	$ 256.6	$ 172.7
Argo Group [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Amount	28.4	28.4
Trust Preferred Debentures [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Amount	$ 172.7	$ 172.7
Trust Preferred Debentures [Member] | PXRE Capital Statutory Trust II [Member] | Argo Group [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	May 15, 2003	May 15, 2003
Junior subordinated debentures, Maturity date	May 15, 2033	May 15, 2033
Junior subordinated debentures, Rate Structure	2017 and 2016: 3M LIBOR + 4.10%
Junior subordinated debentures, interest rate stated percentage	5.52%	5.00%
Junior subordinated debentures, Amount	$ 18.1	$ 18.1
Trust Preferred Debentures [Member] | PXRE Capital Trust VI [Member] | Argo Group [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	Nov. 6, 2003	Nov. 6, 2003
Junior subordinated debentures, Maturity date	Sep. 30, 2033	Sep. 30, 2033
Junior subordinated debentures, Rate Structure	2017 and 2016: 3M LIBOR + 3.90%
Junior subordinated debentures, interest rate stated percentage	5.59%	4.90%
Junior subordinated debentures, Amount	$ 10.3	$ 10.3
Trust Preferred Debentures [Member] | Argonaut Group Statutory Trust I [Member] | Argo Group US [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	May 15, 2003	May 15, 2003
Junior subordinated debentures, Maturity date	May 15, 2033	May 15, 2033
Junior subordinated debentures, Rate Structure	2017 and 2016: 3M LIBOR + 4.10%
Junior subordinated debentures, interest rate stated percentage	5.52%	5.00%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust Preferred Debentures [Member] | Argonaut Group Statutory Trust III [Member] | Argo Group US [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	Dec. 16, 2003	Dec. 16, 2003
Junior subordinated debentures, Maturity date	Jan. 8, 2034	Jan. 8, 2034
Junior subordinated debentures, Rate Structure	2017 and 2016: 3M LIBOR + 4.10%
Junior subordinated debentures, interest rate stated percentage	5.46%	4.98%
Junior subordinated debentures, Amount	$ 12.3	$ 12.3
Trust Preferred Debentures [Member] | Argonaut Group Statutory Trust IV [Member] | Argo Group US [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	Apr. 29, 2004	Apr. 29, 2004
Junior subordinated debentures, Maturity date	Apr. 29, 2034	Apr. 29, 2034
Junior subordinated debentures, Rate Structure	2017 and 2016: 3M LIBOR + 3.85%
Junior subordinated debentures, interest rate stated percentage	5.27%	4.76%
Junior subordinated debentures, Amount	$ 13.4	$ 13.4
Trust Preferred Debentures [Member] | Argonaut Group Statutory Trust V [Member] | Argo Group US [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	May 26, 2004	May 26, 2004
Junior subordinated debentures, Maturity date	May 24, 2034	May 24, 2034
Junior subordinated debentures, Rate Structure	2017 and 2016: 3M LIBOR + 3.85%
Junior subordinated debentures, interest rate stated percentage	5.30%	4.77%
Junior subordinated debentures, Amount	$ 12.3	$ 12.3
Trust Preferred Debentures [Member] | Argonaut Group Statutory Trust VI [Member] | Argo Group US [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	May 12, 2004	May 12, 2004
Junior subordinated debentures, Maturity date	May 12, 2034	May 12, 2034
Junior subordinated debentures, Rate Structure	2017 and 2016: 3M LIBOR + 3.80%
Junior subordinated debentures, interest rate stated percentage	5.40%	4.79%
Junior subordinated debentures, Amount	$ 13.4	$ 13.4
Trust Preferred Debentures [Member] | Argonaut Group Statutory Trust VII [Member] | Argo Group US [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	Sep. 17, 2004	Sep. 17, 2004
Junior subordinated debentures, Maturity date	Dec. 15, 2034	Dec. 15, 2034
Junior subordinated debentures, Rate Structure	2017 and 2016: 3M LIBOR + 3.60%
Junior subordinated debentures, interest rate stated percentage	5.19%	4.56%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust Preferred Debentures [Member] | Argonaut Group Statutory Trust VIII [Member] | Argo Group US [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	Sep. 22, 2004	Sep. 22, 2004
Junior subordinated debentures, Maturity date	Sep. 22, 2034	Sep. 22, 2034
Junior subordinated debentures, Rate Structure	2017 and 2016: 3M LIBOR + 3.55%
Junior subordinated debentures, interest rate stated percentage	5.21%	4.55%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust Preferred Debentures [Member] | Argonaut Group Statutory Trust IX [Member] | Argo Group US [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	Oct. 22, 2004	Oct. 22, 2004
Junior subordinated debentures, Maturity date	Dec. 15, 2034	Dec. 15, 2034
Junior subordinated debentures, Rate Structure	2017 and 2016: 3M LIBOR + 3.60%
Junior subordinated debentures, interest rate stated percentage	5.19%	4.56%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust Preferred Debentures [Member] | Argonaut Group Statutory Trust X [Member] | Argo Group US [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	Sep. 15, 2005	Sep. 15, 2005
Junior subordinated debentures, Maturity date	Sep. 15, 2035	Sep. 15, 2035
Junior subordinated debentures, Rate Structure	2017 and 2016: 3M LIBOR + 3.40%
Junior subordinated debentures, interest rate stated percentage	4.99%	4.36%
Junior subordinated debentures, Amount	$ 30.9	$ 30.9

Junior Subordinated Debentur107

Junior Subordinated Debentures - Schedule of Maybrooke Junior Subordinated Debentures (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Principal	$ 143.8	$ 143.8
Junior Subordinated Debentures, Maybrooke [Member]
Debt Instrument [Line Items]
Issue Date	Sep. 15, 2007
Maturity	Sep. 15, 2037
Rate Structure	3M LIBOR + 3.15%
Interest Rate	4.74%
Principal	$ 91.8
Carrying Value	$ 83.9

Other Indebtedness - Schedule o

Other Indebtedness - Schedule of Terms and Principal Amounts of Each Debt Instrument (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Other indebtedness	$ 184.5	$ 55.4
Term Loan [Member]
Debt Instrument [Line Items]
Other indebtedness	125	0
Floating Rate Loan Stock [Member]
Debt Instrument [Line Items]
Other indebtedness	58.9	54.8
Other Debt [Member]
Debt Instrument [Line Items]
Other indebtedness	$ 0.6	$ 0.6

Other Indebtedness - Additional

Other Indebtedness - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Mar. 03, 2017	Mar. 07, 2014
Debt Instrument [Line Items]
Floating rate loan stock denominated in Euros		$ 42,400,000	$ 38,300,000
New Credit Agreement [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Maturity date		Mar. 3, 2019
New Credit Agreement [Member] | J P Morgan Chase Bank [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing amount				$ 325,000,000
New Credit Agreement [Member] | Revolving credit facility [Member] | J P Morgan Chase Bank [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing amount				200,000,000
Revolving credit facility maturity date		Mar. 3, 2022
Letter of credit facility amount outstanding		$ 0	0
New Credit Agreement [Member] | Term Loan [Member] | J P Morgan Chase Bank [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing amount				125,000,000
New Credit Agreement [Member] | Letter Of Credit [Member] | J P Morgan Chase Bank [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing amount		200,000,000
Letter of credit facility amount		$ 500,000		$ 200,000
Prior Agreement [Member] | Revolving credit facility [Member] | Maybrooke [Member]
Debt Instrument [Line Items]
Repayments of borrowings	$ 125,000,000
Prior Agreement [Member] | Revolving credit facility [Member] | J P Morgan Chase Bank [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing amount					$ 175,000,000
Prior Agreement [Member] | Letter Of Credit [Member] | J P Morgan Chase Bank [Member]
Debt Instrument [Line Items]
Letter of credit facility amount			$ 200,000
Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage		100.00%	100.00%
Other Debt - Note Payable [Member]
Debt Instrument [Line Items]
Note payable as part of ARIS acquisition		$ 600,000	$ 600,000
Junior subordinated debentures, Maturity date		Apr. 1, 2019
Debt instrument, interest rate terms		The note had a variable interest rate of 2.00% above 30-day LIBOR, with the variable interest rate being reset quarterly and subject to certain interest rate ceilings. Interest payments are payable quarterly.
Other Debt - Note Payable [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Variable interest rate of note payable, above 30-day LIBOR		2.00%

Other Indebtedness - Schedul110

Other Indebtedness - Schedule of Floating Rate Loan Stock, Notes Outstanding (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Amount	$ 58.9	$ 54.8
Other indebtedness, Amount	$ 184.5	$ 55.4
Floating Rate Loan Stock, U.S. Dollar, Start Year 2004 [Member]
Debt Instrument [Line Items]
Issue Date	Dec. 8, 2004	Dec. 8, 2004
Maturity	Nov. 15, 2034	Nov. 15, 2034
Rate Structure	2017 and 2016: 6 month LIBOR + 4.2%
Interest Rate	5.66%	5.18%
Amount	$ 6.5	$ 6.5
Floating Rate Loan Stock, Euro, Start Year 2005 [Member]
Debt Instrument [Line Items]
Issue Date	Sep. 6, 2005	Sep. 6, 2005
Maturity	Aug. 22, 2035	Aug. 22, 2035
Rate Structure	2017 and 2016: 3 month LIBOR + 4.0%
Interest Rate	3.67%	3.70%
Amount	$ 14.1	$ 12.8
Floating Rate Loan Stock, U.S. Dollar, Start Year 2006 [Member]
Debt Instrument [Line Items]
Issue Date	Oct. 31, 2006	Oct. 31, 2006
Maturity	Jan. 15, 2036	Jan. 15, 2036
Rate Structure	2017 and 2016: 6 month LIBOR + 4.0%
Interest Rate	5.46%	4.98%
Amount	$ 10	$ 10
Floating Rate Loan Stock, Euro, Start Year 2006 [Member]
Debt Instrument [Line Items]
Issue Date	Oct. 31, 2006	Oct. 31, 2006
Maturity	Nov. 22, 2036	Nov. 22, 2036
Rate Structure	2017 and 2016: 3 month LIBOR + 4.0%
Interest Rate	3.67%	3.70%
Amount	$ 12.4	$ 11.2
Floating Rate Loan Stock, Euro, Start Year 2007 [Member]
Debt Instrument [Line Items]
Issue Date	Jun. 8, 2007	Jun. 8, 2007
Maturity	Sep. 15, 2037	Sep. 15, 2037
Rate Structure	2017 and 2016: 3 month LIBOR + 3.9%
Interest Rate	3.57%	3.58%
Amount	$ 15.9	$ 14.3
New Credit Agreement [Member]
Debt Instrument [Line Items]
Issue Date	Mar. 3, 2017
Maturity	Mar. 3, 2019
Rate Structure	2 month LIBOR + 1.5%
Interest Rate	2.94%
Other indebtedness, Amount	$ 125

Disclosures about Fair Value111

Disclosures about Fair Value of Financial Instruments - Summary of Company's Financial Instruments Whose Carrying Amount Did Not Equal Fair Value (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Junior Subordinated Debentures [Member] | Trust Preferred Debentures [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	$ 172.7	$ 172.7
Junior Subordinated Debentures [Member] | Trust Preferred Debentures [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	172.9	162.4
Junior Subordinated Debentures [Member] | Junior Subordinated Debentures, Maybrooke [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	83.9	0
Junior Subordinated Debentures [Member] | Junior Subordinated Debentures, Maybrooke [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	85	0
Senior Unsecured Fixed Rate Notes [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	139.6	139.5
Senior Unsecured Fixed Rate Notes [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	141.2	139.3
Floating Rate Loan Stock [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	58.9	54.8
Floating Rate Loan Stock [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	$ 59	$ 51.5

Shareholders Equity - Additiona

Shareholders Equity - Additional Information (Detail) - USD ($)	Feb. 20, 2018	May 03, 2016	Feb. 17, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class Of Stock [Line Items]
Stock dividend declared					10.00%	10.00%
Dividend declared per common share				$ 1.08	$ 0.86	$ 0.73
Cash dividends paid - common shares, total				$ 33,200,000	$ 26,600,000	$ 22,700,000
Preferred shares, authorized				30,000,000
Preferred shares, par value				$ 1
Preferred shares, issued				0	0
Preferred shares, outstanding				0	0
Common shares repurchased on open market				756,252	847,111	575,155
Common shares repurchased on open market, total cost				$ 45,200,000	$ 47,100,000	$ 29,700,000
2016 Repurchase Authorization [Member]
Class Of Stock [Line Items]
Remaining number of shares available under repurchase authorization, Value				$ 85,000,000
Common shares repurchased on open market				756,252	847,111
Common shares repurchased on open market, total cost				$ 45,200,000	$ 47,100,000
2016 Repurchase Authorization [Member] | Maximum [Member]
Class Of Stock [Line Items]
Total number of shares authorized for purchase, Value		$ 150,000,000
Subsequent Event [Member]
Class Of Stock [Line Items]
Dividends declaration date	Feb. 20, 2018
Dividends payable date	Mar. 23, 2018
Dividend payable, date of record	Mar. 7, 2018
Dividend declared per common share	$ 0.27
Stock Dividend [Member]
Class Of Stock [Line Items]
Dividends declaration date		May 3, 2016	Feb. 17, 2015
Dividends payable date		Jun. 15, 2016	Mar. 16, 2015
Dividend payable, date of record		Jun. 1, 2016	Mar. 2, 2015
Stock dividend declared		10.00%	10.00%
Additional stock issued as dividend		2,735,542	2,554,506
Stock Dividend [Member] | Subsequent Event [Member]
Class Of Stock [Line Items]
Dividends declaration date	Feb. 20, 2018
Dividends payable date	Mar. 21, 2018
Dividend payable, date of record	Mar. 7, 2018
Stock dividend declared	15.00%
Additional stock issued as dividend	4,400,000
Before 2016 Adjustment Stock Dividend [Member]
Class Of Stock [Line Items]
Dividend declared per common share						$ 0.80

Shareholders Equity - Schedule

Shareholders Equity - Schedule of Repurchase of Shares (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity Class Of Treasury Stock [Line Items]
Common shares repurchased on open market	756,252	847,111	575,155
Average Price of Shares Repurchased	$ 59.76	$ 55.64	$ 51.58
Total Cost (in millions)	$ 45.2	$ 47.1	$ 29.7
10b5-1 Trading Plan Initiated In 06/14/2017 [Member]
Equity Class Of Treasury Stock [Line Items]
Date Trading Plan Initiated	Jun. 14, 2017
Trading Plan Purchase Period Start Date	Jun. 20, 2017
Common shares repurchased on open market	156,570
Average Price of Shares Repurchased	$ 59.53
Total Cost (in millions)	$ 9.4
Repurchase Authorization Year	2,016
Trading Plan Purchase Period End Date	Aug. 10, 2017
10b5-1 Trading Plan Initiated In 09/15/2017 [Member]
Equity Class Of Treasury Stock [Line Items]
Date Trading Plan Initiated	Sep. 15, 2017
Trading Plan Purchase Period Start Date	Sep. 18, 2017
Common shares repurchased on open market	84,711
Average Price of Shares Repurchased	$ 60.71
Total Cost (in millions)	$ 5.1
Repurchase Authorization Year	2,016
Trading Plan Purchase Period End Date	Nov. 7, 2017
10b5-1 Trading Plan Initiated In 12/15/2017 [Member]
Equity Class Of Treasury Stock [Line Items]
Date Trading Plan Initiated	Dec. 15, 2017
Common shares repurchased on open market	1,420
Average Price of Shares Repurchased	$ 60.53
Total Cost (in millions)	$ 0.1
Repurchase Authorization Year	2,016
Trading Plan Purchase Period Date	Dec. 27, 2017
Open Market Repurchase Authorization Two Thousand And Sixteen [Member]
Equity Class Of Treasury Stock [Line Items]
Trading Plan Purchase Period Start Date	Aug. 11, 2017
Common shares repurchased on open market	421,976
Average Price of Shares Repurchased	$ 59.75
Total Cost (in millions)	$ 25.2
Repurchase Authorization Year	2,016
Trading Plan Purchase Period End Date	Sep. 15, 2017
Open Market Repurchase Authorization Two Thousand And Sixteen [Member]
Equity Class Of Treasury Stock [Line Items]
Trading Plan Purchase Period Start Date	Nov. 8, 2017
Common shares repurchased on open market	91,575
Average Price of Shares Repurchased	$ 59.28
Total Cost (in millions)	$ 5.4
Repurchase Authorization Year	2,016
Trading Plan Purchase Period End Date	Nov. 21, 2017

Shareholders' Equity - Schedule

Shareholders' Equity - Schedule of Authorized and Unissued Common Shares Reserved (Detail)	Dec. 31, 2017shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Shares Reserved	3,671,053
2014 Long-Term Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Shares Reserved	3,178,971
2016 Employee Share Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Shares Reserved	492,082

Accumulated Other Comprehens115

Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Beginning Balance	$ 47.7	$ 11.5
Other comprehensive income (loss) before reclassifications	77.1	46.2
Amounts reclassified from accumulated other comprehensive income (loss)	(28.2)	(10)
Other comprehensive income (loss), net of tax	48.9	36.2	$ (96.6)
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Ending Balance	96.6	47.7	11.5
Foreign Currency Translation Adjustments [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Beginning Balance	(17.6)	(21.6)
Other comprehensive income (loss) before reclassifications	(1.4)	4
Amounts reclassified from accumulated other comprehensive income (loss)	0	0
Other comprehensive income (loss), net of tax	(1.4)	4
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Ending Balance	(19)	(17.6)	(21.6)
Unrealized Holding Gains on Securities [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Beginning Balance	72.4	40
Other comprehensive income (loss) before reclassifications	77.7	42.4
Amounts reclassified from accumulated other comprehensive income (loss)	(28.2)	(10)
Other comprehensive income (loss), net of tax	49.5	32.4
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Ending Balance	121.9	72.4	40
Defined Benefit Pension Plans [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Beginning Balance	(7.1)	(6.9)
Other comprehensive income (loss) before reclassifications	0.8	(0.2)
Amounts reclassified from accumulated other comprehensive income (loss)	0	0
Other comprehensive income (loss), net of tax	0.8	(0.2)
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Ending Balance	$ (6.3)	$ (7.1)	$ (6.9)

Accumulated Other Comprehens116

Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrealized gains and losses on securities:
Reclassification adjustment for gains included in net income	$ (41.6)	$ (23.2)	$ (7.2)
Provision for income taxes	13.4	13.2	6.3
Net of taxes	(28.2)	(10)
Unrealized Gains and Losses on Securities [Member]
Unrealized gains and losses on securities:
Net of taxes	(28.2)	(10)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains and Losses on Securities [Member]
Unrealized gains and losses on securities:
Reclassification adjustment for gains included in net income	(41.6)	(23.2)	(7.2)
Provision for income taxes	13.4	13.2	6.3
Net of taxes	$ (28.2)	$ (10)	$ (0.9)

Net Income Per Common Share - N

Net Income Per Common Share - Net Income Per Common Share on Basic and Diluted Basis (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Earnings Per Share [Abstract]
Net income	$ 28.9		$ (61.3)		$ 46		$ 36.7		$ 32.9		$ 55.2		$ 30.9		$ 27.7		$ 50.3		$ 146.7		$ 163.2
Weighted average common shares outstanding - basic																	29,962,524		30,166,440		30,769,089
Effect of dilutive securities:
Equity compensation awards																	794,710		679,270		616,371
Weighted average common shares outstanding - diluted																	30,757,234		30,845,710		31,385,460
Net income per common share - basic	$ 0.98	[1]	$ (2.04)	[1]	$ 1.52	[1]	$ 1.22	[1]	$ 1.10	[1]	$ 1.84	[1]	$ 1.03	[1]	$ 0.91	[1]	$ 1.68	[1]	$ 4.86	[1]	$ 5.31
Net income per common share - diluted	$ 0.95	[1]	$ (2.04)	[1]	$ 1.48	[1]	$ 1.19	[1]	$ 1.07	[1]	$ 1.80	[1]	$ 1	[1]	$ 0.89	[1]	$ 1.64	[1]	$ 4.75	[1]	$ 5.20
[1]	Basic and diluted net income per common share are computed independently for each quarter and full year based on the respective average number of common shares outstanding; therefore, the sum of the quarterly net income per common share data may not equal the net income per common share for the year.

Net Income Per Common Share - A

Net Income Per Common Share - Additional Information (Detail) - shares	Feb. 20, 2018	May 03, 2016	Feb. 17, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Line Items]
Treasury shares, shares				10,785,007	10,028,755	9,181,644
Anti-dilutive securities excluded from computation of diluted net income per common share				0	0	0
Stock dividend declared					10.00%	10.00%
Stock Dividend [Member]
Earnings Per Share [Line Items]
Stock dividend declared		10.00%	10.00%
Dividends declaration date		May 3, 2016	Feb. 17, 2015
Dividends payable date		Jun. 15, 2016	Mar. 16, 2015
Dividend payable, date of record		Jun. 1, 2016	Mar. 2, 2015
Subsequent Event [Member]
Earnings Per Share [Line Items]
Dividends declaration date	Feb. 20, 2018
Dividends payable date	Mar. 23, 2018
Dividend payable, date of record	Mar. 7, 2018
Subsequent Event [Member] | Stock Dividend [Member]
Earnings Per Share [Line Items]
Stock dividend declared	15.00%
Dividends declaration date	Feb. 20, 2018
Dividends payable date	Mar. 21, 2018
Dividend payable, date of record	Mar. 7, 2018

Share-based Compensation - Addi

Share-based Compensation - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Share-based compensation arrangement by share-based payment award, award vesting period, years	5 years	5 years
Stock dividend declared		10.00%	10.00%
Percentage increase in number of awards outstanding due to stock dividend	10.00%
Reduction in exercise price of awards outstanding due to stock dividend	9.09%
Share-based payment expense	$ 12.3	$ 19.8	$ 29.1
Share-based payments expense, net of tax	$ 10.3	$ 15.5	$ 23.7

Share-based Compensation - Fair

Share-based Compensation - Fair Value Assumptions (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free rate of return, minimum	1.83%	1.00%	1.44%
Expected volatility, minimum	18.13%	18.73%	20.04%
Risk-free rate of return, maximum	2.22%	2.02%	1.81%
Expected volatility, maximum	18.70%	19.70%	22.09%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yields	1.63%	1.62%	1.46%
Expected award life (years)	4 years 5 months 23 days	4 years 6 months	4 years 7 months 13 days
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yields	1.72%	1.70%	1.60%
Expected award life (years)	4 years 5 months 26 days	4 years 7 months 10 days	4 years 8 months 16 days

Share-Based Compensation - 2014

Share-Based Compensation - 2014 Long-Term Incentive Plan - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	May 31, 2014	Dec. 31, 2017	Dec. 31, 2016	Nov. 30, 2007
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, number of shares authorized		3,671,053
Share-based compensation arrangement by share-based payment award, award vesting period, years		5 years	5 years
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee service share-based compensation, unrecognized compensation costs on non-vested awards		$ 25.6
Employee service share-based compensation, unrecognized compensation costs weighted-average period, years		2 years
Share-based compensation arrangement by share-based payment award, total fair value of shares vested		$ 7.7
Share based compensation arrangement by share based payment award options, outstanding		681,669	702,030
Restricted Stock Units (RSUs) [Member] | Performance Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation arrangement by share based payment award options, outstanding		406,712
Stock-Settled SARs Activity [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee service share-based compensation, unrecognized compensation costs on non-vested awards		$ 3.6
Employee service share-based compensation, unrecognized compensation costs weighted-average period, years		1 year
Exercised, shares		486,454
Share based compensation arrangement by share based payment award, shares issued		252,580
Share based compensation arrangement by share based payment award options exercises, intrinsic value		$ 26.2
Stock-Settled SARs Activity [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, award vesting period, years		1 year
Stock-Settled SARs Activity [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, award vesting period, years		4 years
Cash-Settled SARs Activity [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercised, shares		318,562
Share based compensation arrangement by share based payment award options exercises, intrinsic value		$ 4.1
Employee service share-based compensation, cash paid to exercise of stock options		10.2
Liability for stock awards		$ 5.7	$ 18.6
2007 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, number of shares authorized				4,500,000
Share-based compensation arrangement by share-based payment award, number of shares available for grant	1,500,000
2014 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, additional number of shares authorized	2,800,000
Count of restricted shares, settle in common shares		2.75

Share-based Compensation - Summ

Share-based Compensation - Summary of Restricted Share Activity (Detail) - Restricted Stock Units (RSUs) [Member]	12 Months Ended
	Dec. 31, 2017$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares, Outstanding at January 1, 2017 | shares	702,030
Granted, Shares | shares	235,431
Vested and issued, Shares | shares	(183,458)
Expired or forfeited, Shares | shares	(72,334)
Shares, Outstanding at December 31, 2017 | shares	681,669
Weighted-Average Grant Date Fair Value, Outstanding at January 1, 2017 | $ / shares	$ 42.69
Granted, Weighted-Average Grant Date Fair Value | $ / shares	62.85
Vested and issued, Weighted-Average Grant Date Fair Value | $ / shares	41.96
Expired or forfeited, Weighted-Average Grant Date Fair Value | $ / shares	50.53
Weighted-Average Grant Date Fair Value, Outstanding at December 31, 2017 | $ / shares	$ 49.02

Share-based Compensation - S123

Share-based Compensation - Summary of Stock-Settled SARs Activity (Detail) - Stock-Settled SARs Activity [Member]	12 Months Ended
	Dec. 31, 2017$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares, Outstanding at January 1, 2017 | shares	1,982,695
Exercised, Shares | shares	(486,454)
Expired or forfeited, Shares | shares	(186,089)
Shares, Outstanding at December 31, 2017 | shares	1,310,152
Vested or expected to vest as of year end, Shares | shares	1,236,514
Exercisable at end of year, Shares | shares	891,067
Weighted-Average Exercise Price, Outstanding at January 1, 2017 | $ / shares	$ 34.80
Exercised, Weighted-Average Exercise Price | $ / shares	30.77
Expired or forfeited, Weighted-Average Exercise Price | $ / shares	40.75
Weighted-Average Exercise Price, Outstanding at December 31, 2017 | $ / shares	35.45
Vested or expected to vest as of year end, Weighted-Average Exercise Price | $ / shares	19.80
Exercisable at end of year, Weighted-Average Exercise Price | $ / shares	$ 22.80

Share-based Compensation - S124

Share-based Compensation - Summary of Cash-Settled SARs Activity (Detail) - Cash-Settled SARs Activity [Member]	12 Months Ended
	Dec. 31, 2017$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares, Outstanding at January 1, 2017 | shares	500,486
Exercised, Shares | shares	(318,562)
Expired or forfeited, Shares | shares	(14,628)
Shares, Outstanding at December 31, 2017 | shares	167,296
Weighted-Average Exercise Price, Outstanding at January 1, 2017 | $ / shares	$ 32.08
Exercised, Weighted-Average Exercise Price | $ / shares	31.87
Expired or forfeited, Weighted-Average Exercise Price | $ / shares	23.40
Weighted-Average Exercise Price, Outstanding at December 31, 2017 | $ / shares	$ 33.23

Share-Based Compensation - Empl

Share-Based Compensation - Employees Share Purchase Plans - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Share Repurchase Programs [Line Items]
Share-based compensation arrangement by share-based payment award, award vesting period, years	5 years	5 years
Share-based payment expense	$ 12.3	$ 19.8	$ 29.1
2007 Employee Share Purchase Plan [Member]
Schedule Of Share Repurchase Programs [Line Items]
Share-based compensation arrangement by share-based payment award, discount from market price	85.00%
Share-based compensation arrangement by share-based payment award, award vesting period, years	3 months
Earn Plan [Member]
Schedule Of Share Repurchase Programs [Line Items]
Share-based compensation arrangement by share-based payment award, discount from market price	85.00%
Earn Plan [Member] | Minimum [Member]
Schedule Of Share Repurchase Programs [Line Items]
Share-based compensation arrangement by share-based payment award, award vesting period, years	3 years
Earn Plan [Member] | Maximum [Member]
Schedule Of Share Repurchase Programs [Line Items]
Share-based compensation arrangement by share-based payment award, award vesting period, years	5 years
Stock Compensation Plan [Member]
Schedule Of Share Repurchase Programs [Line Items]
Share-based payment expense	$ 0.4	$ 0.3	$ 0.4

Share-Based Compensation - Defe

Share-Based Compensation - Deferred Compensation Plan for Non-Employee Directors - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2013
Schedule Of Share Repurchase Programs [Line Items]
Deferred compensation arrangement with individual, description	Under the Directors Plan, non-employee directors could elect each year to defer payment of 0%, 50% or 100% of their cash compensation payable during the next calendar year.
Share-based payment expense	$ 12.3	$ 19.8	$ 29.1
2007 Plan [Member]
Schedule Of Share Repurchase Programs [Line Items]
Share-based compensation arrangement by share-based payment award, discount from market price				50.00%
2007 Plan [Member] | Minimum [Member]
Schedule Of Share Repurchase Programs [Line Items]
Share-based compensation arrangement by share-based payment award, discount from market price				0.00%
2007 Plan [Member] | Maximum [Member]
Schedule Of Share Repurchase Programs [Line Items]
Share-based compensation arrangement by share-based payment award, discount from market price				100.00%
Deferred Compensation Plan [Member]
Schedule Of Share Repurchase Programs [Line Items]
Employee service share-based compensation, cash paid to exercise of stock options	$ 5.1
Share-based payment expense		$ 0.9	$ 0.9

Underwriting, Acquisition an127

Underwriting, Acquisition and Insurance Expenses - Underwriting, Acquisition and Insurance Expenses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Underwriting Acquisition And Insurance Expenses [Abstract]
Commissions	$ 255.9	$ 234	$ 232.2
General expenses	359.1	292.1	295.6
Premium taxes, boards and bureaus	32.3	25.2	9.8
Underwriting, acquisition and insurance expenses, total	647.3	551.3	537.6
Net deferral of policy acquisition costs	(11.9)	(4.3)	(0.9)
Total underwriting, acquisition and insurance expenses	$ 635.4	$ 547	$ 536.7

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes Disclosure [Line Items]
US federal tax rate		35.00%
Tax cuts and jobs act of 2017, incomplete accounting, change in tax rate, provisional income tax benefit, revaluing deferred tax assets and liabilities		$ 20,200,000
Benefit related to deferred taxes previously recognized in accumulated other comprehensive income		$ 20,900,000
Net operating losses carryback period		2 years
Capital losses carryback period		3 years
Net change in valuation allowance, for deferred tax asset		$ 3,400,000
Net operating loss carryforward		7,200,000
Loss carryforwards per year		2,800,000
Valuation allowance		20,100,000	$ 23,500,000
Unrecognized tax benefits		0	$ 0	$ 0
Maybrooke Holdings SA [Member]
Income Taxes Disclosure [Line Items]
Valuation allowance		3,500,000
Brazil [Member]
Income Taxes Disclosure [Line Items]
Valuation allowance		7,700,000
Malta [Member]
Income Taxes Disclosure [Line Items]
Valuation allowance		1,100,000
ARIS [Member]
Income Taxes Disclosure [Line Items]
Net operating loss carryforward		$ 600,000
Scenario Forecast [Member]
Income Taxes Disclosure [Line Items]
US federal tax rate	21.00%

Income Taxes - Income Tax (Bene

Income Taxes - Income Tax (Benefit) Provision Expense (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current income tax (benefit) provision related to:
Total current income tax (benefit) provision	$ 7.5	$ 36.3	$ 6
Deferred income tax (benefit) provision related to:
Deferred income tax (benefit) provision, net	(17.9)	(1.1)	8.3
Income tax (benefit) provision	(10.4)	35.2	14.3
United States [Member]
Current income tax (benefit) provision related to:
Total current income tax (benefit) provision	(0.3)	30.4	5.1
Deferred income tax (benefit) provision related to:
Deferred income tax (benefit) provision, net	(0.3)	3.3	7.8
United Kingdom [Member]
Current income tax (benefit) provision related to:
Total current income tax (benefit) provision	7.6	3.9	(1.5)
Deferred income tax (benefit) provision related to:
Deferred income tax (benefit) provision, net	(17.6)	(4.4)	0.5
Other jurisdictions [Member]
Current income tax (benefit) provision related to:
Total current income tax (benefit) provision	0.2	2	2.4
Deferred income tax (benefit) provision related to:
Deferred income tax (benefit) provision, net	$ 0	$ 0	$ 0

Income Taxes - Schedule of Pre-

Income Taxes - Schedule of Pre-Tax Income (Loss) and Effective Income Tax Rates (Detail) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ 12.3	$ (65.9)	$ 50.8	$ 42.7	$ 47	$ 62.2	$ 39.6	$ 33.1	$ 39.9		$ 181.9		$ 177.5
Effective Tax Rate										(26.10%)		19.40%		8.10%
Bermuda [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)										$ 30.2		$ 99.6		$ 94.3
Effective Tax Rate										0.00%		0.00%		0.00%
United States [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)										$ 67.5		$ 115.2		$ 64.8
Effective Tax Rate										(0.90%)		29.20%		19.90%
United Kingdom [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)										$ (53.8)		$ (34.8)		$ 21.3
Effective Tax Rate										18.70%		1.20%		(4.70%)
Belgium [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)										$ 0.1		$ (0.1)		$ 0	[1]
Effective Tax Rate										75.00%		23.20%		237.00%
Brazil [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)										$ 0.8		$ 0.6		$ (3.2)
Effective Tax Rate										0.00%		0.00%		0.00%
United Arab Emirates [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)										$ 0.2		$ 0	[1]	$ 0.2
Effective Tax Rate										0.00%		0.00%		0.00%
Ireland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	[2]									$ (0.2)		$ (0.2)		$ (0.1)
Effective Tax Rate	[2]									0.00%		5.00%		5.00%
Luxembourg [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)										$ (5.2)		$ 0		$ 0
Effective Tax Rate										0.00%		0.00%		0.00%
Malta [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)										$ 0.3		$ 1.6		$ 0.1
Effective Tax Rate										0.00%		0.30%		0.00%
Switzerland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)										$ 0	[1]	$ 0	[1]	$ 0.1
Effective Tax Rate										21.10%		0.00%		20.50%
[1]	Pre-tax income for the respective year was less than $0.1 million.
[2]	Effective tax rate of 5 percent on intercompany dividends of $40.0 million for the year ended December 31, 2016. Dividends eliminated in consolidation.

Income Taxes - Schedule of P131

Income Taxes - Schedule of Pre-Tax Income (Loss) and Effective Income Tax Rates (Parenthetical) (Detail) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ 12.3	$ (65.9)	$ 50.8	$ 42.7	$ 47	$ 62.2	$ 39.6	$ 33.1	$ 39.9	$ 181.9	$ 177.5
Effective tax rate percent on intercompany dividends										(26.10%)	19.40%	8.10%
Belgium [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)										$ 0.1	$ (0.1)	$ 0	[1]
Effective tax rate percent on intercompany dividends										75.00%	23.20%	237.00%
Ireland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	[2]									$ (0.2)	$ (0.2)	$ (0.1)
Effective tax rate percent on intercompany dividends	[2]									0.00%	5.00%	5.00%
Intercompany dividends											$ 40
Maximum [Member] | Belgium [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)										$ 0.1	$ 0.1	$ 0.1
[1]	Pre-tax income for the respective year was less than $0.1 million.
[2]	Effective tax rate of 5 percent on intercompany dividends of $40.0 million for the year ended December 31, 2016. Dividends eliminated in consolidation.

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Difference Between Provision for Income Taxes and Expected Tax Provision at Weighted Average Tax Rate (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income tax provision at expected rate	$ 12.7	$ 34.1	$ 25.8
Nontaxable investment income	(4.7)	(5.5)	(6.5)
Foreign exchange adjustments	2.1	5.3	(0.1)
Withholding taxes	0.4	2.4	2.9
Change in valuation allowance	(0.9)	0.7	(2.6)
Impact of change in tax rate related to TCJA	(20.2)	0	0
Other	0.2	(1.8)	(5.2)
Income tax (benefit) provision	$ (10.4)	$ 35.2	$ 14.3

Income Taxes - Schedule of Net

Income Taxes - Schedule of Net Deferred Tax Liability Comprises Tax Effect Related to Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Losses and loss adjustment expense reserve discounting	$ 17.2	$ 21.1
Unearned premiums	21.4	29.1
Net operating loss carryforwards	21.9	26.7
Impairment of investment values	4	8.6
Accrued bonus	2.6	7.4
Stock option expense	2	8.1
United Kingdom underwriting results	3.8	0
Other	20	32.5
Deferred tax assets, gross	92.9	133.5
Unrealized gains on equity securities	(30.5)	(39.1)
Unrealized gains on fixed maturities and other investment securities	(5)	(3.9)
Deferred acquisition costs	(16.9)	(22.2)
TCJA reserve transitional liability	(5.5)	0
United Kingdom underwriting results	0	(5.4)
Deferred gain on like-kind exchange	(8)	(13.3)
Depreciable fixed assets	(14.5)	(21)
Unrealized gains on limited partnership interests	(14.6)	(17.6)
Other	(9.1)	(11.6)
Deferred tax liabilities, gross	(104.1)	(134.1)
Deferred tax (liabilities) assets, net before valuation allowance	(11.2)	(0.6)
Valuation allowance	(20.1)	(23.5)
Deferred tax liabilities, net	(31.3)	(24.1)
Net deferred tax (liabilities) assets - Other jurisdictions	1.2	(6.6)
Net deferred tax liabilities - United States	$ (32.5)	$ (17.5)

Income Taxes - Schedule of N134

Income Taxes - Schedule of Net Operating Loss Carryforwards Amounts by Jurisdiction and Year of Expiration (Detail) $ in Millions	Dec. 31, 2017USD ($)
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards by jurisdiction	$ 7.2
Brazil [Member] | Indefinite Expiration [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards by jurisdiction	17
Luxembourg [Member] | Indefinite Expiration [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards by jurisdiction	7.8
Luxembourg [Member] | Expiration Year 2034 [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards by jurisdiction	5.2
Malta [Member] | Indefinite Expiration [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards by jurisdiction	3
United States [Member] | Expiration Year From 2025 - 2036 [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards by jurisdiction	$ 51.5

Pension Benefits and Savings135

Pension Benefits and Savings Plans - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 0.2	$ 0.1	$ 0.1
Employee Savings Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributions by employer towards savings plan	7	6	$ 6.3
Qualified Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Amount of pension related to plan	2.7	3.7
Non- Qualified Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Amount of pension related to plan	$ 2.2	$ 2.2

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
Commitments And Contingencies [Line Items]
Contractual commitments related to its limited partnership investments	$ 129.9
Maximum [Member]
Commitments And Contingencies [Line Items]
Contractual commitments period	13 years

Leases - Schedule of Future Min

Leases - Schedule of Future Minimum Payments Under Non-Cancelable Operating Leases (Detail) $ in Millions	Dec. 31, 2017USD ($)
Leases [Abstract]
2,018	$ 16.2
2,019	13.5
2,020	10.9
2,021	9.5
2,022	8.2
Thereafter	52.8
Total	$ 111.1

Lease - Additional Information

Lease - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Leases [Abstract]
Operating leases expense	$ 21	$ 12.2	$ 14.2

Segment Information - Additiona

Segment Information - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)Segment	Dec. 31, 2016USD ($)
Segment Reporting [Abstract]
Number of reportable segments | Segment	2
Assets associated with trade capital providers | $	$ 731.8	$ 630.4

Segment Information - Revenue a

Segment Information - Revenue and Income (Loss) Before Income Taxes for Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Earned premiums									$ 1,572.3	$ 1,410.8	$ 1,371.9
Net investment income									140	115.1	88.6
Fee and other income									22.5	24.5	22.2
Net realized investment and other gains									39.3	26.1	24.1
Total revenue	$ 455.8	$ 439.2	$ 451	$ 428.1	$ 405.4	$ 416.7	$ 384.3	$ 370.1	1,774.1	1,576.5	1,506.8
Income (loss) before income taxes	$ 12.3	$ (65.9)	$ 50.8	$ 42.7	$ 47	$ 62.2	$ 39.6	$ 33.1	39.9	181.9	177.5
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Net investment income									10.8	3.2	8
Income (loss) before income taxes									(64.2)	(55)	(28)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Income (loss) before income taxes									64.8	210.8	181.4
U.S. Operations [Member]
Segment Reporting Information [Line Items]
Earned premiums									936.6	849.5	815.4
Net investment income									87.2	71.9	52.2
U.S. Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Income (loss) before income taxes									169.4	174.4	124.4
International Operations [Member]
Segment Reporting Information [Line Items]
Earned premiums									635.8	560.9	556.1
Net investment income									32.7	28.7	20.3
International Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Income (loss) before income taxes									(86.7)	51.6	64.4
Run Off Lines [Member]
Segment Reporting Information [Line Items]
Earned premiums									(0.1)	0.4	0.4
Net investment income									9.3	11.3	8.1
Run Off Lines [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Income (loss) before income taxes									$ (17.9)	$ (15.2)	$ (7.4)

Segment Information - Schedule

Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues From External Customers And Long Lived Assets [Line Items]
Total earned premiums	$ 1,572.3	$ 1,410.8	$ 1,371.9
Bermuda [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Total earned premiums	92.1	112.6	103.4
Brazil [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Total earned premiums	49	39.2	43.1
Malta [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Total earned premiums	11.1	2.1	1.9
United Kingdom [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Total earned premiums	489.3	406.9	407.4
United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Total earned premiums	$ 930.8	$ 850	$ 816.1

Segment Information - Identifia

Segment Information - Identifiable Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 8,764	$ 7,205
U.S. Operations [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	4,298.4	3,961.2
International Operations [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	3,553.8	2,356.9
Run Off Lines [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	449.6	537
Corporate and Other [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 462.2	$ 349.9

Segment Information - Schedu143

Segment Information - Schedule of Goodwill and Intangible Assets Net of Accumulated Amortization (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Goodwill	$ 161.4	$ 152.2
Intangible assets, net of accumulated amortization	96.8	67.7
U.S. Operations [Member]
Segment Reporting Information [Line Items]
Goodwill	123.5	123.5
Intangible assets, net of accumulated amortization	1.5	3.7
International Operations [Member]
Segment Reporting Information [Line Items]
Goodwill	37.9	28.7
Intangible assets, net of accumulated amortization	$ 95.3	$ 64

Statutory Accounting Principle

Statutory Accounting Principle - Statutory Capital and Surplus for Principal Operating Subsidiaries (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Bermuda [Member]
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	$ 1,694.4	$ 1,480.1
United Kingdom [Member]
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	330.3	216.6
United States [Member]
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	$ 992.8	$ 886.7

Statutory Accounting Princip145

Statutory Accounting Principles - Statutory Net Income (Loss) for Principal Operating Subsidiaries (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Bermuda [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	$ 37.4	$ 158.2	$ 182.2
United Kingdom [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	(95.5)	1.8	34.8
United States [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	$ 57	$ 99	$ 94.4

Statutory Accounting Princip146

Statutory Accounting Principles - Additional Information (Detail) - USD ($)	1 Months Ended		12 Months Ended
	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statutory Accounting Practices [Line Items]
Cash dividends				$ 0
Argo Re [Member]
Statutory Accounting Practices [Line Items]
Maximum permissible limit for dividend				25.00%
Reduction in total statutory capital				15.00%
Maximum permitted amount of dividends				$ 382,400,000
Cash dividends				0	$ 41,000,000	$ 41,000,000
Rockwood Casualty Insurance Company [Member]
Statutory Accounting Practices [Line Items]
Cash dividends	$ 18,100,000
Rockwood Casualty Insurance Company [Member] | Scenario Forecast [Member]
Statutory Accounting Practices [Line Items]
Maximum permitted amount of dividends			$ 33,200,000
Argonaut Insurance Company [Member]
Statutory Accounting Practices [Line Items]
Value of ordinary dividends	41,600,000
Argonaut Insurance Company [Member] | Scenario Forecast [Member]
Statutory Accounting Practices [Line Items]
Maximum permitted amount of dividends			$ 87,300,000
Colony Insurance Company [Member]
Statutory Accounting Practices [Line Items]
Cash dividends		$ 19,900,000
Dividends, Securities		15,100,000
Value of ordinary dividends		$ 35,000,000
Minimum [Member]
Statutory Accounting Practices [Line Items]
Minimum statutory capital and surplus balance	$ 362,100,000			$ 403,800,000	$ 362,100,000

Insurance Assessments - Additio

Insurance Assessments - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Insurance [Abstract]
Insurance assessments on current insolvencies	$ 4.8	$ 4.7

Transactions with Related Pa148

Transactions with Related Parties - Additional Information (Detail) - Kinetica [Member] $ in Millions	Dec. 31, 2017USD ($)
Schedule Of Other Related Party Transactions [Line Items]
Surety bonds issued	$ 13
Percentage of investment ownership	10.00%

Unaudited Quarterly Financia149

Unaudited Quarterly Financial Data - Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 455.8		$ 439.2		$ 451		$ 428.1		$ 405.4		$ 416.7		$ 384.3		$ 370.1		$ 1,774.1		$ 1,576.5		$ 1,506.8
Net income (loss) before income taxes	12.3		(65.9)		50.8		42.7		47		62.2		39.6		33.1		39.9		181.9		177.5
Net income (loss)	$ 28.9		$ (61.3)		$ 46		$ 36.7		$ 32.9		$ 55.2		$ 30.9		$ 27.7		$ 50.3		$ 146.7		$ 163.2
Net income (loss) per common share :
Basic	$ 0.98	[1]	$ (2.04)	[1]	$ 1.52	[1]	$ 1.22	[1]	$ 1.10	[1]	$ 1.84	[1]	$ 1.03	[1]	$ 0.91	[1]	$ 1.68	[1]	$ 4.86	[1]	$ 5.31
Diluted	$ 0.95	[1]	$ (2.04)	[1]	$ 1.48	[1]	$ 1.19	[1]	$ 1.07	[1]	$ 1.80	[1]	$ 1	[1]	$ 0.89	[1]	$ 1.64	[1]	$ 4.75	[1]	$ 5.20
Comprehensive income (loss)	$ 28.5		$ (44.4)		$ 64.5		$ 50.6		$ 9.4		$ 57.3		$ 59.7		$ 56.5		$ 99.2		$ 182.9		$ 66.6
[1]	Basic and diluted net income per common share are computed independently for each quarter and full year based on the respective average number of common shares outstanding; therefore, the sum of the quarterly net income per common share data may not equal the net income per common share for the year.

Senior Unsecured Fixed Rate 150

Senior Unsecured Fixed Rate Notes - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Equity Method Investments [Line Items]
Junior subordinated debentures principal balance		$ 143,800,000	$ 143,800,000
Senior Unsecured Fixed Rate Notes [Member]
Schedule Of Equity Method Investments [Line Items]
Junior subordinated debentures principal balance	$ 143,750,000
Debt instrument interest rate stated percentage	6.50%
Junior subordinated debentures, Maturity date		Sep. 15, 2042
Initial maturity date		Sep. 15, 2017
Debt Instrument, Redemption Price, Percentage	100.00%

Senior Unsecured Fixed Rate 151

Senior Unsecured Fixed Rate Notes - Schedule Of Unamortized Debt Issuance Costs Deducted From Carrying Value Of Debt Liability (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Principal	$ 143.8	$ 143.8
Less: unamortized debt issuance costs	(4.2)	(4.3)
Senior unsecured fixed rate notes, less unamortized debt issuance costs	$ 139.6	$ 139.5

Senior Unsecured Fixed Rate 152

Senior Unsecured Fixed Rate Notes - Condensed Consolidating Balance Sheet (Detail) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Investments		$ 4,742.9	$ 4,320.3
Cash		176.6	86	$ 121.7	$ 81
Accrued investment income		23.5	20.7
Premiums receivable		598.6	463.8
Reinsurance recoverables		2,093.3	1,385.6
Goodwill and other intangible assets, net		258.2	219.9
Current income taxes receivable, net		1.4	0
Deferred acquisition costs, net		160.4	139.1
Ceded unearned premiums		399.5	302.8
Other assets		309.6	266.8
Intercompany note receivable		0	0
Investments in subsidiaries		0	0
Total assets		8,764	7,205
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses		4,201	3,350.8	3,123.6
Unearned premiums		1,207.7	970
Funds held and ceded reinsurance payable, net		776.7	543.7
Long-term debt		580.7	367.6
Current income taxes payable, net		0	8.1
Deferred tax liabilities, net		31.3	24.1
Accrued underwriting expenses and other liabilities		146.9	148
Due to affiliates		0	0
Total liabilities		6,944.3	5,412.3
Total shareholders' equity		1,819.7	1,792.7	1,668.1	1,646.7
Total liabilities and shareholders' equity		8,764	7,205
Argo Group [Member]
Assets
Investments		0.4	1.9
Cash		0.9	0	0	0
Accrued investment income		0	0
Premiums receivable		0	0
Reinsurance recoverables		0	0
Goodwill and other intangible assets, net		43.2	0
Current income taxes receivable, net		0
Deferred acquisition costs, net		0	0
Ceded unearned premiums		0	0
Other assets		9.2	9
Intercompany note receivable		0	0
Investments in subsidiaries		1,940	1,834.4
Total assets		1,993.7	1,845.3
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses		0	0
Unearned premiums		0	0
Funds held and ceded reinsurance payable, net		0	0
Long-term debt		153.4	28.4
Current income taxes payable, net			0
Deferred tax liabilities, net		0	0
Accrued underwriting expenses and other liabilities		8.9	13.7
Due to affiliates		11.7	10.5
Total liabilities		174	52.6
Total shareholders' equity		1,819.7	1,792.7
Total liabilities and shareholders' equity		1,993.7	1,845.3
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Assets
Investments		3,079	2,833.5
Cash		47.8	53.7	88.8	49.3
Accrued investment income		18	16
Premiums receivable		216.5	204.9
Reinsurance recoverables		1,487.3	1,348.4
Goodwill and other intangible assets, net		124.9	127.1
Current income taxes receivable, net		2.4
Deferred acquisition costs, net		80.7	63.5
Ceded unearned premiums		198.5	168.9
Other assets		171.5	168.7
Intercompany note receivable		50.9	50.2
Investments in subsidiaries		0	0
Total assets		5,477.5	5,034.9
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses		2,483.9	2,322.4
Unearned premiums		704	580
Funds held and ceded reinsurance payable, net		799.4	750.2
Long-term debt		284.5	284.4
Current income taxes payable, net			8.5
Deferred tax liabilities, net		32.5	17.6
Accrued underwriting expenses and other liabilities		95	92
Due to affiliates		(0.4)	1.8
Total liabilities		4,398.9	4,056.9
Total shareholders' equity		1,078.6	978
Total liabilities and shareholders' equity		5,477.5	5,034.9
Other Subsidiaries and Eliminations [Member]
Assets
Investments	[1]	1,663.5	1,484.9
Cash	[1]	127.9	32.3	32.9	31.7
Accrued investment income	[1]	5.5	4.7
Premiums receivable	[1]	382.1	258.9
Reinsurance recoverables	[1]	606	37.2
Goodwill and other intangible assets, net	[1]	90.1	92.8
Current income taxes receivable, net	[1]	(1)
Deferred acquisition costs, net	[1]	79.7	75.6
Ceded unearned premiums	[1]	201	133.9
Other assets	[1]	128.9	89.1
Intercompany note receivable	[1]	(50.9)	(50.2)
Investments in subsidiaries	[1]	0	0
Total assets	[1]	3,232.8	2,159.2
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	[1]	1,717.1	1,028.4
Unearned premiums	[1]	503.7	390
Funds held and ceded reinsurance payable, net	[1]	(22.7)	(206.5)
Long-term debt	[1]	142.8	54.8
Current income taxes payable, net	[1]		(0.4)
Deferred tax liabilities, net	[1]	(1.2)	6.5
Accrued underwriting expenses and other liabilities	[1]	43	42.3
Due to affiliates	[1]	0.4	(1.8)
Total liabilities	[1]	2,383.1	1,313.3
Total shareholders' equity	[1]	849.7	845.9
Total liabilities and shareholders' equity	[1]	3,232.8	2,159.2
Consolidating Adjustments [Member]
Assets
Investments	[2]	0	0
Cash	[2]	0	0	$ 0	$ 0
Accrued investment income	[2]	0	0
Premiums receivable	[2]	0	0
Reinsurance recoverables	[2]	0	0
Goodwill and other intangible assets, net	[2]	0	0
Current income taxes receivable, net	[2]	0
Deferred acquisition costs, net	[2]	0	0
Ceded unearned premiums	[2]	0	0
Other assets	[2]	0	0
Intercompany note receivable	[2]	0	0
Investments in subsidiaries	[2]	(1,940)	(1,834.4)
Total assets	[2]	(1,940)	(1,834.4)
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	[2]	0	0
Unearned premiums	[2]	0	0
Funds held and ceded reinsurance payable, net	[2]	0	0
Long-term debt	[2]	0	0
Current income taxes payable, net	[2]		0
Deferred tax liabilities, net	[2]	0	0
Accrued underwriting expenses and other liabilities	[2]	0	0
Due to affiliates	[2]	(11.7)	(10.5)
Total liabilities	[2]	(11.7)	(10.5)
Total shareholders' equity	[2]	(1,928.3)	(1,823.9)
Total liabilities and shareholders' equity	[2]	$ (1,940)	$ (1,834.4)
[1]	Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations.
[2]	Includes all Argo Group parent company eliminations.

Senior Unsecured Fixed Rate 153

Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Income (Detail) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Premiums and other revenue:
Earned premiums										$ 1,572.3	$ 1,410.8	$ 1,371.9
Net investment income										140	115.1	88.6
Fee and other income										22.5	24.5	22.2
Net realized investment and other gains										39.3	26.1	24.1
Total revenue		$ 455.8	$ 439.2	$ 451	$ 428.1	$ 405.4	$ 416.7	$ 384.3	$ 370.1	1,774.1	1,576.5	1,506.8
Expenses:
Losses and loss adjustment expenses										1,050.2	810.1	766.1
Underwriting, acquisition and insurance expenses										635.4	547	536.7
Interest expense										27.7	19.6	19
Fee and other expense										14.6	22.4	25.8
Foreign currency exchange losses (gains)										6.3	(4.5)	(18.3)
Total expenses										1,734.2	1,394.6	1,329.3
Income before income taxes		12.3	(65.9)	50.8	42.7	47	62.2	39.6	33.1	39.9	181.9	177.5
Provision for income taxes										(10.4)	35.2	14.3
Net (loss) income before equity in earnings of subsidiaries										50.3	146.7	163.2
Equity in undistributed earnings of subsidiaries										0	0	0
Net income		$ 28.9	$ (61.3)	$ 46	$ 36.7	$ 32.9	$ 55.2	$ 30.9	$ 27.7	50.3	146.7	163.2
Argo Group [Member]
Premiums and other revenue:
Earned premiums										0	0	0
Net investment income										(4.5)	37.2	40.1
Fee and other income										0	0	0
Net realized investment and other gains										0.4	0.6	0
Total revenue										(4.1)	37.8	40.1
Expenses:
Losses and loss adjustment expenses										0	0	0
Underwriting, acquisition and insurance expenses										14.3	12.8	19
Interest expense										4.3	1.4	1.5
Fee and other expense										0	0	0
Foreign currency exchange losses (gains)										0.1	0	0
Total expenses										18.7	14.2	20.5
Income before income taxes										(22.8)	23.6	19.6
Provision for income taxes										0	0	0
Net (loss) income before equity in earnings of subsidiaries										(22.8)	23.6	19.6
Equity in undistributed earnings of subsidiaries										73.1	123.1	143.6
Net income										50.3	146.7	163.2
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Premiums and other revenue:
Earned premiums										555.9	497.3	497.3
Net investment income										87.5	78.9	59
Fee and other income										18.8	21.4	19
Net realized investment and other gains										40.8	50	31.8
Total revenue										703	647.6	607.1
Expenses:
Losses and loss adjustment expenses										337.9	290.4	304.2
Underwriting, acquisition and insurance expenses										266.6	204.4	198.4
Interest expense										17.2	15.8	15.3
Fee and other expense										12.4	21.7	23.3
Foreign currency exchange losses (gains)										0.1	0.2	1
Total expenses										634.2	532.5	542.2
Income before income taxes										68.8	115.1	64.9
Provision for income taxes										(0.6)	33.6	12.9
Net (loss) income before equity in earnings of subsidiaries										69.4	81.5	52
Equity in undistributed earnings of subsidiaries										0	0	0
Net income										69.4	81.5	52
Other Subsidiaries and Eliminations [Member]
Premiums and other revenue:
Earned premiums	[1]									1,016.4	913.5	874.6
Net investment income	[1]									57	40	30.5
Fee and other income	[1]									3.7	3.1	3.2
Net realized investment and other gains	[1]									(1.9)	(24.5)	(7.7)
Total revenue	[1]									1,075.2	932.1	900.6
Expenses:
Losses and loss adjustment expenses	[1]									712.3	519.7	461.9
Underwriting, acquisition and insurance expenses	[1]									354.5	329.8	319.3
Interest expense	[1]									6.2	2.4	2.2
Fee and other expense	[1]									2.2	0.7	2.5
Foreign currency exchange losses (gains)	[1]									6.1	(4.7)	(19.3)
Total expenses	[1]									1,081.3	847.9	766.6
Income before income taxes	[1]									(6.1)	84.2	134
Provision for income taxes	[1]									(9.8)	1.6	1.4
Net (loss) income before equity in earnings of subsidiaries	[1]									3.7	82.6	132.6
Equity in undistributed earnings of subsidiaries	[1]									0	0	0
Net income	[1]									3.7	82.6	132.6
Consolidating Adjustments [Member]
Premiums and other revenue:
Earned premiums	[2]									0	0	0
Net investment income	[2]									0	(41)	(41)
Fee and other income	[2]									0	0	0
Net realized investment and other gains	[2]									0	0	0
Total revenue	[2]									0	(41)	(41)
Expenses:
Losses and loss adjustment expenses	[2]									0	0	0
Underwriting, acquisition and insurance expenses	[2]									0	0	0
Interest expense	[2]									0	0	0
Fee and other expense	[2]									0	0	0
Foreign currency exchange losses (gains)	[2]									0	0	0
Total expenses	[2]									0	0	0
Income before income taxes	[2]									0	(41)	(41)
Provision for income taxes	[2]									0	0	0
Net (loss) income before equity in earnings of subsidiaries	[2]									0	(41)	(41)
Equity in undistributed earnings of subsidiaries	[2]									(73.1)	(123.1)	(143.6)
Net income	[2]									$ (73.1)	$ (164.1)	$ (184.6)
[1]	Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations.
[2]	Includes all Argo Group parent company eliminations.

Senior Unsecured Fixed Rate 154

Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Cash Flows (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities		$ 165	$ 182	$ 283.2
Cash flows from investing activities:
Proceeds from sales of investments		1,729.9	1,443.5	967.9
Maturities and mandatory calls of fixed maturity investments		678.3	1,002.7	843.9
Purchases of investments		(2,660.8)	(2,380.5)	(2,034.1)
Change in short-term investments and foreign regulatory deposits		299.5	(195.2)	49.6
Settlements of foreign currency exchange forward contracts		(2.9)	(5.4)	(10.1)
Issuance of intercompany note, net		0		0
Redemption of PXRE Capital Trust V				0
Acquisition of subsidiaries, net of cash		(105.2)	0	0
Purchases of fixed assets and other, net		(60.1)	(10.2)	(10.8)
Cash used in investing activities		(121.3)	(145.1)	(193.6)
Cash flows from financing activities:
Redemption of PXRE Capital Trust V				0
Additional long-term borrowings		125	0	0
Borrowing under the intercompany note		0
Activity under stock incentive plans		1.4	1	1.8
Redemption of trust preferred securities, net		0
Payment on note payable		0
Repurchase of Company's common shares		(45.2)	(47.1)	(29.7)
Payment of cash dividends to common shareholders		(33.2)	(26.6)	(22.7)
Intercompany cash dividend			0
Cash provided by (used in) financing activities		48	(72.7)	(50.6)
Effect of exchange rate changes on cash		(1.1)	0.1	1.7
Change in cash		90.6	(35.7)	40.7
Cash, beginning of year		86	121.7	81
Cash, end of period		176.6	86	121.7
Argo Group [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities		20.8	26.5	32.7
Cash flows from investing activities:
Proceeds from sales of investments		0	0	0
Maturities and mandatory calls of fixed maturity investments		0	0	0
Purchases of investments		0	0	0
Change in short-term investments and foreign regulatory deposits		1.5	(0.9)	0.9
Settlements of foreign currency exchange forward contracts		0.9	0	1.5
Issuance of intercompany note, net		0		0
Redemption of PXRE Capital Trust V				0
Acquisition of subsidiaries, net of cash		(235.3)	0	0
Purchases of fixed assets and other, net		(0.1)	0	3.8
Cash used in investing activities		(233)	(0.9)	6.2
Cash flows from financing activities:
Redemption of PXRE Capital Trust V				(18)
Additional long-term borrowings		125
Borrowing under the intercompany note		120
Activity under stock incentive plans		1.4	1	1.8
Redemption of trust preferred securities, net		0	0	(18)
Payment on note payable		0
Repurchase of Company's common shares		(0.1)	0	0
Payment of cash dividends to common shareholders		(33.2)	(26.6)	(22.7)
Intercompany cash dividend			0
Cash provided by (used in) financing activities		213.1	(25.6)	(38.9)
Effect of exchange rate changes on cash		0	0	0
Change in cash		0.9	0	0
Cash, beginning of year		0	0	0
Cash, end of period		0.9	0	0
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities		149.5	72.3	117
Cash flows from investing activities:
Proceeds from sales of investments		809.5	1,035.9	631.1
Maturities and mandatory calls of fixed maturity investments		483.1	543.2	681.8
Purchases of investments		(1,495.6)	(1,450)	(1,384.5)
Change in short-term investments and foreign regulatory deposits		67.1	(138.1)	14.9
Settlements of foreign currency exchange forward contracts		(8.2)	0	0
Issuance of intercompany note, net		0		7.5
Redemption of PXRE Capital Trust V				18
Acquisition of subsidiaries, net of cash		0
Purchases of fixed assets and other, net		(26.2)	(11.3)	(16.6)
Cash used in investing activities		(170.3)	(20.3)	(47.8)
Cash flows from financing activities:
Redemption of PXRE Capital Trust V				0
Additional long-term borrowings		0
Borrowing under the intercompany note		60
Activity under stock incentive plans		0	0	0
Redemption of trust preferred securities, net		0
Payment on note payable		0
Repurchase of Company's common shares		(45.1)	(47.1)	(29.7)
Payment of cash dividends to common shareholders		0	0	0
Intercompany cash dividend			(40)
Cash provided by (used in) financing activities		14.9	(87.1)	(29.7)
Effect of exchange rate changes on cash		0	0	0
Change in cash		(5.9)	(35.1)	39.5
Cash, beginning of year		53.7	88.8	49.3
Cash, end of period		47.8	53.7	88.8
Other Subsidiaries and Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	[1]	(5.3)	83.2	133.5
Cash flows from investing activities:
Proceeds from sales of investments	[1]	920.4	407.6	336.8
Maturities and mandatory calls of fixed maturity investments	[1]	195.2	459.5	162.1
Purchases of investments	[1]	(1,165.2)	(930.5)	(649.6)
Change in short-term investments and foreign regulatory deposits	[1]	230.9	(56.2)	33.8
Settlements of foreign currency exchange forward contracts	[1]	4.4	(5.4)	(11.6)
Issuance of intercompany note, net	[1]	(120)		(7.5)
Redemption of PXRE Capital Trust V	[1]			(18)
Acquisition of subsidiaries, net of cash	[1]	130.1
Purchases of fixed assets and other, net	[1]	(33.8)	1.1	2
Cash used in investing activities	[1]	162	(123.9)	(152)
Cash flows from financing activities:
Redemption of PXRE Capital Trust V	[1]			18
Additional long-term borrowings	[1]	0
Borrowing under the intercompany note	[1]	(60)
Activity under stock incentive plans	[1]	0	0	0
Redemption of trust preferred securities, net	[1]	0
Payment on note payable	[1]	0
Repurchase of Company's common shares	[1]	0	0	0
Payment of cash dividends to common shareholders	[1]	0	0	0
Intercompany cash dividend	[1]		40
Cash provided by (used in) financing activities	[1]	(60)	40	18
Effect of exchange rate changes on cash	[1]	(1.1)	0.1	1.7
Change in cash	[1]	95.6	(0.6)	1.2
Cash, beginning of year	[1]	32.3	32.9	31.7
Cash, end of period	[1]	127.9	32.3	32.9
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	[2]	0	0	0
Cash flows from investing activities:
Proceeds from sales of investments	[2]	0	0	0
Maturities and mandatory calls of fixed maturity investments	[2]	0	0	0
Purchases of investments	[2]	0	0	0
Change in short-term investments and foreign regulatory deposits	[2]	0	0	0
Settlements of foreign currency exchange forward contracts	[2]	0	0	0
Issuance of intercompany note, net	[2]	120		0
Redemption of PXRE Capital Trust V	[2]			0
Acquisition of subsidiaries, net of cash	[2]	0
Purchases of fixed assets and other, net	[2]	0	0	0
Cash used in investing activities	[2]	120	0	0
Cash flows from financing activities:
Redemption of PXRE Capital Trust V	[2]			0
Additional long-term borrowings	[2]	0
Borrowing under the intercompany note	[2]	(120)
Activity under stock incentive plans	[2]	0	0	0
Redemption of trust preferred securities, net	[2]	0
Payment on note payable	[2]	0
Repurchase of Company's common shares	[2]	0	0	0
Payment of cash dividends to common shareholders	[2]	0	0	0
Intercompany cash dividend	[2]		0
Cash provided by (used in) financing activities	[2]	(120)	0	0
Effect of exchange rate changes on cash	[2]	0	0	0
Change in cash	[2]	0	0	0
Cash, beginning of year	[2]	0	0	0
Cash, end of period	[2]	$ 0	$ 0	$ 0
[1]	Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations.
[2]	Includes all Argo Group parent company eliminations.

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - USD ($) $ in Millions	Feb. 20, 2018	May 03, 2016	Feb. 17, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Subsequent Event [Line Items]
Stock issued during period, value, stock dividend				$ 4.4	$ 0	$ 0
Increase in additional paid in capital due to stock dividend				255.7
Decrease in retained earnings due to stock dividend				$ 260.1
Subsequent Event [Member]
Subsequent Event [Line Items]
Dividends declaration date	Feb. 20, 2018
Dividends payable date	Mar. 23, 2018
Dividend payable, date of record	Mar. 7, 2018
Stock Dividend [Member]
Subsequent Event [Line Items]
Dividends declaration date		May 3, 2016	Feb. 17, 2015
Dividends payable date		Jun. 15, 2016	Mar. 16, 2015
Dividend payable, date of record		Jun. 1, 2016	Mar. 2, 2015
Additional stock issued as dividend		2,735,542	2,554,506
Stock Dividend [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Dividends declaration date	Feb. 20, 2018
Percentage of stock dividend declared	15.00%
Dividends payable date	Mar. 21, 2018
Dividend payable, date of record	Mar. 7, 2018
Additional stock issued as dividend	4,400,000

Subsequent Events - Pro forma B

Subsequent Events - Pro forma Basic and Diluted Net Income Per Common Share(Detail) - $ / shares	3 Months Ended																12 Months Ended
	Dec. 31, 2017	[1]	Sep. 30, 2017	[1]	Jun. 30, 2017	[1]	Mar. 31, 2017	[1]	Dec. 31, 2016	[1]	Sep. 30, 2016	[1]	Jun. 30, 2016	[1]	Mar. 31, 2016	[1]	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Subsequent Event [Line Items]
Basic	$ 0.98		$ (2.04)		$ 1.52		$ 1.22		$ 1.10		$ 1.84		$ 1.03		$ 0.91		$ 1.68	[1]	$ 4.86	[1]	$ 5.31
Diluted	$ 0.95		$ (2.04)		$ 1.48		$ 1.19		$ 1.07		$ 1.80		$ 1		$ 0.89		1.64	[1]	4.75	[1]	5.20
Pro Forma
Subsequent Event [Line Items]
Basic																	1.46		4.23		4.61
Diluted																	$ 1.42		$ 4.13		$ 4.52
[1]	Basic and diluted net income per common share are computed independently for each quarter and full year based on the respective average number of common shares outstanding; therefore, the sum of the quarterly net income per common share data may not equal the net income per common share for the year.

Schedule II - Schedule of Balan

Schedule II - Schedule of Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Short-term investments	$ 368.5	$ 405.5
Investment in subsidiaries	0	0
Cash	176.6	86	$ 121.7	$ 81
Goodwill and other intangible assets, net	258.2	219.9
Other assets	309.6	266.8
Total assets	8,764	7,205
Junior subordinated debentures, Amount	256.6	172.7
Other indebtedness	184.5	55.4
Accrued underwriting expenses and other liabilities	146.9	148
Due to subsidiaries	0	0
Total liabilities	6,944.3	5,412.3
Shareholders' equity	1,819.7	1,792.7	1,668.1	1,646.7
Total liabilities and shareholders' equity	8,764	7,205
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Condensed Financial Statements, Captions [Line Items]
Short-term investments	0.4	1.9
Investment in subsidiaries	1,940	1,834.4
Cash	0.9	0	$ 0	$ 0
Goodwill and other intangible assets, net	43.2	0
Other assets	9.2	9
Total assets	1,993.7	1,845.3
Junior subordinated debentures, Amount	28.4	28.4
Other indebtedness	125	0
Accrued underwriting expenses and other liabilities	8.9	13.7
Due to subsidiaries	11.7	10.5
Total liabilities	174	52.6
Shareholders' equity	1,819.7	1,792.7
Total liabilities and shareholders' equity	$ 1,993.7	$ 1,845.3

Schedule II - Schedule of State

Schedule II - Schedule of Statements of Income (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net investment income									$ 140	$ 115.1	$ 88.6
Net realized investment and other gains									39.3	26.1	24.1
Total revenue	$ 455.8	$ 439.2	$ 451	$ 428.1	$ 405.4	$ 416.7	$ 384.3	$ 370.1	1,774.1	1,576.5	1,506.8
Interest expense									27.7	19.6	19
Other expenses									635.4	547	536.7
Foreign currency exchange losses (gains)									6.3	(4.5)	(18.3)
Net (loss) income before equity in earnings of subsidiaries									50.3	146.7	163.2
Equity in undistributed earnings of subsidiaries									0	0	0
Net income	$ 28.9	$ (61.3)	$ 46	$ 36.7	$ 32.9	$ 55.2	$ 30.9	$ 27.7	50.3	146.7	163.2
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Condensed Financial Statements, Captions [Line Items]
Net investment income									(4.5)	37.2	40.1
Net realized investment and other gains									0.4	0.6	0
Total revenue									(4.1)	37.8	40.1
Interest expense									4.3	1.4	1.5
Other expenses									14.3	12.8	19
Foreign currency exchange losses (gains)									0.1	0	0
Total expenses									18.7	14.2	20.5
Net (loss) income before equity in earnings of subsidiaries									(22.8)	23.6	19.6
Equity in undistributed earnings of subsidiaries									73.1	123.1	143.6
Net income									$ 50.3	$ 146.7	$ 163.2

Schedule II - Schedule of St159

Schedule II - Schedule of Statements of Income (Parenthetical) (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Intercompany dividend	$ 0
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Condensed Financial Statements, Captions [Line Items]
Intercompany dividend		$ 41,000,000	$ 41,000,000

Schedule II - Schedule of St160

Schedule II - Schedule of Statements of Cash Flows (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net income	$ 28.9	$ (61.3)	$ 46	$ 36.7	$ 32.9	$ 55.2	$ 30.9	$ 27.7	$ 50.3	$ 146.7	$ 163.2
Amortization and depreciation									33.8	35.4	38.7
Share-based payments expense									12.3	19.8	29.1
Net realized investment and other gains									(39.3)	(26.1)	(24.1)
Undistributed earnings of subsidiaries									0	0	0
Accrued underwriting expenses									(24.4)	(18.9)	(24.6)
Other, net									(0.6)	(48.9)	0.6
Cash provided by operating activities									165	182	283.2
Change in short-term investments									306.7	(193.9)	46.1
Settlements of foreign currency exchange forward contracts									(2.9)	(5.4)	(10.1)
Acquisition of subsidiaries, net of cash									(105.2)	0	0
Purchases of fixed assets and other, net									(60.1)	(10.2)	(10.8)
Cash used in investing activities									(121.3)	(145.1)	(193.6)
Activity under stock incentive plans									1.4	1	1.8
Redemption of trust preferred securities, net									0
Repurchase of Company's common shares									(45.2)	(47.1)	(29.7)
Payment of cash dividends to common shareholders									(33.2)	(26.6)	(22.7)
Cash provided by (used in) financing activities									48	(72.7)	(50.6)
Change in cash									90.6	(35.7)	40.7
Cash, beginning of year				86				121.7	86	121.7	81
Cash, end of period	176.6				86				176.6	86	121.7
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Condensed Financial Statements, Captions [Line Items]
Net income									50.3	146.7	163.2
Amortization and depreciation									1.3	0.2	0.2
Share-based payments expense									3.3	4.2	8.2
Net realized investment and other gains									(0.4)	(0.6)	0
Undistributed earnings of subsidiaries									(73.1)	(123.1)	(143.6)
Prepaid assets									(0.3)	(0.2)	0.2
Accrued underwriting expenses									(2.3)	0.9	(2.4)
Due to subsidiaries									48.1	(0.6)	12.5
Other, net									(6.1)	(1)	(5.6)
Cash provided by operating activities									20.8	26.5	32.7
Change in short-term investments									1.5	(0.9)	0.9
Settlements of foreign currency exchange forward contracts									0.9	0	1.5
Acquisition of subsidiaries, net of cash									(235.3)	0	0
Purchases of fixed assets and other, net									(0.1)	0	3.8
Cash used in investing activities									(233)	(0.9)	6.2
Additional borrowings									125	0	0
Borrowings under intercompany note payable, net									120	0	0
Activity under stock incentive plans									1.4	1	1.8
Redemption of trust preferred securities, net									0	0	(18)
Repurchase of Company's common shares									(0.1)	0	0
Payment of cash dividends to common shareholders									(33.2)	(26.6)	(22.7)
Cash provided by (used in) financing activities									213.1	(25.6)	(38.9)
Change in cash									0.9	0	0
Cash, beginning of year				$ 0				$ 0	0	0	0
Cash, end of period	$ 0.9				$ 0				$ 0.9	$ 0	$ 0

Schedule III - Supplemental Ins

Schedule III - Supplemental Insurance Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information By Segment [Line Items]
Deferred acquisition costs	$ 160.4	$ 139.1	$ 132.4
Reserves for Losses and Loss Adjustment Expenses	4,201	3,350.8	3,123.6
Unearned Premiums	1,207.7	970	886.7
Earned premiums	1,572.3	1,410.8	1,371.9
Net investment income	140	115.1	88.6
Benefits, Claims and Claim Adjustment Expenses	1,050.2	810.1	766.1
Amortization (Deferral) of Deferred Acquisition Costs	(11.9)	(4.3)	(0.9)
Other Insurance Expenses	647.3	551.3	537.6
Net Premiums Written	1,653.5	1,440.2	1,402.1
U.S. Operations [Member]
Supplementary Insurance Information By Segment [Line Items]
Deferred acquisition costs	80.8	63.5	58.4
Reserves for Losses and Loss Adjustment Expenses	2,196.1	2,028.4	1,888.6
Unearned Premiums	695.1	575.4	501.2
Earned premiums	936.6	849.5	815.4
Net investment income	87.2	71.9	52.2
Benefits, Claims and Claim Adjustment Expenses	528.1	467.5	471.1
Amortization (Deferral) of Deferred Acquisition Costs	(17.3)	(5.1)	1
Other Insurance Expenses	336.4	275.6	258.4
Net Premiums Written	1,031.8	883.5	838.5
International Operations [Member]
Supplementary Insurance Information By Segment [Line Items]
Deferred acquisition costs	79.6	75.6	74
Reserves for Losses and Loss Adjustment Expenses	1,723	1,031.5	928.7
Unearned Premiums	512.6	394.6	385.5
Earned premiums	635.8	560.9	556.1
Net investment income	32.7	28.7	20.3
Benefits, Claims and Claim Adjustment Expenses	504.8	324	286.4
Amortization (Deferral) of Deferred Acquisition Costs	5.4	0.8	(1.9)
Other Insurance Expenses	236.8	210.3	222.3
Net Premiums Written	621.7	556.4	563.2
Run Off Lines [Member]
Supplementary Insurance Information By Segment [Line Items]
Deferred acquisition costs	0	0	0
Reserves for Losses and Loss Adjustment Expenses	281.9	290.9	306.3
Unearned Premiums	0	0	0
Earned premiums	(0.1)	0.4	0.4
Net investment income	9.3	11.3	8.1
Benefits, Claims and Claim Adjustment Expenses	17.3	18.6	8.6
Amortization (Deferral) of Deferred Acquisition Costs	0	0	0
Other Insurance Expenses	8.3	6.9	5.9
Net Premiums Written	0	0.3	0.4
Corporate and Other [Member]
Supplementary Insurance Information By Segment [Line Items]
Deferred acquisition costs	0	0	0
Reserves for Losses and Loss Adjustment Expenses	0	0	0
Unearned Premiums	0	0	0
Earned premiums	0	0	0
Net investment income	10.8	3.2	8
Benefits, Claims and Claim Adjustment Expenses	0	0	0
Amortization (Deferral) of Deferred Acquisition Costs	0	0	0
Other Insurance Expenses	65.8	58.5	51
Net Premiums Written	$ 0	$ 0	$ 0

Schedule V - Valuation and Qual

Schedule V - Valuation and Qualifying Accounts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation And Qualifying Accounts [Abstract]
Valuation allowance for deferred tax asset, Balance at Beginning of Year	$ 23.5	$ 22.8	$ 25.4
Valuation allowance for deferred tax asset, Charged to Cost and Expense	(6.2)	(1)	(1)
Valuation allowance for deferred tax asset, Capital Loss Carryforward	0	0	0
Valuation allowance for deferred tax asset, Net Operating Loss Carryforward	2.7	0	0
Valuation allowance for deferred tax asset, Charged to Other Accounts	0.1	1.7	(1.6)
Valuation allowance for deferred tax asset, Deductions	0	0	0
Valuation allowance for deferred tax asset, Balance at End of Year	$ 20.1	$ 23.5	$ 22.8

Schedule VI - Supplemental Info

Schedule VI - Supplemental Information for Property-Casualty Insurance Companies (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Information For Property Casualty Insurance Underwriters [Abstract]
Deferred acquisition costs	$ 160.4	$ 139.1	$ 132.4
Reserves for losses and loss adjustment expenses	4,201	3,350.8	3,123.6
Unamortized discount in reserves for losses	17.6	19.4	21.5
Unearned premiums	1,207.7	970	886.7
Premiums earned	1,572.3	1,410.8	1,371.9
Net investment income	140	115.1	88.6
Losses and loss adjustment expenses incurred: Current Year	1,058.4	843.4	798.5
Prior accident years	(8.2)	(33.3)	(32.4)
Losses and loss adjustment expenses incurred	1,050.2	810.1	766.1
Amortization (Deferral) of Deferred Acquisition Costs	(11.9)	(4.3)	(0.9)
Paid losses and loss adjustment expenses, net of reinsurance	889.4	716.5	733.5
Gross premiums written	$ 2,697.2	$ 2,164.8	$ 2,012.1