Argo Group International (ARGO-PA) 10-K 26 Feb 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Feb. 21, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
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		33,969,883
Entity Public Float			$ 1,926

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investments:
Fixed maturities available-for-sale, at fair value (cost: 2018 - $3,529.1; 2017 - $3,320.6)	$ 3,460.4	$ 3,343.4
Equity securities, at fair value (cost: 2018 - $310.6; 2017 - $338.2)	354.5	487.4
Other investments (cost: 2018 - $482.0; 2017 - $534.1)	489.8	543.6
Short-term investments, at fair value (cost: 2018 - $482.3; 2017 - $368.5)	482.3	368.5
Total investments	4,787	4,742.9
Cash	139.2	176.6	$ 86
Accrued investment income	27.2	23.5
Premiums receivable	649.9	598.6
Reinsurance recoverables	2,688.3	2,093.3
Goodwill	177	161.4
Intangible assets, net of accumulated amortization	93.5	96.8
Current income taxes receivable, net	8.2	1.4
Deferred acquisition costs, net	167.3	160.4
Ceded unearned premiums	457.7	399.5
Other assets	362.9	309.6
Total assets	9,558.2	8,764
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	4,654.6	4,201	3,350.8
Unearned premiums	1,300.9	1,207.7
Accrued underwriting expenses	141.4	115.3
Ceded reinsurance payable, net	970.5	734
Funds held	37.2	42.7
Senior unsecured fixed rate notes	139.8	139.6
Other indebtedness	183.4	184.5
Junior subordinated debentures	257	256.6
Deferred tax liabilities, net	6.2	31.3
Other liabilities	120.5	31.6
Total liabilities	7,811.5	6,944.3
Commitments and contingencies (Note 17)
Shareholders' equity:
Common shares - $1.00 par, 500,000,000 shares authorized; 45,276,999 and 40,385,309 shares issued at December 31, 2018 and December 31, 2017, respectively	45.3	40.4
Additional paid-in capital	1,372	1,129.1
Treasury shares (11,315,889 and 10,785,007 shares at December 31, 2018 and December 31, 2017, respectively)	(455.1)	(423.4)
Retained earnings	862.6	977
Accumulated other comprehensive (loss) income, net of taxes	(78.1)	96.6
Total shareholders' equity	1,746.7	1,819.7	$ 1,792.7
Total liabilities and shareholders' equity	$ 9,558.2	$ 8,764

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Available-for-sale cost	$ 3,529.1	$ 3,320.6
Equity securities cost	310.6	338.2
Other investments cost	482	534.1
Short-term investments, Cost	$ 482.3	$ 368.5
Common shares, par value	$ 1	$ 1
Common shares, shares authorized	500,000,000	500,000,000
Common shares, shares issued	45,276,999	40,385,309
Treasury shares (in shares)	11,315,889	10,785,007

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Premiums and other revenue:
Earned premiums	$ 1,731.7	$ 1,572.3	$ 1,410.8
Net investment income	133.1	140	115.1
Fee and other income	9	22.5	24.5
Net realized investment gains (losses):
Net realized investment gains	33.1	39.3	26.1
Change in fair value of equity securities	(105.1)	0	0
Net realized investment and other (losses) gains	(72)	39.3	26.1
Total revenue	1,801.8	1,774.1	1,576.5
Expenses:
Losses and loss adjustment expenses	1,040.8	1,050.2	810.1
Underwriting, acquisition and insurance expenses	654.7	635.4	547
Interest expense	31.6	27.7	19.6
Fee and other expense	7.1	14.6	22.4
Foreign currency exchange (gains) losses	(0.1)	6.3	(4.5)
Total expenses	1,734.1	1,734.2	1,394.6
Income before income taxes	67.7	39.9	181.9
Income tax provision (benefit)	4.1	(10.4)	35.2
Net income	$ 63.6	$ 50.3	$ 146.7
Net income per common share:
Basic (in dollars per share)	$ 1.87	$ 1.46	$ 4.23
Diluted (in dollars per share)	1.83	1.42	4.13
Dividend declared per common share (in dollars per share)	$ 1.08	$ 0.94	$ 0.75
Weighted average common shares:
Basic (in shares)	33,922,009	34,457,098	34,691,618
Diluted (in shares)	34,678,781	35,371,644	35,472,779
Net realized investment (losses) gains before other-than-temporary impairment losses	$ (64.4)	$ 41.8	$ 36.3
Other-than-temporary impairment losses recognized in earnings:
Other-than-temporary impairment losses on fixed maturities	(6.6)	(0.8)	(1.7)
Other-than-temporary impairment losses on equity securities	0	(1.7)	(8.5)
Other-than-temporary impairment losses on other invested assets	(1)	0	0
Impairment losses recognized in earnings	$ (7.6)	$ (2.5)	$ (10.2)

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 63.6	$ 50.3	$ 146.7
Other comprehensive income:
Foreign currency translation adjustments	(3.4)	(1.4)	4
Defined benefit pension plans:
Net gain (loss) arising during the year	1.2	1.2	(0.3)
Unrealized (losses) gains on securities:
(Losses) gains arising during the year	(93.9)	105.7	58.1
Reclassification adjustment for loss (gains) included in net income	5.4	(41.6)	(23.2)
Other comprehensive (loss) income before tax	(90.7)	63.9	38.6
Defined benefit pension plans:
Net gain (loss) arising during the year	0.2	0.4	(0.1)
Unrealized (losses) gains on securities:
(Losses) gains arising during the year	(13.5)	28	15.7
Reclassification adjustment for losses (gains) included in net income	0.5	(13.4)	(13.2)
Income tax (benefit) provision related to other comprehensive income	(12.8)	15	2.4
Other comprehensive (loss) income, net of tax	(77.9)	48.9	36.2
Comprehensive (loss) income	$ (14.3)	$ 99.2	$ 182.9

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, 2015	$ 1,668.1	$ 37.1	$ 964.9	$ (331.1)	$ 985.7	$ 11.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	146.7				146.7
Other comprehensive income (loss), net of tax	36.2					36.2
Repurchase of common shares	(47.1)			(47.1)
Activity under stock incentive plans	15.4	0.2	15.2
Retirement of common shares (tax payments on equity compensation)	(2.1)		(2.1)
Deferred tax - share-based payments	0.7		0.7
Employee stock purchase plan	1.4		1.4
Stock Dividend		2.7	143.2		(145.9)
Cash dividend declared - common shares	(26.6)				(26.6)
Ending Balance at Dec. 31, 2016	1,792.7	40	1,123.3	(378.2)	959.9	47.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	50.3				50.3
Other comprehensive income (loss), net of tax	48.9					48.9
Repurchase of common shares	(45.2)			(45.2)
Activity under stock incentive plans	12.4	0.4	12
Retirement of common shares (tax payments on equity compensation)	(8.2)	(0.1)	(8.1)
Employee stock purchase plan	2	0.1	1.9
Cash dividend declared - common shares	(33.2)				(33.2)
Ending Balance at Dec. 31, 2017	1,819.7	40.4	1,129.1	(423.4)	977	96.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adoption of ASU, net of taxes | Accounting Standards Update 2016-01 [Member]					117.5	(117.5)
Cumulative effect of adoption of ASU, net of taxes | Accounting Standards Update 2018-02 [Member]					(20.7)	20.7
Net income	63.6				63.6
Other comprehensive income (loss), net of tax	(77.9)					(77.9)
Repurchase of common shares	(31.7)			(31.7)
Activity under stock incentive plans	15.9	0.6	15.3
Retirement of common shares (tax payments on equity compensation)	(7.4)	(0.1)	(7.3)
Employee stock purchase plan	2		2
Stock Dividend		4.4	232.9		(237.3)
Cash dividend declared - common shares	(37.5)				(37.5)
Ending Balance at Dec. 31, 2018	$ 1,746.7	$ 45.3	$ 1,372	$ (455.1)	$ 862.6	$ (78.1)

Consolidated Statements of Sh_2

Consolidated Statements of Shareholders' Equity (Parenthetical) - $ / shares	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	May 03, 2016
Statement of Stockholders' Equity [Abstract]
Common shares repurchased (in shares)									530,882	756,252	847,111
Repurchase of common shares, weighted average price (in dollars per share)									$ 59.83	$ 59.76	$ 55.64
Stock dividend declared, percent	15.00%								15.00%		10.00%	10.00%
Cash dividend declared - common shares, per share (in dollars per share)	$ 0.27	$ 0.27	$ 0.27	$ 0.27	$ 0.24	$ 0.24	$ 0.23	$ 0.23	$ 1.08	$ 0.94	$ 0.75

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net income	$ 63.6	$ 50.3	$ 146.7
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Amortization and depreciation	31.9	33.8	35.4
Share-based payments expense	18.3	12.3	19.8
Deferred income tax benefit, net	(12.6)	(17.9)	(1.1)
Net realized investment (loss) gains	72	(39.3)	(26.1)
Undistributed earnings from alternative investment portfolio	(19.8)	(49.5)	(23.9)
Loss (gains) on disposals of fixed assets, net	0.3	2.1	(0.1)
Change in:
Accrued investment income	(3.5)	(2.7)	0.9
Receivables	(557.5)	(602.7)	(318)
Deferred acquisition costs	(7.3)	(11.5)	(6.4)
Ceded unearned premiums	(47.6)	(4.2)	(51.2)
Reserves for losses and loss adjustment expenses	338.3	653.9	220.2
Unearned premiums	76.4	85.5	80.1
Ceded reinsurance payable and funds held	226.5	88.8	153.6
Income taxes	(6.1)	(8.9)	19.9
Accrued underwriting expenses	15.2	(24.4)	(18.9)
Other, net	113.2	(0.6)	(48.9)
Cash provided by operating activities	301.3	165	182
Cash flows from investing activities:
Sales of fixed maturity investments	1,259.1	1,433.3	1,138.4
Maturities and mandatory calls of fixed maturity investments	418.6	678.3	1,002.7
Sales of equity securities	238.9	201.1	208.3
Sales of other investments	101.8	95.5	96.8
Purchases of fixed maturity investments	(1,936)	(2,464.1)	(2,137.3)
Purchases of equity securities	(170.5)	(157.7)	(146.9)
Purchases of other investments	(42.6)	(39)	(96.3)
Change in foreign regulatory deposits and voluntary pools	13	(7.2)	(1.3)
Change in short-term investments	(132.2)	306.7	(193.9)
Settlements of foreign currency exchange forward contracts	(1.5)	(2.9)	(5.4)
Acquisition of Maybrooke, net of cash acquired		(105.2)
Cash acquired with acquisition of Ariscom	15.6
Purchases of fixed assets	(32.2)	(30.6)	(32.1)
Other, net	(0.3)	(29.5)	21.9
Cash used in investing activities	(268.3)	(121.3)	(145.1)
Cash flows from financing activities:
Additional long-term borrowings	0	125	0
Activity under stock incentive plans	1.6	1.4	1
Repurchase of Company's common shares	(31.7)	(45.2)	(47.1)
Payment of cash dividends to common shareholders	(37.5)	(33.2)	(26.6)
Cash (used in) provided by financing activities	(67.6)	48	(72.7)
Effect of exchange rate changes on cash	(2.8)	(1.1)	0.1
Change in cash	(37.4)	90.6	(35.7)
Cash, beginning of year	176.6	86	121.7
Cash, end of period	139.2	176.6	86
Maybrooke Holdings S A [Member]
Cash flows from investing activities:
Acquisition of Maybrooke, net of cash acquired	0	(105.2)	0
Ariscom Compagnia Di Assicurazioni S P A [Member]
Cash flows from investing activities:
Cash acquired with acquisition of Ariscom	$ 15.6	$ 0	$ 0

Business and Significant Accoun

Business and Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Business and Significant Accounting Policies	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 U.S., Inc. (“Argo Group U.S.”) 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 March 5, 2018, we acquired 100% of the capital stock of Ariscom Compagnia di Assicurazioni S.p.A. (“Ariscom”) upon its release from extraordinary administration by the Italian insurance supervisory authority (“IVASS”). The acquisition is being accounted for in accordance with Accounting Standards Codification (“ASC”) 805, “Business Combinations.” See Note 2, “Acquisition of Ariscom,” for additional discussion regarding the acquisition. The Consolidated Financial Statements as of and for the year ended December 31, 2018 and the Notes to the Consolidated Financial Statements reflect the consolidated results of Argo Group and Ariscom commencing on the date of acquisition. We conduct our ongoing business through two segments. U.S. Operations is comprised of the Excess and Surplus Lines businesses focusing on the U.S.-based risks that the standard, admitted insurance market is unwilling or unable to write, and through other specialized admitted and non-admitted business distributed through retail, wholesale, and managing general brokers/agents in the specialty insurance market. Excess and Surplus Lines products are underwritten by Colony Insurance Company (“Colony”). The other U.S. specialized admitted and non-admitted businesses consist of the following operations: Argo Insurance, Rockwood Casualty Insurance Company (“Rockwood”), Argo Pro, Argo Surety, U.S. Specialty Programs, Inland Marine and Trident Insurance Services. International Operations is comprised of the Lloyd's Syndicate platform (Syndicate 1200 and Syndicate 1910), Argo Insurance Bermuda, Continental Europe and Latin America. Syndicate 1200 and Syndicate 1910 insurance and reinsurance products are underwritten by Argo Underwriting Agency Limited based in London, under the Lloyd’s of London (“Lloyd’s”) global franchise. The additional International Operations business include Argo Insurance Bermuda, ArgoGlobal SE in Malta, ArgoGlobal Assicurazioni S.p.A (formerly Ariscom) in Italy, and Argo Seguros in Brazil. These businesses provide a broad range of commercial property, casualty, professional liability and specialty coverages in a number of countries and jurisdictions outside the United States. The International Specialty businesses include Reinsurance, Argo Insurance Bermuda, ArgoGlobal SE, Argo Seguros business in Brazil and ArgoGlobal Assicurazioni in Italy. 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. Our Run-off Lines segment includes liabilities associated with other liability policies that were issued in the 1960s, 1970s and into the 1980s, as well as the former risk management business and other business no longer underwritten. 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 both Syndicate 1200 and 1910, 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. As a result of the stock dividend, 4,397,520 additional shares were issued. Cash was paid in lieu of fractional shares of our common shares. Excluding repurchased shares, all references to common shares and related per share amounts in this document and related disclosures have been adjusted to reflect the stock dividend for all periods presented. 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. Excluding repurchased shares, all references to common shares and related 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, 2018 and 2017 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 are considered available-for-sale and are carried at fair value. As available-for-sale investments, changes in the fair value fixed maturities 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, 2018 , 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. Our investments in equity securities are carried at fair value. Beginning with the adoption of Accounting Standards Update ("ASU") 2016-01, effective January 1, 2018, the changes in the fair value of equity securities are now included in "Net realized investment (losses) gains" in consolidated statements of income. See "Recently Issued Accounting Pronouncements" below for further information about ASU 2016-01 and the related impact on our consolidated financial statements. 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 $4.0 million and $3.2 million at December 31, 2018 and 2017 , 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 $1.8 million and $2.1 million at December 31, 2018 and 2017 , 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) 2018 2017 2016 Premiums receivable $ 2.1 $ 1.5 $ 1.1 Reinsurance recoverables — — — Net written off $ 2.1 $ 1.5 $ 1.1 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 o f premiums under retrospectively rated policies is included in “Unearned premiums” in our Consolidated Balance Sheets and was $6.9 million and $7.9 million at December 31, 2018 and 2017 , respectively. The estimated amount included in premiums receivables for additional premiums due under retrospectively rated policies was $0.4 million and $0.1 million at December 31, 2018 an d 2017 , 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 2018 and 2017 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. We perform our goodwill impairment test on the first day of the fourth quarter of each year, or October 1 of each year. As a result of the reviews performed on each of the entity’s reporting units for the three years ended December 31, 2018, 2017 and 2016, the Company determined that the estimated fair value substantially exceeded the respective carrying value of its reporting units for those years and goodwill was not impaired. In 2018, we recorded $15.6 million of goodwill as a result of the acquisition of Ariscom. This goodwill is included in the International Operations Segment. See Note 2, “Acquisition of Ariscom” for further discussion. Other indefinite-lived intangible assets and intangible assets with finite lives were also reviewed for impairment as of October 1, 2018. As a result of the reviews performed on each of the entity’s reporting units for the three years ended December 31, 2018, 2017 and 2016, the Company determined that the other indefinite-lived intangible assets and finite-lived intangible assets were not impaired. The following table presents our intangible assets and accumulated amortization at December 31: December 31, 2018 December 31, 2017 (in millions) Gross Carrying Accumulated Gross Carrying Accumulated Lloyd's capacity $ 89.0 n/a $ 89.0 n/a Distribution network 50.2 47.0 50.2 44.1 Additional Lloyd's capacity 4.8 4.8 4.8 4.8 Other 3.3 2.0 3.3 1.6 $ 147.3 $ 53.8 $ 147.3 $ 50.5 The remaining weighted average useful life by category at December 31, 2018 was 9.7 years for the distribution network and 6.3 years for other. As of December 31, 2018, the additional Lloyd's capacity was fully amortized. The remaining weighted average useful life for all assets that have not yet been fully amortized was 9.5 years at December 31, 2018 . During the years ended December 31, 2018 , 2017 and 2016 , amortization expense was $3.3 million , $5.9 million and $5.5 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, 2019, 2020, 2021 and 2022 is $1.7 million , $1.3 million , $1.3 million and $0.2 million , respectively. As of December 31, 2018, we have no estimated amortization expense after the year ended December 31, 2022. 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 $146.0 million and $124.1 million at December 31, 2018 and 2017 , respectively. The net book value of our property and equipment at December 31, 2018 and 2017 was $154.8 million and $151.0 million , respectively. The depreciation expense for the years ended December 31, 2018 , 2017 and 2016 was $24.5 million , $24.1 million and $20.9 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 assets” at December 31, 2018 and 2017 , respectively. The realized and unrealized gains and losses are included in “Net realized investment and other (losses) 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 three 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 income (loss). Translation gains and losses related to our operations in Brazil, Malta and Italy are recorded as a component of shareholders’ equity in our Consolidated Balance Sheets. At December 31, 2018 and 2017 , the foreign currency translation adjustments were a loss of $22.4 million and $19.0 million , respectively. Income Taxes On December 22, 2017, the Tax Cuts and Jobs Act (“TCJA”) was enacted in the United States. 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 Interest paid and income taxes paid (recovered) were as follows: For the Years Ended December 31, (in millions) 2018 2017 2016 Senior unsecured fixed rate notes $ 9.3 $ 9.3 $ 9.3 Junior subordinated debentures 15.5 12.6 7.7 Other indebtedness 6.5 4.9 2.3 Revolving credit facility — 0.3 — Total interest paid $ 31.3 $ 27.1 $ 19.3 Income taxes paid 24.8 16.5 16.6 Income taxes recovered — (2.5 ) (0.5 ) Income taxes paid, net $ 24.8 $ 14.0 $ 16.1 Recently Issued Accounting Pronouncements On August 28, 2018, the Financial Accounting Standards Board (“FASB”) issued ASU 2018-13, "Fair Value Measurement (Topic 820)." ASU 2018-13 eliminates, adds and modifies certain disclosure requirements on fair value measurements. The guidance is effective for fiscal years beginning after December 15, 2019, including interim periods within the year of adoption. The amendments on changes in unrealized gains and losses, the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements, and the narrative description of measurement uncertainty are applied prospectively for only the most recent interim or annual period presented in the initial fiscal year adoption. All other amendments are applied retrospectively to all periods presented upon their effective date. Early adoption is permitted. We are currently in the process of evaluating the impact that the adoption of the ASU will have on our financial disclosures. On February 14, 2018, the FASB issued ASU 2018-02, “Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income” that allows a reclassification from accumulated other comprehensive income ("AOCI") to retained earnings of the stranded tax effects in AOCI resulting from the Tax Cuts and Jobs Act (“TCJA”). Current guidance required the effect of a change in tax laws or rates on deferred tax balances to be reported in income from continuing operations in the accounting period that includes the period of enactment, even if the related income tax effects were originally charged or credited directly to AOCI. The amount of the reclassification would include the effect of the change in the U.S. federal corporate income tax rate on the gross deferred tax amounts and related valuation allowances, if any, at the date of the enactment of TCJA related to items in AOCI. The updated guidance is effective for reporting periods beginning after December 15, 2018 and is to be applied retrospectively to each period in which the effect of the TCJA related to items remaining in AOCI are recognized or at the beginning of the period of adoption. Early adoption is permitted, including adoption in any interim period for public business entities for reporting periods for which financial statements have not yet been issued. We have adopted the guidance effective January 1, 2018. The adoption of this ASU does not affect the Company’s results of operations, financial position, or liquidity. As a result of adopting this ASU, we reclassified $20.7 million of previously recognized deferred taxes from accumulated other comprehensive income into retained earnings as of January 1, 2018. 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 2

Acquisition of Ariscom

Acquisition of Ariscom	12 Months Ended
	Dec. 31, 2018
Business Combinations [Abstract]
Acquisition of Ariscom	Acquisition of Ariscom Effective March 5, 2018, we acquired 100% of the capital stock of Ariscom upon its release from extraordinary administration by IVASS. We injected an amount of capital into Ariscom necessary to meet certain regulatory requirements and thresholds. As part of this capital infusion, we have become the sole shareholder of Ariscom. 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 2018. Provisional fair values were recorded in the Company’s interim consolidated financial statements and notes for the periods ending March 31, June 30 and September 30, 2018. Ariscom’s financial position, results of operations, and cash flows were not material to our consolidated financial results as of and for the year ended December 31, 2018. The excess of the purchase price over the fair value of the net assets acquired was $15.6 million and has been allocated to goodwill. The goodwill is included in the International Operations segment in Note 19, “Segment Information”. Goodwill is not deductible for tax purposes. The acquisition provides Argo Group with an in-market Italian insurance platform and access to Ariscom’s client network throughout Italy, with longer-term opportunities to expand our presence in continental Europe, particularly Spain and Portugal. There were no identifiable intangible assets recognized as part of the acquisition.

Investments

Investments	12 Months Ended
	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Investments	Investments Included in “Total investments” in our Consolidated Balance Sheets at December 31, 2018 and December 31, 2017 is $133.4 million and $130.8 million , respectively, of assets managed on behalf of the trade capital providers, who are third-party participants that provide underwriting capital to the operations of Syndicate 1200. Fixed Maturities The amortized cost, gross unrealized gains, gross unrealized losses and fair value in fixed maturity investments were as follows: December 31, 2018 (in millions) Amortized Gross Gross Fair Fixed maturities U.S. Governments $ 240.9 $ 0.2 $ 4.9 $ 236.2 Foreign Governments 224.1 0.5 7.8 216.8 Obligations of states and political subdivisions 236.7 4.3 1.2 239.8 Corporate bonds 1,808.7 7.5 58.7 1,757.5 Commercial mortgage-backed securities 205.3 0.7 3.2 202.8 Residential mortgage-backed securities 413.1 3.4 5.7 410.8 Asset-backed securities 173.6 0.4 1.2 172.8 Collateralized loan obligations 226.7 0.5 3.5 223.7 Total fixed maturities 3,529.1 17.5 86.2 3,460.4 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 Contractual Maturity The amortized cost and fair values of fixed maturity investments as of December 31, 2018 , by contractual maturity, were as follows: (in millions) Amortized Cost Fair Value Due in one year or less $ 270.5 $ 266.3 Due after one year through five years 1,517.0 1,486.2 Due after five years through ten years 581.5 557.1 Thereafter 141.4 140.7 Structured securities 1,018.7 1,010.1 Total $ 3,529.1 $ 3,460.4 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 and unfunded investment commitments of the other invested assets portfolio as of December 31, 2018 and 2017 were as follows: December 31, 2018 (in millions) Carrying Value Unfunded Commitments Investment Type Hedge funds $ 120.6 $ — Private equity 211.8 120.5 Long only funds 153.0 — Other 4.4 — Total other investments $ 489.8 $ 120.5 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 4.3 — Total other invested assets $ 543.6 $ 129.9 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 and funds that primarily own investment-grade corporate and sovereign fixed income securities. • Other: Other includes participation in investment pools. Unrealized Losses and Other-than-temporary Impairments An aging of unrealized losses on our investments in fixed maturities is presented below: December 31, 2018 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 $ 28.2 $ 0.2 $ 173.0 $ 4.7 $ 201.2 $ 4.9 Foreign Governments 73.4 3.6 125.0 4.2 198.4 7.8 Obligations of states and political subdivisions 53.3 0.6 25.3 0.6 78.6 1.2 Corporate bonds 964.3 45.7 440.8 13.0 1,405.1 58.7 Commercial mortgage-backed securities 48.5 0.6 90.6 2.6 139.1 3.2 Residential mortgage-backed securities 63.5 0.7 176.1 5.0 239.6 5.7 Asset-backed securities 73.6 0.6 64.2 0.6 137.8 1.2 Collateralized loan obligations 209.5 3.3 10.3 0.2 219.8 3.5 Total fixed maturities 1,514.3 55.3 1,105.3 30.9 2,619.6 86.2 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 (1) 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 (1) 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. We also recognize other-than-temporary losses on fixed maturity securities that we intend to sell. Effective January 1, 2018, the Company adopted ASU 2016-1. As a result, changes in the fair value of equity securities are recognized in net realized investment gains in the Consolidated Statement of Income. We hold a total of 9,241 securities, of which 3,113 were in an unrealized loss position for less than one year and 1,267 were in an unrealized loss position for a period one year or greater as of December 31, 2018 . 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-than-temporarily impaired or not, we also considered that we do not intend to sell the investments and it is unlikely that we will be required to sell the investments before recovery of their amortized cost bases, which may be maturity. We do not consider these investments to be other-than-temporarily impaired at December 31, 2018 . We recognized other-than-temporary losses on our fixed maturities and equity portfolios as follows: For the Years Ended December 31, (in millions) 2018 2017 2016 Other-than-temporary impairment: Obligations of states and political subdivisions $ — $ (0.1 ) $ — Corporate bonds (6.6 ) (0.7 ) (1.7 ) Equity securities — (1.7 ) (8.5 ) Other invested assets (1.0 ) — — Other-than-temporary impairment losses $ (7.6 ) $ (2.5 ) $ (10.2 ) Net Investment Income and Realized Gains and Losses Investment income and expenses were as follows: For the Years Ended December 31, (in millions) 2018 2017 2016 Investment income: Interest on fixed maturities $ 115.0 $ 97.1 $ 88.9 Dividends on equity securities 12.5 13.9 15.6 Income on alternative investments 19.8 49.5 29.9 Income on short-term and other investments 9.5 7.7 0.4 Investment income 156.8 168.2 134.8 Investment expenses (23.7 ) (28.2 ) (19.7 ) Net investment income $ 133.1 $ 140.0 $ 115.1 Net Realized Investment Gains and Losses The following table presents our gross realized investment gains (losses): For the Years Ended December 31, (in millions) 2018 2017 2016 Realized gains on fixed maturities and other Fixed maturities $ 17.7 $ 25.7 $ 21.5 Other investments 41.4 25.7 47.5 Short-term investments 0.2 0.7 0.5 Other assets — — 1.2 59.3 52.1 70.7 Realized losses on fixed maturities and other Fixed maturities (16.0 ) (20.0 ) (35.9 ) Other investments (39.5 ) (36.2 ) (46.0 ) Short-term investments (0.5 ) (0.2 ) (0.3 ) Other Assets — — — Other-than-temporary impairment losses on fixed maturities (6.6 ) (0.8 ) (1.7 ) Other-than-temporary impairment losses on other assets (1.0 ) — — (63.6 ) (57.2 ) (83.9 ) Equity securities (1) Net realized gains on equity securities 37.4 46.1 47.8 Other-than-temporary impairment losses on equity securities — (1.7 ) (8.5 ) Change in unrealized (losses) gains on equity securities held at the end of the period (105.1 ) — — Net realized (losses) gains on equity securities (67.7 ) 44.4 39.3 Net realized investment and other (losses) gains (72.0 ) 39.3 26.1 Income tax expense (11.2 ) (12.0 ) (11.6 ) After tax $ (83.2 ) $ 27.3 $ 14.5 (1) Effective January 1, 2018, we adopted ASU 2016-1. As a result, unrealized gains (losses) at the date of adoption have been reclassified from accumulated other comprehensive income to retained earnings. Additionally, all changes in the fair value of equity securities are recognized in net realized investment gains (losses). Prior periods have not been restated to conform to the current presentation. See Note 2, "Recently Issued Accounting Pronouncements." The cost of securities sold is based on the specific identification method. Changes in unrealized appreciation (depreciation) related to investments are summarized as follows: For the Years Ended December 31, (in millions) 2018 2017 2016 Change in unrealized (losses) gains Fixed maturities $ (88.1 ) $ 25.4 $ 36.5 Equity securities — 36.6 (2.6 ) Other investments 0.1 2.1 0.6 Short-term investments (0.5 ) — 0.4 Net unrealized investment (losses) gains before income taxes (88.5 ) 64.1 34.9 Income tax benefit (expense) 13.0 (14.6 ) (2.4 ) Net unrealized investment (losses) gains, net of income taxes $ (75.5 ) $ 49.5 $ 32.5 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, 2018 and 2017 , 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, 2018 December 31, 2017 Operational currency exposure $ 4.4 $ (0.2 ) Asset manager investment exposure (0.3 ) (0.9 ) Total return strategy (1.5 ) 0.7 Total $ 2.6 $ (0.4 ) 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) 2018 2017 2016 Realized gains Operational currency exposure 9.7 12.4 10.9 Asset manager investment exposure 5.8 1.5 9.0 Total return strategy 26.7 10.4 25.6 Gross realized investment gains 42.2 24.3 45.5 Realized losses Operational currency exposure (7.9 ) (13.8 ) (18.0 ) Asset manager investment exposure (3.0 ) (11.3 ) (4.5 ) Total return strategy (28.6 ) (7.6 ) (21.0 ) Gross realized investment losses (39.5 ) (32.7 ) (43.5 ) Net realized investment gains (losses) on foreign currency exchange forward contracts $ 2.7 $ (8.4 ) $ 2.0 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, 2018 December 31, 2017 Securities on deposit for regulatory and other purposes $ 172.6 $ 175.3 Securities pledged as collateral for letters of credit 120.9 78.1 Securities and cash on deposit supporting Lloyd’s business 376.8 355.5 Total restricted investments $ 670.3 $ 608.9 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, 2018 and 2017 . 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 term loans that are 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 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 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 2018 or 2017 . Based on an analysis of the inputs, our financial assets measured at fair value on a recurring basis have been categorized as follows: Fair Value Measurements at Reporting Date Using (in millions) December 31, 2018 Level 1 (a) Level 2 (b) Level 3 (c) Fixed maturities U.S. Governments $ 236.2 $ 226.7 $ 9.5 $ — Foreign Governments 216.8 — 216.8 — Obligations of states and political subdivisions 239.8 — 239.8 — Corporate bonds 1,757.5 — 1,755.3 2.2 Commercial mortgage-backed securities 202.8 — 202.8 — Residential mortgage-backed securities 410.8 — 410.8 — Asset-backed securities 172.8 — 172.8 — Collateralized loan obligations 223.7 — 223.7 — Total fixed maturities 3,460.4 226.7 3,231.5 2.2 Equity securities 354.5 346.3 — 8.2 Other investments 114.4 — 114.4 — Short-term investments 482.3 453.9 28.4 — $ 4,411.6 $ 1,026.9 $ 3,374.3 $ 10.4 (a) Quoted prices in active markets for identical asset (b) Significant other observable inputs (c) Significant unobservable inputs Fair Value Measurements at Reporting Date Using (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 asset (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, 2018 $ 1.9 $ 2.3 $ 4.2 Transfers into Level 3 — — — Transfers out of Level 3 — — — Total gains or losses (realized/unrealized): Included in net income (loss) — 0.2 0.2 Included in other comprehensive income (loss) 0.3 — 0.3 Purchases, issuances, sales, and settlements: Purchases — 7.3 7.3 Issuances — — — Sales — (1.6 ) (1.6 ) Settlements — — — Ending balance, Ending balance, December 31, 2018 $ 2.2 $ 8.2 $ 10.4 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, 2018 $ — $ — $ — (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, 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 $ — $ — $ — At December 31, 2018 and 2017 , 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, 2018
Insurance [Abstract]
Reinsurance	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 $1.8 million and $2.1 million as of December 31, 2018 and 2017 , respectively. 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 $944.0 million and $907.7 million at December 31, 2018 and 2017 , 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 $596.6 million and $380.3 million as of December 31, 2018 and 2017 , 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 $1,040.8 million , $1,050.2 million and $810.1 million for the years ended December 31, 2018 , 2017 and 2016 , respectively, are net of amounts ceded to reinsurers of $888.9 million , $992.6 million and $419.3 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) 2018 2017 2016 Direct written premiums $ 2,293.8 $ 2,029.2 $ 1,792.5 Reinsurance ceded to other companies (1,189.7 ) (1,043.7 ) (724.6 ) Reinsurance assumed from other companies 661.4 668.0 372.3 Net written premiums $ 1,765.5 $ 1,653.5 $ 1,440.2 Direct earned premiums $ 2,201.9 $ 1,912.2 $ 1,722.8 Reinsurance ceded to other companies (1,137.9 ) (1,033.6 ) (675.8 ) Reinsurance assumed from other companies 667.7 693.7 363.8 Net earned premiums $ 1,731.7 $ 1,572.3 $ 1,410.8 Percentage of reinsurance assumed to net earned premiums 38.6 % 44.1 % 25.8 %

Reserves for Losses and Loss Ad

Reserves for Losses and Loss Adjustment Expenses	12 Months Ended
	Dec. 31, 2018
Insurance [Abstract]
Reserves for Losses and Loss Adjustment Expenses	Reserves for Losses and Loss Adjustment Expenses The following table provides a reconciliation of reserves for losses and loss adjustment expenses (“LAE”): For the Years Ended December 31, (in millions) 2018 2017 2016 Net reserves beginning of the year $ 2,488.0 $ 2,180.2 $ 2,133.3 Net Maybrooke reserves acquired — 131.8 Net Ariscom reserves acquired 43.4 — — Add: Losses and LAE incurred during current calendar year, net of reinsurance: Current accident year 1,058.8 1,058.4 843.4 Prior accident years (18.0 ) (8.2 ) (33.3 ) Losses and LAE incurred during calendar year, net of reinsurance 1,040.8 1,050.2 810.1 Deduct: Losses and LAE payments made during current calendar year, net of reinsurance: Current accident year 273.3 289.6 178.9 Prior accident years 665.6 599.8 537.6 Losses and LAE payments made during current calendar year, net of reinsurance: 938.9 889.4 716.5 Change in participation interest (1) (25.5 ) (23.2 ) (36.3 ) Foreign exchange adjustments (44.9 ) 38.4 (10.4 ) Net reserves - end of year 2,562.9 2,488.0 2,180.2 Add: Reinsurance recoverables on unpaid losses and LAE, end of year 2,091.7 1,713.0 1,170.6 Gross reserves - end of year $ 4,654.6 $ 4,201.0 $ 3,350.8 (1) Amount represents (decrease) increase in reserves due to change in our Syndicate 1200 and Syndicate 1910 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 Years Ended December 31, (in millions) 2018 2017 2016 U.S. Operations $ (20.8 ) $ (38.7 ) $ (35.9 ) International Operations (9.5 ) 13.2 (16.0 ) Run-off Lines 12.3 17.3 18.6 Total unfavorable (favorable) prior-year development $ (18.0 ) $ (8.2 ) $ (33.3 ) The following describes the primary factors behind each segment’s prior accident year reserve development for the years ended December 31, 2018 , 2017 and 2016 : Year ended December 31, 2018 : • U.S. Operations: Favorable development in general liability and surety lines, partially offset by unfavorable development in commercial multi-peril lines. • International Operations: Favorable development in property partially offset by unfavorable development within specialty and liability lines. • Run-off Lines: Unfavorable development in liability lines as well as asbestos and environmental. 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. 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 have 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 Specialty Admitted, while International Operations' primary operating divisions are Syndicate 1200, Reinsurance, and Argo Insurance Bermuda. Each of these 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. Specialty Admitted This Specialty Admitted 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. Argo Insurance Bermuda Argo Insurance Bermuda offers casualty, property and professional lines, which serves the needs of global clients by providing the following coverages: property, general and products liability, directors and officers liability, errors and omissions liability and employment practices liability. Reinsurance The Reinsurance division operates in two areas - treaty property and specialty. This business is focused on mainly North American commercial properties and writes on both a primary and excess basis. Business is written on an open market basis through retail and wholesale brokers. Treaty property reinsurance is predominantly catastrophe-focused. Specialty reinsurance encompasses marine, energy, aviation, terrorism and property. This reinsurance portfolio is focused on treaties where high-quality exposure and experience data allow our underwriters to quantify the risk. 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. 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 Argo Bermuda Insurance 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 Argo Bermuda Insurance 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 Reinsurance 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 the significant lines of business for each operating division, as disclosed in the following tables, 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 Specialty Admitted • Liability: workers compensation, general liability, auto liability, and various public entity liability risks • Professional: management liability and errors and omissions liability • Specialty: surety and inland marine Argo Insurance Bermuda • Liability: long-tail excess casualty and general liability Reinsurance • Property: property catastrophe reinsurance and excess property direct and facultative insurance 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 insurance, North American and international binders, and residential collateral protection for lending institutions • Specialty: personal accident, aviation, cargo, yachts, and onshore and offshore marine 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 6% 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 ten 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 of Ariel Re 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. Beginning with the 2017 calendar year, we began accumulating such claims information by accident year and line of business, and have included such in the tabular disclosures below for the Reinsurance operating division, Property line of business disaggregation category. Accordingly, calendar years prior to 2017 for the aforementioned tabular disclosures relate only to our Reinsurance business prior to the acquisition of Ariel Re. Foreign Currency Portions of the business we write in the Syndicate 1200, Argo Bermuda Insurance and Reinsurance divisions are denominated in foreign currencies. We have used the December 31, 2018 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 used 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, 2018 . 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, 2018 . 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 Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ 202.9 $ 206.0 $ 205.8 $ 200.0 $ 193.5 $ 192.8 $ 189.0 $ 187.8 2012 189.6 196.0 189.7 183.6 184.4 182.1 182.3 2013 217.9 222.6 224.3 227.2 220.4 216.0 2014 213.0 215.2 213.2 211.9 212.3 2015 232.5 237.1 228.6 226.4 2016 246.4 250.6 243.1 2017 253.3 244.3 2018 278.8 Total $ 1,791.0 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ 17.6 $ 53.8 $ 91.0 $ 122.9 $ 146.6 $ 162.4 $ 170.0 $ 174.3 2012 17.2 52.8 89.1 120.8 142.4 157.5 163.4 2013 17.6 60.2 100.4 135.2 163.7 179.6 2014 15.0 52.2 95.9 131.6 154.5 2015 16.5 51.9 91.4 131.5 2016 17.4 52.8 95.5 2017 11.5 38.7 2018 15.0 Total $ 952.5 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance 54.7 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 893.2 As of December 31, 2018 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ 187.8 $ 10.0 8,454 2012 182.3 11.0 7,389 2013 216.0 21.0 7,297 2014 212.3 36.2 6,502 2015 226.4 60.3 6,000 2016 243.1 100.3 5,548 2017 244.3 153.3 5,302 2018 278.8 227.9 4,026 (1) Information presented for calendar years prior to 2018 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: Specialty Admitted Line of Business: Liability (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ 140.3 $ 155.1 $ 159.0 $ 157.5 $ 158.2 $ 154.0 $ 153.7 $ 154.0 2012 140.3 146.3 149.7 153.3 151.5 147.7 146.7 2013 126.6 133.2 136.7 133.2 131.1 130.6 2014 115.6 121.9 116.9 114.5 111.5 2015 107.3 106.7 101.7 102.3 2016 96.1 99.9 99.3 2017 121.5 129.5 2018 147.3 Total $ 1,021.2 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ 23.2 $ 57.5 $ 85.9 $ 111.3 $ 126.1 $ 135.1 $ 139.8 $ 143.1 2012 20.1 51.0 80.7 105.8 120.8 127.9 131.8 2013 18.9 49.4 74.0 93.6 102.8 109.7 2014 17.4 38.8 58.7 75.3 86.1 2015 17.2 35.0 48.8 64.2 2016 11.1 31.7 48.6 2017 16.3 44.4 2018 19.4 Total $ 647.3 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance 48.7 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 422.6 As of December 31, 2018 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ 154.0 $ 6.3 28,160 2012 146.7 8.0 23,627 2013 130.6 10.3 18,948 2014 111.5 13.9 16,316 2015 102.3 21.1 14,530 2016 99.3 22.1 11,583 2017 129.5 50.2 12,897 2018 147.3 82.8 13,351 (1) Information presented for calendar years prior to 2018 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: Specialty Admitted Line of Business: Professional (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ 35.0 $ 35.0 $ 35.0 $ 32.5 $ 28.2 $ 26.9 26.6 $ 26.0 2012 27.8 28.3 28.6 25.8 24.0 24.5 24.9 2013 20.9 21.5 21.1 19.0 19.8 19.5 2014 22.4 22.4 26.0 33.7 36.2 2015 29.9 29.5 33.2 34.0 2016 44.2 44.8 45.1 2017 60.1 61.8 2018 70.8 Total $ 318.3 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ 3.2 $ 11.8 $ 17.8 $ 22.0 $ 24.0 $ 25.4 $ 25.7 $ 25.8 2012 2.3 8.6 16.9 19.9 21.4 22.6 23.5 2013 1.9 6.3 10.9 14.2 17.6 17.5 2014 2.3 5.4 15.1 24.1 25.5 2015 1.8 8.3 15.6 20.8 2016 2.4 11.9 24.6 2017 3.5 24.9 2018 4.5 Total $ 167.1 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance 0.1 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 151.3 As of December 31, 2018 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ 26.0 $ 0.3 820 2012 24.9 0.4 640 2013 19.5 1.2 620 2014 36.2 2.8 1,043 2015 34.0 2.1 1,816 2016 45.1 13.1 3,226 2017 61.8 15.9 3,669 2018 70.8 55.0 3,603 (1) Information presented for calendar years prior to 2018 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: Specialty Admitted Line of Business: Specialty (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ 0.2 $ 3.9 $ 3.4 $ 3.4 $ 3.6 $ 2.6 $ 2.0 $ 1.7 2012 7.5 6.7 4.9 4.3 4.0 3.9 3.5 2013 10.0 8.6 4.6 2.5 1.7 0.9 2014 13.1 13.1 8.9 6.0 4.8 2015 14.8 14.3 9.5 5.5 2016 15.0 15.0 11.2 2017 16.2 16.2 2018 20.8 Total $ 64.6 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ — $ 1.6 $ 1.4 $ 1.3 $ 1.2 $ 1.7 $ 1.7 $ 1.7 2012 3.6 3.3 3.3 3.3 3.3 3.4 3.3 2013 0.4 0.9 0.9 0.9 0.9 0.9 2014 1.1 3.3 4.0 4.0 4.1 2015 0.2 0.1 0.2 0.3 2016 1.3 1.6 2.2 2017 0.3 0.1 2018 — Total $ 12.6 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance — Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 52.0 As of December 31, 2018 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ 1.7 $ — 80 2012 3.5 0.2 129 2013 0.9 — 50 2014 4.8 0.8 50 2015 5.5 2.9 24 2016 11.2 5.5 58 2017 16.2 16.2 84 2018 20.8 19.8 71 (1) Information presented for calendar years prior to 2018 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: Reinsurance Line of Business: Property (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ 116.3 $ 105.0 $ 107.1 $ 105.1 $ 104.4 $ 104.3 $ 132.3 $ 138.9 2012 47.2 51.3 50.3 51.6 46.4 62.7 66.9 2013 32.1 33.6 33.1 31.4 33.0 32.4 2014 26.5 26.5 24.2 35.3 38.1 2015 27.1 23.3 32.6 27.1 2016 43.7 175.5 169.1 2017 157.8 158.0 2018 67.6 Total $ 698.1 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ 40.9 $ 66.5 $ 87.7 $ 95.5 $ 97.8 $ 99.0 $ 127.2 $ 134.1 2012 12.4 31.1 40.6 49.6 44.0 58.7 64.6 2013 4.2 16.7 26.5 29.0 31.0 31.1 2014 2.8 12.7 18.3 36.4 36.9 2015 4.3 11.0 20.3 20.6 2016 13.6 142.3 153.0 2017 84.2 139.5 2018 24.7 Total $ 604.5 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance 3.6 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 97.2 As of December 31, 2018 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ 138.9 $ 0.3 459 2012 66.9 0.2 277 2013 32.4 0.3 219 2014 38.1 0.5 221 2015 27.1 3.2 219 2016 169.1 11.0 381 2017 158.0 (28.8 ) 774 2018 67.6 4.2 391 (1) Information presented for calendar years prior to 2018 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: Argo Insurance Bermuda Line of Business: Liability (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ 6.6 $ 6.6 $ 6.6 $ 4.4 $ 2.2 $ 1.6 $ 1.0 $ — 2012 7.4 7.4 7.4 5.6 4.4 1.7 — 2013 8.5 8.5 8.5 8.5 4.9 2.2 2014 9.8 9.8 9.8 6.2 1.5 2015 11.3 14.3 24.8 35.4 2016 13.9 14.0 14.0 2017 17.1 17.3 2018 8.9 Total $ 79.3 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ — $ — $ — $ — $ — $ — $ — $ — 2012 — — — — — — — 2013 — — — — 2.3 2.3 2014 — — 0.1 0.1 1.2 2015 — — 16.1 20.3 2016 — — — 2017 — 3.3 2018 — Total $ 27.1 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance — Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 52.2 As of December 31, 2018 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ — $ — 1,423 2012 — — 1,381 2013 2.2 — 1,190 2014 1.5 0.2 1,329 2015 35.4 1.0 1,567 2016 14.0 13.9 1,874 2017 17.3 12.2 1,900 2018 8.9 8.8 839 (1) Information presented for calendar years prior to 2018 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 Year For the Years Ended December 31, 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2010 $ 5.7 $ 4.8 $ 4.4 $ 6.1 $ 5.9 $ 5.8 $ 5.7 $ 5.8 $ 5.7 2011 8.0 8.6 10.8 10.9 10.2 10.1 10.6 10.7 2012 8.7 10.8 14.9 14.2 13.8 14.5 15.0 2013 22.7 26.7 26.2 24.4 24.3 25.1 2014 37.1 35.9 33.6 32.5 33.8 2015 34.2 29.6 29.2 29.6 2016 25.5 26.4 26.1 2017 24.5 23.3 2018 21.4 Total $ 190.7 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2010 $ — $ 0.2 $ 0.6 $ 1.1 $ 2.0 $ 2.8 $ 3.7 $ 4.0 $ 4.3 2011 0.2 0.8 1.6 3.4 5.4 6.9 7.8 8.7 2012 0.4 1.1 2.6 5.8 8.2 10.1 11.8 2013 1.5 3.2 7.0 11.3 15.6 19.7 2014 1.9 4.5 9.8 13.6 20.2 2015 0.8 5.1 7.4 12.3 2016 1.8 5.7 10.6 2017 1.9 6.6 2018 2.2 Total $ 96.4 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 94.3 As of December 31, 2018 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims 2010 $ 5.7 $ 0.1 2011 10.7 0.2 2012 15.0 0.6 2013 25.1 2.5 2014 33.8 5.0 2015 29.6 8.2 2016 26.1 10.2 2017 23.3 15.2 2018 21.4 17.6 (1) Information presented for calendar years prior to 2018 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 Year For the Years Ended December 31, 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2010 $ 15.5 $ 12.8 $ 12.7 $ 11.2 $ 10.1 $ 9.4 $ 9.5 $ 9.5 $ 9.7 2011 19.1 21.0 18.5 15.5 14.7 14.7 15.1 15.7 2012 13.9 13.8 14.1 14.0 14.0 14.9 15.4 2013 22.5 22.4 22.6 22.2 22.3 23.1 2014 34.5 35.7 35.9 39.0 40.9 2015 37.8 37.3 38.5 38.7 2016 32.8 26.9 26.4 2017 24.5 22.0 2018 21.2 Total $ 213.1 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2010 $ 0.1 $ 0.9 $ 2.0 $ 3.0 $ 3.8 $ 5.1 $ 6.5 $ 6.8 $ 7.6 2011 1.0 2.5 4.1 6.6 8.3 10.6 11.3 12.8 2012 0.6 1.8 4.3 5.7 8.1 9.5 11.2 2013 1.7 3.7 7.0 11.6 15.4 17.6 2014 1.6 6.3 14.6 23.7 27.9 2015 2.2 8.2 14.8 19.7 2016 2.0 5.8 10.6 2017 1.2 5.1 2018 1.0 Total $ 113.5 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 99.6 As of December 31, 2018 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims 2010 $ 9.7 $ 0.1 2011 15.7 0.4 2012 15.4 0.8 2013 23.1 3.0 2014 40.9 6.5 2015 38.7 11.3 2016 26.4 10.8 2017 22.0 14.6 2018 21.2 17.6 (1) Information presented for calendar years prior to 2018 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 Year For the Years Ended December 31, 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2010 $ 50.7 $ 58.2 $ 54.6 $ 54.2 $ 51.5 $ 51.2 $ 50.7 $ 51.4 $ 51.1 20

Run-off Lines

Run-off Lines	12 Months Ended
	Dec. 31, 2018
Discontinued Operations and Disposal Groups [Abstract]
Run-off Lines	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) 2018 2017 Asbestos and Environmental: Reinsurance assumed $ 27.7 $ 29.3 Other 27.1 26.6 Total Asbestos and Environmental 54.8 55.9 Risk management 197.0 219.6 Run-off reinsurance lines 1.6 1.8 Other run-off lines 12.2 4.6 Gross reserves - Run-off Lines $ 265.6 $ 281.9 We have received asbestos and environmental liability claims arising from other liability coverage primarily written in the 1960s, 1970s and into the early 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) 2018 2017 2016 Direct written Case reserves $ 2.7 $ 2.1 $ 2.8 Unallocated loss adjustment expense ("ULAE") 0.5 0.5 0.5 Incurred but not reported ("IBNR") 19.1 18.8 12.1 Total direct written reserves 22.3 21.4 15.4 Assumed domestic Case reserves 8.7 9.8 10.5 ULAE 0.8 0.8 0.8 IBNR 12.0 13.7 10.5 Total assumed domestic reserves 21.5 24.3 21.8 Assumed London Case reserves 1.5 2.3 3.3 ULAE — — — IBNR 1.5 0.6 1.4 Total assumed London reserves 3.0 2.9 4.7 Total asbestos reserves $ 46.8 $ 48.6 $ 41.9 The following table presents our underwriting losses for Run-off Lines: For the Years Ended December 31, (in millions) 2018 2017 2016 Asbestos and Environmental: Reinsurance assumed $ (3.9 ) $ (8.7 ) $ (1.3 ) Other (4.1 ) (6.7 ) (9.6 ) Total Asbestos and Environmental (8.0 ) (15.4 ) (10.9 ) Risk management (2.6 ) (8.8 ) (13.1 ) Run-off reinsurance lines — (0.1 ) 0.1 Other run-off lines (5.3 ) (1.4 ) (1.2 ) Total underwriting loss - Run-off Lines $ (15.9 ) $ (25.7 ) $ (25.1 ) 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 using 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, 2018
Debt Disclosure [Abstract]
Junior Subordinated Debentures	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, 2018 and 2017 , 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: December 31, 2018 (in millions) Issue Date Trust Preferred Pools Maturity Rate Structure Interest Rate at Amount Argo Group 5/15/2003 PXRE Capital Statutory Trust II 5/15/2033 3M LIBOR + 4.10% 6.72% $ 18.1 11/6/2003 PXRE Capital Trust VI 9/30/2033 3M LIBOR + 3.90% 6.70% 10.3 Argo Group US 5/15/2003 Argonaut Group Statutory Trust I 5/15/2033 3M LIBOR + 4.10% 6.72% 15.5 12/16/2003 Argonaut Group Statutory Trust III 1/8/2034 3M LIBOR + 4.10% 6.54% 12.3 4/29/2004 Argonaut Group Statutory Trust IV 4/29/2034 3M LIBOR + 3.85% 6.47% 13.4 5/26/2004 Argonaut Group Statutory Trust V 5/24/2034 3M LIBOR + 3.85% 6.54% 12.3 5/12/2004 Argonaut Group Statutory Trust VI 5/12/2034 3M LIBOR + 3.80% 6.59% 13.4 9/17/2004 Argonaut Group Statutory Trust VII 12/15/2034 3M LIBOR + 3.60% 6.39% 15.5 9/22/2004 Argonaut Group Statutory Trust VIII 9/22/2034 3M LIBOR + 3.55% 6.37% 15.5 10/22/2004 Argonaut Group Statutory Trust IX 12/15/2034 3M LIBOR + 3.60% 6.39% 15.5 9/14/2005 Argonaut Group Statutory Trust X 9/15/2035 3M LIBOR + 3.40% 6.19% 30.9 Total Outstanding $ 172.7 December 31, 2017 (in millions) Issue Date Trust Preferred Pools Maturity Rate Structure Interest Rate at Amount Argo Group 5/15/2003 PXRE Capital Statutory Trust II 5/15/2033 3M LIBOR + 4.10% 5.52% $ 18.1 11/6/2003 PXRE Capital Trust VI 9/30/2033 3M LIBOR + 3.90% 5.59% 10.3 Argo Group US 5/15/2003 Argonaut Group Statutory Trust I 5/15/2033 3M LIBOR + 4.10% 5.52% 15.5 12/16/2003 Argonaut Group Statutory Trust III 1/8/2034 3M LIBOR + 4.10% 5.46% 12.3 4/29/2004 Argonaut Group Statutory Trust IV 4/29/2034 3M LIBOR + 3.85% 5.27% 13.4 5/26/2004 Argonaut Group Statutory Trust V 5/24/2034 3M LIBOR + 3.85% 5.30% 12.3 5/12/2004 Argonaut Group Statutory Trust VI 5/12/2034 3M LIBOR + 3.80% 5.40% 13.4 9/17/2004 Argonaut Group Statutory Trust VII 12/15/2034 3M LIBOR + 3.60% 5.19% 15.5 9/22/2004 Argonaut Group Statutory Trust VIII 9/22/2034 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 9/14/2005 Argonaut Group Statutory Trust X 9/15/2035 3M LIBOR + 3.40% 4.99% 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 2018, the acquired debt was ultimately assigned to Argo Re and is carried on our consolidated balance sheet at $84.3 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 . At December 31, 2018 , 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 is presented below: December 31, 2018 (in millions) Issue Date Maturity Rate Structure Interest Rate at Principal at Carrying Value at 9/15/2007 9/15/2037 3 month LIBOR + 3.15% 5.94 % $ 91.8 $ 84.3 December 31, 2017 (in millions) Issue Date Maturity Rate Structure Interest Rate at Principal at Carrying Value at 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, 2018
Debt Disclosure [Abstract]
Other Indebtedness	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, 2018 December 31, 2017 Floating rate loan stock $ 57.8 $ 58.9 Term loan 125.0 125.0 Other debt 0.6 0.6 Total other indebtedness $ 183.4 $ 184.5 Floating Rate Loan Stock This debt was assumed through the acquisition of Argo Underwriting Agency, Ltd. These notes are unsecured. At December 31, 2018 and 2017 , 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, 2018 and 2017 is presented below: December 31, 2018 (in millions) Currency Issue Date Maturity Rate Structure Interest Rate at Amount U.S. Dollar 12/8/2004 11/15/2034 6 month LIBOR + 4.2% 6.72% $ 6.5 U.S. Dollar 10/31/2006 1/15/2036 6 month LIBOR + 4.0% 6.52% 10.0 Total U.S. Dollar notes 16.5 Euro 9/6/2005 8/22/2035 3 month LIBOR + 4.0% 3.68% 13.8 Euro 10/31/2006 11/24/2036 3 month LIBOR + 4.0% 3.68% 12.0 Euro 6/8/2007 9/15/2037 3 month LIBOR + 3.9% 3.59% 15.5 Total Euro notes 41.3 Total notes outstanding $ 57.8 December 31, 2017 (in millions) Currency Issue Date Maturity Rate Structure Interest Rate at Amount U.S. Dollar 12/8/2004 11/15/2034 6 month LIBOR + 4.2% 5.66% $ 6.5 U.S. Dollar 10/31/2006 1/15/2036 6 month LIBOR + 4.0% 5.46% 10.0 Total U.S. Dollar notes 16.5 Euro 9/6/2005 8/22/2035 3 month LIBOR + 4.0% 3.67% 14.1 Euro 10/31/2006 11/22/2036 3 month LIBOR + 4.0% 3.67% 12.4 Euro 6/8/2007 9/15/2037 3 month LIBOR + 3.9% 3.57% 15.9 Total Euro notes 42.4 Total notes outstanding $ 58.9 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 $41.3 million and $42.4 million as of December 31, 2018 and 2017 , respectively. The foreign currency translation adjustment is recorded in our Consolidated Statements of Income. Borrowing Under Revolving Credit Facility On November 2, 2018, each of Argo Group, Argo Group U.S., Inc., Argo International Holdings Limited, and Argo Underwriting Agency Limited (the “Borrowers”) entered into a new $325 million credit agreement (the "New Credit Agreement") with JPMorgan Chase Bank, N.A., as administrative agent. The New Credit Agreement replaced the prior $325 million Credit Agreement (the "Prior Agreement"), dated as of March 3, 2017. In connection with the consummation of the New Credit Agreement, Argo Group International Holdings, Ltd. borrowed $125 million as a term loan due on November 2, 2021, which amount was used on November 2, 2018 to pay off in its entirety the $125 million of borrowings previously outstanding under the Prior Agreement. In addition, the New Credit Agreement provides for a $200 million revolving credit facility, and the commitments thereunder shall expire on November 2, 2023 unless extended in accordance with the terms of the New Credit Agreement. 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, 2018 and December 31, 2017 is presented below: December 31, 2018 (in millions) Issue Date Maturity Rate Structure Interest Rate at Amount 11/2/2018 11/2/2021 3 month LIBOR + 1.25% 3.86% $ 125.0 December 31, 2017 (in millions) Issue Date Maturity Rate Structure Interest Rate at 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 November 2, 2018, the $0.5 million LOC outstanding under the Prior Agreement was transferred to the New Credit Agreement. At December 31, 2018 and 2017, there were no borrowings outstanding under the revolving portions of the credit facilities. At December 31, 2018 and 2017, there was $0.5 million in LOCs against the New and Prior Agreement, respectively. Other Debt Argo Re has entered into two secured, bilateral committed letter of credit facilities with commercial banks to issue LOCs in support of its non-admitted reinsurance obligations. These facilities have a term of one year and include customary conditions and event of default provisions. The availability of letters of credit under these secured facilities are subject to a borrowing base requirement, determined on the basis of specified percentages of the market value of eligible categories of securities pledged to the lender. Argo Re has also used LOCs issued from commercial banks on a secured, uncommitted basis in order to satisfy these requirements. On December 31, 2018, reinsurance LOCs totaling $99.7 million were outstanding, of which $80.7 million were issued against the secured bilateral LOC facilities and $19.0 million were issued by a commercial bank on an uncommitted basis. Collateral with a market value of $120.9 million was pledged to these banks as security against these LOCs. See Note 3, “Investments,” for additional information. In November 2018 Argo Group executed an LOC facility with a commercial bank to issue LOCs in favor of Lloyd’s to support its Funds at Lloyd's ("FAL") requirements. This facility has a term of one year , is unsecured, and includes customary conditions and event of default provisions. During 2018, an LOC in the amount of £23.3 million was issued in favor of Lloyd’s, which allowed the company to reduce its other collateral pledged to Lloyd’s by a comparable amount. See Note 3, “Investments,” for additional information. As part of the ARIS Title Insurance Corporation (“ARIS”) acquisition, at December 31, 2018 and 2017 , 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, 2018
Fair Value Disclosures [Abstract]
Disclosures about Fair Value of Financial Instruments	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, 2018 and 2017 , 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, 2018 December 31, 2017 (in millions) Carrying Amount Fair Value Carrying Amount Fair Value Junior subordinated debentures: Trust preferred debentures $ 172.7 $ 163.2 $ 172.7 $ 172.9 Subordinated debentures acquired with Maybrooke 84.3 85.0 83.9 85.0 Total junior subordinated debentures 257.0 248.2 256.6 257.9 Senior unsecured fixed rate notes 139.8 139.5 139.6 141.2 Floating rate loan stock 57.8 54.5 58.9 59.0

Shareholders' Equity

Shareholders' Equity	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Shareholders' Equity	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. As a result of the stock dividend, 4,397,520 additional shares were issued. Cash was paid in lieu of fractional shares of our common shares. Excluding repurchased shares, all references to common shares and related per share amounts in this document and related disclosures have been adjusted to reflect the stock dividend for all periods presented. 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. Excluding repurchased shares, all references to common shares and related per share amounts in this document and related disclosures have been adjusted to reflect the stock dividend for all periods presented. During 2018 , 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, 2018 , we paid cash dividends totaling $37.5 million to our shareholders. During 2017 , our Board of Directors declared quarterly cash dividends totaling $0.94 on each share of common stock outstanding. 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 $0.75 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. We are authorized to issue 30 million shares of $1.00 par value preferred shares. As of December 31, 2018 and 2017 , 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, 2018 , availability under the 2016 Repurchase Authorization for future repurchases of our common shares was $53.3 million . For the year ended December 31, 2018 and 2017 , we repurchased 530,882 common shares and 756,252 common shares, respectively for $31.7 million and $45.2 million , respectively. A summary of common shares repurchased for the twelve months ended December 31, 2018 is shown below: Repurchase Type Date 2018 Purchase Period Number of Average Price Total Cost Repurchase 10b5-1 Trading Plan 12/18/2017 01/02/2018-02/16/2018 225,281 $ 59.31 $ 13.3 2016 10b5-1 Trading Plan 6/18/2018 06/22/2018- 08/07/2018 75,661 $ 59.30 4.5 2016 10b5-1 Trading Plan 9/18/2018 09/19/2018-10/24/2018 37,385 $ 59.75 2.2 2016 Open Market N/A 02/20/2018-03/02/2018 89,305 $ 59.03 5.3 2016 Open Market N/A 08/13/2018-09/17/2018 103,250 $ 62.07 6.4 2016 Total 530,882 $ 59.83 $ 31.7

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income (Loss)	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Accumulated Other Comprehensive Income (Loss)	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 Currency Translation Adjustments Unrealized Defined Benefit Pension Plans Total Balance, January 1, 2017 $ (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 Other comprehensive income (loss) before reclassifications (3.4 ) (80.4 ) 1.0 (82.8 ) Amounts reclassified from accumulated other comprehensive income (loss) — 4.9 — 4.9 Net current-period other comprehensive income (loss) (3.4 ) (75.5 ) 1.0 (77.9 ) Cumulative effect of adoption of ASU 2016-01 — (117.5 ) — (117.5 ) Cumulative effect of adoption of ASU 2018-02 — 22.1 (1.4 ) 20.7 Balance, December 31, 2018 $ (22.4 ) $ (49.0 ) $ (6.7 ) $ (78.1 ) 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) 2018 2017 2016 Unrealized gains and losses on securities: Net realized investment losses (gains) $ 5.4 $ (41.6 ) $ (23.2 ) Provision (benefit) for income taxes (0.5 ) 13.4 13.2 Net of taxes $ 4.9 $ (28.2 ) $ (10.0 )

Net Income Per Common Share

Net Income Per Common Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Net Income Per Common Share	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) 2018 2017 2016 Net income $ 63.6 $ 50.3 $ 146.7 Weighted average common shares outstanding - basic 33,922,009 34,457,098 34,691,618 Effect of dilutive securities: Equity compensation awards 756,772 914,546 781,161 Weighted average common shares outstanding - diluted 34,678,781 35,371,644 35,472,779 Net income per common share: Basic $ 1.87 $ 1.46 $ 4.23 Diluted $ 1.83 $ 1.42 $ 4.13 Excluded from the weighted average common shares outstanding calculation at December 31, 2018 , 2017 and 2016 are 11,315,889 shares, 10,785,007 shares and 10,028,755 shares, respectively, which are held as treasury shares. The shares are excluded as of their repurchase date. In 2018, 2017 and 2016, there were no anti-dilutive shares of common stock to be excluded from the computation of diluted net income per common share.

Share-based Compensation

Share-based Compensation	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-based Compensation	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, 2018 2017 2016 Risk-free rate of return 2.61% to 2.96% 1.83% to 2.22% 1.00% to 2.02% Expected dividend yields 1.71% to 1.87% 1.63% to 1.72% 1.62% to 1.70% Expected award life (years) 4.48 to 4.49 4.48 to 4.49 4.50 to 4.61 Expected volatility 17.82% to 18.44% 18.13% to 18.70% 18.73% to 19.70% All outstanding awards were adjusted to reflect the 15% stock dividend in 2018, resulting in a 15% increase to the number of awards outstanding and an 13.04% reduction in the exercise price. 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 $18.3 million ( $16.5 million , net of tax), $12.3 million ( $10.3 million , net of tax) and $19.8 million ( $15.5 million , net of tax) for the years ended December 31, 2018 , 2017 and 2016 , 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 operating 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, 2018 and changes during the year then ended is as follows: Shares Weighted-Average Grant Date Fair Value Outstanding at January 1, 2018 767,140 $ 42.91 Granted 458,715 $ 51.84 Vested and issued (211,170 ) $ 43.35 Expired or forfeited (117,680 ) $ 46.32 Outstanding at December 31, 2018 897,005 $ 46.92 As of December 31, 2018 , there was $27.8 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 1.8 years. The total fair value of shares vested during the year ended December 31, 2018 was $9.2 million . A summary of stock-settled SARs activity as of December 31, 2018 and changes during the year then ended is as follows: Shares Weighted-Average Exercise Price Outstanding at January 1, 2018 1,494,458 $ 30.85 Exercised (665,875 ) $ 27.00 Expired or forfeited (17,824 ) $ 36.51 Outstanding at December 31, 2018 810,759 $ 33.88 Vested or expected to vest as of end of year 781,124 $ 33.73 Exercisable at end of year 618,935 $ 32.65 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, 2018 , 665,875 stock-settled SARs were exercised resulting in 372,705 shares being issued. As of December 31, 2018 , there was $0.3 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 1.5 years. Aggregate intrinsic value of the stock-settled SARs at December 31, 2018 was $27.1 million . The remaining weighted average contractual term at December 31, 2018 was 2.48 years . A summary of cash-settled SARs activity as of December 31, 2018 and changes during the year then ended is as follows: Shares Weighted-Average Exercise Price Outstanding at January 1, 2018 189,568 $ 28.95 Exercised (128,572 ) $ 28.31 Expired or forfeited (2,568 ) $ 21.23 Outstanding at December 31, 2018 58,428 $ 30.71 As of December 31, 2018 , 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, 2018 , 128,572 cash-settled SARs were exercised resulting in $4.3 million in cash payments. Aggregate intrinsic value of the cash-settled SARs at December 31, 2018 was $2.1 million . The liability for cash-settled SARs was $2.1 million and $5.7 million at December 31, 2018 and 2017 , respectively. Included in the total shares outstanding at December 31, 2018 are 307,335 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. 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, 2018 , 2017 and 2016 was $0.4 million , $0.4 million and $0.3 million , respectively.

Underwriting, Acquisition and I

Underwriting, Acquisition and Insurance Expenses	12 Months Ended
	Dec. 31, 2018
Underwriting Acquisition And Insurance Expenses [Abstract]
Underwriting, Acquisition and Insurance Expenses	Underwriting, Acquisition and Insurance Expenses Underwriting, acquisition and insurance expenses were as follows: For the Years Ended December 31, (in millions) 2018 2017 2016 Commissions $ 280.4 $ 255.9 $ 234.0 General expenses 351.7 359.1 292.1 Premium taxes, boards and bureaus 32.5 32.3 25.2 664.6 647.3 551.3 Net deferral of policy acquisition costs (9.9 ) (11.9 ) (4.3 ) Total underwriting, acquisition and insurance expenses $ 654.7 $ 635.4 $ 547.0

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	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. As such, these subsidiaries are now subject to the minimum BEAT computation imposed by the TCJA on this underwriting business. Relief, exclusive of any BEAT, 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, Italy, 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. In 2017, the SEC issued Staff Accounting Bulletin (“SAB”) No. 118 which permitted the recording of provisional amounts related to the impact of the U.S. Tax Cuts and Jobs Act of 2017 (the “TCJA”) during a measurement period not to exceed one year from the enactment date of the TCJA. In 2017 and 2018 we recorded provisional amounts for certain enactment-date effects of the TCJA by applying guidance in SAB 118 because we had not yet completed the enactment-date accounting for these effects. Upon further analysis of the TCJA and notices and regulations issued and proposed by the United States Department of Treasury and the Internal Revenue Service, we finalized our calculations and completed our accounting for the enactment-date income tax effects of the TCJA in December 2018. Our accounting for these items is now complete. Current period adjustments related to the provisional items were immaterial. 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) 2018 2017 2016 Current income tax (benefit) provision related to: United States $ 13.4 $ (0.3 ) $ 30.4 United Kingdom 3.2 7.6 3.9 Other jurisdictions 0.1 0.2 2.0 Total current income tax provision 16.7 7.5 36.3 Deferred income tax (benefit) provision related to: United States (13.8 ) (0.3 ) 3.3 United Kingdom 1.1 (17.6 ) (4.4 ) Other jurisdictions 0.1 — — Total deferred income tax (benefit) provision (12.6 ) (17.9 ) (1.1 ) Income tax (benefit) provision $ 4.1 $ (10.4 ) $ 35.2 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, 2018 , 2017 and 2016 , pre-tax income (loss) attributable to our operations and the operations’ effective tax rates were as follows: (in millions) 2018 2017 2016 Pre-Tax Income (Loss) Effective Tax Rate Pre-Tax Income (Loss) Effective Tax Rate Pre-Tax Income (Loss) Effective Tax Rate Bermuda $ 26.0 — % $ 30.2 — % $ 99.6 — % United States 13.9 (5.5 )% 67.5 (0.9 )% 115.2 29.2 % United Kingdom 25.0 18.8 % (53.8 ) 18.7 % (34.8 ) 1.2 % Belgium — (1) — % (3) 0.1 75.0 % (0.1 ) 23.2 % Brazil (0.5 ) — % 0.8 — % 0.6 — % United Arab Emirates 0.8 — % 0.2 — % — (1) — % Ireland (2) (0.2 ) — % (0.2 ) — % (0.2 ) 5.0 % Italy 0.9 — % — — % — — % Malta 1.7 — % 0.3 — % 1.6 0.3 % Luxembourg — (1) — % (5.2 ) — % — — % Switzerland 0.1 18.4 % — (1) 21.1 % — (1) — % Pre-tax income $ 67.7 6.1 % $ 39.9 (26.1 )% $ 181.9 19.4 % (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. (3) Not Meaningful 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 inherent 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) 2018 2017 2016 Income tax provision at expected rate $ 8.2 $ 12.7 $ 34.1 Tax effect of: Nontaxable investment income (1.9 ) (4.7 ) (5.5 ) Foreign exchange adjustments (0.6 ) 2.1 5.3 Withholding taxes 0.4 0.4 2.4 Change in valuation allowance (1.5 ) (0.9 ) 0.7 Impact of change in tax rate related to TCJA (1.6 ) (20.2 ) — Other 1.1 0.2 (1.8 ) Income tax provision $ 4.1 $ (10.4 ) $ 35.2 The net deferred tax liability comprises the tax effects of temporary differences related to the following assets and liabilities: December 31, (in millions) 2018 2017 Deferred tax assets: Losses and loss adjustment expense reserve discounting $ 19.3 $ 17.2 Unearned premiums 23.2 21.4 Net operating loss carryforwards 31.6 21.9 Investment in Limited Partnership Interests 12.0 10.4 Unrealized losses on fixed maturities and other investment securities 9.0 — Impairment of investment values 2.2 4.0 Accrued bonus 4.5 2.6 Stock option expense 1.7 2.0 United Kingdom underwriting results — 3.8 Other 8.0 9.6 Deferred tax assets, gross 111.5 92.9 Deferred tax liabilities: Unrealized gains on equity securities (13.1 ) (30.5 ) Unrealized gains on fixed maturities and other investment securities — (5.0 ) Unrealized gains on limited partnership interests (17.3 ) (14.6 ) Depreciable fixed assets (27.0 ) (22.5 ) Deferred acquisition costs (18.1 ) (16.9 ) TCJA reserve transitional liability (4.5 ) (5.5 ) United Kingdom underwriting results (0.6 ) — Other (7.2 ) (9.1 ) Deferred tax liabilities, gross (87.8 ) (104.1 ) Deferred tax (liabilities) assets, net before valuation allowance $ 23.7 $ (11.2 ) Valuation allowance (29.9 ) (20.1 ) Deferred tax liabilities, net $ (6.2 ) $ (31.3 ) Net deferred tax (liabilities) assets - Other jurisdictions $ (0.6 ) $ 1.2 Net deferred tax liabilities - United States (5.6 ) (32.5 ) Deferred tax liabilities, net $ (6.2 ) $ (31.3 ) 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 U.S. property and casualty insurers two years for net operating losses and for all our U.S. 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 an increase of $9.8 million in 2018 , 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 $6.6 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. Valuation allowances were acquired in our 2017 Ariel Re transaction as well as in the 2018 Ariscom transaction. Accordingly, a valuation allowance is required as of December 31, 2018 of which $6.5 million relates to Brazil operations, $1.9 million relates to Malta operations, $1.4 million relates to Ariel Re operations, and $12.9 million relates to Ariscom, which is attributable to net operating loss ("NOL") carryforwards created before our acquisition of the entity and recorded as part of purchase accounting. For tax return purposes, as of December 31, 2018 , we had NOL carryforwards in Brazil, Italy, 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 has been recognized as mentioned above in the consolidated financial statements and is included in net deferred tax liabilities. The NOL amounts by jurisdiction and year of expiration are as follows: (in millions) December 31, 2018 Expiration Net operating loss carryforwards by jurisdiction Brazil $ 11.5 Indefinite Italy 41.0 Indefinite Malta 5.5 Indefinite United States 48.9 2025 - 2037 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, 2018 , 2017 and 2016 . 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 2016.

Pension Benefits and Savings Pl

Pension Benefits and Savings Plans	12 Months Ended
	Dec. 31, 2018
Retirement Benefits [Abstract]
Pension Benefits and Savings Plans	Pension Benefits and Savings Plans Argo Group U.S. 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, 2018 and 2017 , the qualified pension plan was underfunded by $3.5 million and $2.7 million , respectively. The non-qualified pension plans were unfunded by $2.0 million and $2.2 million at December 31, 2018 and 2017 , 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, 2019. Net periodic benefit costs were minimal for the year ended December 31, 2018. Net periodic benefit cost were $0.2 million and $0.1 million for the years ended December 31, 2017 and 2016 , 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 $7.9 million , $7.0 million and $6.0 million in 2018 , 2017 and 2016 , respectively.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Argo Group’s subsidiaries are parties to legal actions incidental to their business. Based on the opinion of legal 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 $120.5 million related to our limited partnership investments at December 31, 2018 . 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, 2018
Leases [Abstract]
Leases	Leases We lease office space and other 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, 2018 , the future minimum payments under non-cancelable operating leases are as follows: (in millions) Amount Due 2019 $ 18.7 2020 18.6 2021 17.5 2022 14.7 2023 12.3 Thereafter 80.1 Total $ 161.9 We incurred lease expense of $21.6 million , $21.0 million and $12.2 million for the years ended December 31, 2018 , 2017 and 2016 , respectively.

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
Segment Information	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. 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 before income taxes for each segment were as follows: For the Years Ended December 31, (in millions) 2018 2017 2016 Revenue: Earned premiums U.S. Operations $ 1,078.9 $ 936.6 $ 849.5 International Operations 652.5 635.8 560.9 Run-off Lines 0.3 (0.1 ) 0.4 Total earned premiums 1,731.7 1,572.3 1,410.8 Net investment income U.S. Operations 82.9 87.2 71.9 International Operations 32.9 32.7 28.7 Run-off Lines 8.1 9.3 11.3 Corporate and Other 9.2 10.8 3.2 Total net investment income 133.1 140.0 115.1 Fee and other income 9.0 22.5 24.5 Net realized investment (losses) gains (72.0 ) 39.3 26.1 Total revenue $ 1,801.8 $ 1,774.1 $ 1,576.5 For the Years Ended December 31, (in millions) 2018 2017 2016 Income before income taxes U.S. Operations $ 161.4 $ 169.4 $ 174.4 International Operations 32.9 (86.7 ) 51.6 Run-off Lines (9.3 ) (17.9 ) (15.2 ) Total segment income before taxes 185.0 64.8 210.8 Corporate and Other (45.3 ) (64.2 ) (55.0 ) Net realized investment (losses) gains (72.0 ) 39.3 26.1 Total income before income taxes $ 67.7 $ 39.9 $ 181.9 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) 2018 2017 2016 United States $ 1,073.1 $ 930.8 $ 850.0 United Kingdom 455.8 489.3 406.9 Bermuda 85.4 92.1 112.6 Malta 61.0 11.1 2.1 All other jurisdictions 56.4 49.0 39.2 Total earned premiums $ 1,731.7 $ 1,572.3 $ 1,410.8 The following table represents identifiable assets: December 31, (in millions) 2018 2017 U.S. Operations $ 4,707.8 $ 4,298.4 International Operations 3,984.7 3,553.8 Run-off Lines 444.8 449.6 Corporate and Other 420.9 462.2 Total $ 9,558.2 $ 8,764.0 Included in total assets at December 31, 2018 and 2017 are $880.4 million and $731.8 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 (in millions) 2018 2017 2018 2017 U.S. Operations $ 123.5 $ 123.5 $ 0.5 $ 1.5 International Operations 53.5 37.9 93.0 95.3 Total $ 177.0 $ 161.4 $ 93.5 $ 96.8

Statutory Accounting Principles

Statutory Accounting Principles	12 Months Ended
	Dec. 31, 2018
Statutory Accounting Principles [Abstract]
Statutory Accounting Principles	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) 2018 2017 Bermuda $ 1,451.3 $ 1,489.1 United Kingdom (2) 357.3 330.3 United States 1,003.8 992.8 (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) 2018 2017 2016 Bermuda $ 47.4 $ 55.1 $ 158.2 United Kingdom (2) (9.5 ) (95.5 ) 1.8 United States 110.8 57.0 99.0 (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, 2018 . Argo Re is the direct subsidiary of Argo Group, and therefore, has direct dividend paying capabilities to the parent. As of December 31, 2018 , Argo Re’s solvency and liquidity margins and statutory capital and surplus were in excess of the minimum levels required by the Insurance Act. As of December 31, 2018 and 2017 , the minimum statutory capital and surplus required to be maintained by Argo Re was $320.4 million and $403.8 million , respectively. 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 $362.8 million . In 2018 and 2016, Argo Re paid a cash dividend of $36.5 million and $41.0 million , respectively, to Argo Group. The proceeds of the dividends were used to repay intercompany balances related primarily to the funding of dividend and interest payments and other corporate expenses. In 2017, Argo Re did not pay a dividend to Argo Group. 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 underwriting business. 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 U.S. 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 U.S. 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. In December 2018, Argo Group U.S. received an ordinary dividend in the amount of $20.0 million in cash from Rockwood. In December 2016, Argo Group U.S. received an ordinary dividend in the amount of $18.1 million in cash from Rockwood. In December 2016, Argo Group U.S. received an ordinary dividend of $41.6 million from Argonaut Insurance Company. In March 2016, Argo Group U.S. 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 U.S. did not receive dividends from its subsidiaries in 2017. Argonaut Insurance Company is a direct subsidiary of Argo Group U.S. and is regulated by the Illinois Division of Insurance. During 2019, Argonaut Insurance Company may be permitted to pay dividends of up to $89.6 million without approval from the Illinois Division of Insurance. Rockwood, a direct subsidiary of Argo Group U.S., is regulated by the Pennsylvania Department of Insurance. Rockwood may be permitted to pay dividends of up to $12.6 million without approval from the Pennsylvania Department of Insurance during 2019. 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 U.S. Argo Underwriting Agency Ltd. (“AUA”) is our wholly-owned subsidiary through which we conduct the operations of Syndicates 1200 and 1910. Dividend payments from AUA to the immediate parent are not restricted by regulatory authority. Dividend payments will be subject to the earnings, operations, financial condition, capital and general business requirements of AUA. During 2018 we realigned out internal ownership structure so that Ariel Corporate Member Ltd. ("ACML") became a direct subsidiary of AUA. Prior to 2018, ACML had been a subsidiary of Maybrooke Holdings, S.A. 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, 2018
Insurance [Abstract]
Insurance Assessments	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 $6.4 million and $4.8 million at December 31, 2018 and 2017 , respectively.

Transactions with Related Parti

Transactions with Related Parties	12 Months Ended
	Dec. 31, 2018
Related Party Transactions [Abstract]
Transactions with Related Parties	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, 2018 , 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, 2018
Quarterly Financial Information Disclosure [Abstract]
Unaudited Quarterly Financial Data	Unaudited Quarterly Financial Data The following tables represent unaudited quarterly financial data for the years ended December 31, 2018 and 2017 . In the opinion of management, all adjustments necessary to present fairly the results of operations for such periods have been made. (in millions, except per share amounts) 1st Quarter 2nd Quarter 3rd Quarter 4th Quarter Year 2018 Earned premiums $ 414.7 $ 417.7 $ 446.9 $ 452.4 $ 1,731.7 Losses and loss adjustment expenses 237.2 245.5 277.5 280.6 1,040.8 Underwriting, acquisition and insurance expenses 160.2 156.8 168.0 169.7 654.7 Underwriting income 17.3 15.4 1.4 2.1 36.2 Net income (loss) before income taxes 25.0 57.1 45.3 (59.7 ) 67.7 Net income (loss) 24.8 41.8 40.6 (43.6 ) 63.6 Net income (loss) per common share: Basic (1) $ 0.73 $ 1.23 $ 1.20 $ (1.29 ) $ 1.87 Diluted (1) $ 0.71 $ 1.20 $ 1.17 $ (1.29 ) $ 1.83 Dividends per common share $ 0.27 $ 0.27 $ 0.27 $ 0.27 $ 1.08 (in millions, except per share amounts) 1st Quarter 2nd Quarter 3rd Quarter 4th Quarter Year 2017 Earned premiums $ 379.4 $ 399.1 $ 389.3 $ 404.5 $ 1,572.3 Losses and loss adjustment expenses 222.5 230.6 326.4 270.7 1,050.2 Underwriting, acquisition and insurance expenses 153.6 154.7 166.1 161.0 635.4 Underwriting income (loss) 3.3 13.8 (103.2 ) (27.2 ) (113.3 ) 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 (2) : Basic (1) $ 1.06 $ 1.32 $ (1.78 ) $ 0.85 $ 1.46 Diluted (1) $ 1.03 $ 1.29 $ (1.78 ) $ 0.83 $ 1.42 Dividends per common share (2) $ 0.23 $ 0.23 $ 0.24 $ 0.24 $ 0.94 (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. (2) On February 20, 2018, our Board of Directors declared at 15% stock dividend, payable on March 21, 2018, to shareholders of record at the close of business on March 7, 2018. The net income (loss) per common share and dividends declared per share have been adjusted to reflect the effect of the stock dividend

Senior Unsecured Fixed Rate Not

Senior Unsecured Fixed Rate Notes	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Senior Unsecured Fixed Rate Notes	Senior Unsecured Fixed Rate Notes In September 2012, Argo Group (the “Parent Guarantor”), through its subsidiary Argo Group U.S. (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 2015-3, “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, 2018 and 2017 , the Notes consisted of the following: (in millions) December 31, 2018 December 31, 2017 Senior unsecured fixed rate notes Principal $ 143.8 $ 143.8 Less: unamortized debt issuance costs (4.0 ) (4.2 ) Senior unsecured fixed rate notes, less unamortized debt issuance costs $ 139.8 $ 139.6 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, 2018 and 2017 and for the three years ended December 31, 2018 , 2017 and 2016 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, 2018 (in millions) Argo Group Argo Group US, Inc. Other Subsidiaries Eliminations (1) Consolidating (2) Total Assets Investments $ 3.8 $ 3,175.9 $ 1,607.3 $ — $ 4,787.0 Cash 1.7 31.7 105.8 — 139.2 Accrued investment income — 20.3 6.9 — 27.2 Premiums receivable — 229.5 420.4 — 649.9 Reinsurance recoverables — 1,635.2 1,053.1 — 2,688.3 Goodwill and other intangible assets, net 41.9 123.8 104.8 — 270.5 Current income taxes receivable, net — 9.1 (0.9 ) — 8.2 Deferred acquisition costs, net — 86.2 81.1 — 167.3 Ceded unearned premiums — 250.4 207.3 — 457.7 Other assets 15.7 165.3 181.9 — 362.9 Intercompany note receivable — 53.7 (53.7 ) — — Investments in subsidiaries 1,852.7 — — (1,852.7 ) — Total assets $ 1,915.8 $ 5,781.1 $ 3,714.0 $ (1,852.7 ) $ 9,558.2 Liabilities and Shareholders' Equity Reserves for losses and loss adjustment expenses $ — $ 2,771.4 $ 1,883.2 $ — $ 4,654.6 Unearned premiums — 797.4 503.5 — 1,300.9 Funds held and ceded reinsurance payable, net — 739.3 268.4 — 1,007.7 Debt 153.4 284.7 142.1 — 580.2 Deferred tax liabilities, net — 5.6 0.6 — 6.2 Accrued underwriting expenses and other liabilities 7.2 112.4 142.3 — 261.9 Due to (from) affiliates 8.5 2.0 (2.0 ) (8.5 ) — Intercompany note payable — 19.1 (19.1 ) — — Total liabilities 169.1 4,731.9 2,919.0 (8.5 ) 7,811.5 Total shareholders' equity 1,746.7 1,049.2 795.0 (1,844.2 ) 1,746.7 Total liabilities and shareholders' equity $ 1,915.8 $ 5,781.1 $ 3,714.0 $ (1,852.7 ) $ 9,558.2 (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 BALANCE SHEET DECEMBER 31, 2017 (in millions) Argo Group Guarantor) Argo Group US, Inc. Other Subsidiaries Eliminations (1) Consolidating (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 (from) 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 STATEMENT OF INCOME FOR THE YEAR ENDED DECEMBER 31, 2018 (in millions) Argo Group Guarantor) Argo Group US, Inc. Other Subsidiaries Eliminations (1) Consolidating (2) Total Premiums and other revenue: Earned premiums $ — $ 861.9 $ 869.8 $ — $ 1,731.7 Net investment income (expense) 33.8 79.4 56.4 (36.5 ) 133.1 Fee and other income — 4.4 4.6 — 9.0 Net realized investment gains (losses) 2.5 (51.3 ) (20.5 ) (2.7 ) (72.0 ) Total revenue 36.3 894.4 910.3 (39.2 ) 1,801.8 Expenses: Losses and loss adjustment expenses — 523.7 517.1 — 1,040.8 Underwriting, acquisition and insurance expenses 11.3 333.6 309.8 — 654.7 Interest expense 6.2 18.2 7.2 — 31.6 Fee and other expense — 5.3 1.8 — 7.1 Foreign currency exchange losses (gains) — 0.2 (0.3 ) — (0.1 ) Total expenses 17.5 881.0 835.6 — 1,734.1 Income (loss)before income taxes 18.8 13.4 74.7 (39.2 ) 67.7 Provision for income taxes — (0.8 ) 4.9 — 4.1 Net income (loss) before equity in earnings of subsidiaries 18.8 14.2 69.8 (39.2 ) 63.6 Equity in undistributed earnings of subsidiaries 44.8 — — (44.8 ) — Net income $ 63.6 $ 14.2 $ 69.8 $ (84.0 ) $ 63.6 (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 INCOME FOR THE YEAR ENDED DECEMBER 31, 2017 (in millions) Argo Group Guarantor) Argo Group US, Inc. Other Subsidiaries Eliminations (1) Consolidating (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 gains (losses) 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 losses 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 (benefit) 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 Guarantor) Argo Group US, Inc. Other Subsidiaries Eliminations (1) Consolidating (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 gains (losses) 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 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 losses (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 CASH FLOWS FOR YEAR ENDED DECEMBER 31, 2018 (in millions) Argo Group Guarantor) Argo Group US, Inc. Other Subsidiaries Eliminations (1) Consolidating (2) Total Net cash flows from operating activities $ 72.4 $ 182.4 $ 46.5 $ — $ 301.3 Cash flows from investing activities: Proceeds from sales of investments — 1,067.7 532.1 — 1,599.8 Maturities and mandatory calls of fixed maturity investments — 344.9 73.7 — 418.6 Purchases of investments — (1,508.3 ) (640.8 ) — (2,149.1 ) Change in short-term investments and foreign regulatory deposits (3.4 ) (105.0 ) (10.8 ) — (119.2 ) Settlements of foreign currency exchange forward contracts (0.5 ) 2.2 (3.2 ) — (1.5 ) Cash included in acquisition of Ariscom — — 15.6 — 15.6 Purchases of fixed assets and other, net (0.1 ) (19.0 ) (13.4 ) — (32.5 ) Cash used in investing activities (4.0 ) (217.5 ) (46.8 ) — (268.3 ) Cash flows from financing activities: Borrowing under the intercompany note — 19.0 (19.0 ) — — Activity under stock incentive plans 1.6 — — — 1.6 Repurchase of Company's common shares (31.7 ) — — — (31.7 ) Payment of cash dividend to common shareholders (37.5 ) — — — (37.5 ) Cash used in financing activities (67.6 ) 19.0 (19.0 ) — (67.6 ) Effect of exchange rate changes on cash — — (2.8 ) — (2.8 ) Change in cash 0.8 (16.1 ) (22.1 ) — (37.4 ) Cash, beginning of year 0.9 47.8 127.9 — 176.6 Cash, end of period $ 1.7 $ 31.7 $ 105.8 $ — $ 139.2 (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 Guarantor) Argo Group US, Inc. Other Subsidiaries Eliminations (1) Consolidating (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 intercompany note, net 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 period $ 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 Guarantor) Argo Group US, Inc. Other Subsidiaries Eliminations (1) Consolidating ( 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 provided by (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.

Schedule II Condensed Financial

Schedule II Condensed Financial Information of Registrant	12 Months Ended
	Dec. 31, 2018
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Schedule II Condensed Financial Information of Registrant	SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT (in millions) BALANCE SHEETS December 31, 2018 2017 Assets Short-term investments $ 3.8 $ 0.4 Investment in subsidiaries 1,852.7 1,940.0 Cash 1.7 0.9 Goodwill and other intangible assets, net 41.9 43.2 Other assets 15.7 9.2 Total assets $ 1,915.8 $ 1,993.7 Liabilities and Shareholders' Equity Junior subordinated debentures $ 28.4 $ 28.4 Other indebtedness 125.0 125.0 Accrued underwriting expenses and other liabilities 7.2 8.9 Due to subsidiaries 8.5 11.7 Total liabilities 169.1 174.0 Shareholders' equity 1,746.7 1,819.7 Total liabilities and shareholders' equity $ 1,915.8 $ 1,993.7 STATEMENTS OF INCOME For the Years Ended December 31, 2018 2017 2016 Revenue: Net investment income (expense) (1) $ 33.8 $ (4.5 ) $ 37.2 Net realized investment gains 2.5 0.4 0.6 Total revenue 36.3 (4.1 ) 37.8 Expenses: Interest expense 6.2 4.3 1.4 Other expenses 11.3 14.3 12.8 Foreign currency exchange loss — 0.1 — Total expenses 17.5 18.7 14.2 Net income before equity in earnings of subsidiaries (2) 18.8 (22.8 ) 23.6 Equity in undistributed earnings of subsidiaries 44.8 73.1 123.1 Net income $ 63.6 $ 50.3 $ 146.7 (1) For the year ended December 31, 2018 and 2016, net investment income includes intercompany dividends of $36.5 million and $41.0 million , respectively. (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, 2018 2017 2016 Cash flows from operating activities: Net income $ 63.6 $ 50.3 $ 146.7 Adjustments to reconcile net income to net cash provided by (used in) operating activities: Amortization and depreciation 1.4 1.3 0.2 Share-based payments expense 7.0 3.3 4.2 Net realized investment and other gains (2.5 ) (0.4 ) (0.6 ) Undistributed earnings of subsidiaries (44.8 ) (73.1 ) (123.1 ) Change in: Prepaid assets (2.3 ) (0.3 ) (0.2 ) Accrued underwriting expenses (4.4 ) (2.3 ) 0.9 Due to subsidiaries 57.6 48.1 (0.6 ) Other, net (3.2 ) (6.1 ) (1.0 ) Cash provided by operating activities 72.4 20.8 26.5 Cash flows from investing activities: Change in short-term investments (3.4 ) 1.5 (0.9 ) Settlements of foreign currency exchange forward contracts (0.5 ) 0.9 — Acquisition of subsidiaries, net of cash — (235.3 ) — Purchases of fixed assets and other, net (0.1 ) (0.1 ) — Cash used in investing activities (4.0 ) (233.0 ) (0.9 ) Cash flows from financing activities: Additional borrowings — 125.0 — Borrowings under intercompany note payable, net — 120.0 — Activity under stock incentive plans 1.6 1.4 1.0 Repurchase of Company's common shares (31.7 ) (0.1 ) — Payment of cash dividend to common shareholders (37.5 ) (33.2 ) (26.6 ) Cash (used in) provided by financing activities (67.6 ) 213.1 (25.6 ) Change in cash 0.8 0.9 — Cash, beginning of year 0.9 — — Cash, end of year $ 1.7 $ 0.9 $ —

Schedule III Supplemental Insur

Schedule III Supplemental Insurance Information	12 Months Ended
	Dec. 31, 2018
Supplementary Insurance Information [Abstract]
Schedule III Supplemental Insurance Information	SCHEDULE III SUPPLEMENTAL INSURANCE INFORMATION FOR THE YEARS ENDED DECEMBER 31, 2018 , 2017 AND 2016 (in millions) Segment DAC (a) Reserves for (b) UPR Premium (d) Net (l) Loss (e) Amortization (f) (2) Other (3) Net (g) Year Ended December 31, 2018 U.S. Operations 87.2 2,498.9 793.3 1,078.9 82.9 628.2 (6.4 ) 361.2 1,125.7 International Operations 80.1 1,890.1 507.6 652.5 32.9 400.3 (3.5 ) 249.3 639.5 Run-off Lines — 265.6 — 0.3 8.1 12.3 — 3.9 0.3 Corporate and Other — — — — 9.2 — — 50.2 — Total $ 167.3 $ 4,654.6 $ 1,300.9 $ 1,731.7 $ 133.1 $ 1,040.8 $ (9.9 ) $ 664.6 $ 1,765.5 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 (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, 2018
Valuation and Qualifying Accounts [Abstract]
Schedule V Valuation and Qualifying Accounts	SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS (in millions) Balance at Charged to Capital Loss Net Operating Charged to Deductions Balance at Year Ended December 31, 2018 Deducted from assets: Valuation allowance for deferred tax asset $ 20.1 $ (1.5 ) $ — $ 11.3 $ — $ — $ 29.9 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

Schedule VI Supplemental Inform

Schedule VI Supplemental Information for Property-Casualty Insurance Companies	12 Months Ended
	Dec. 31, 2018
Supplemental Information for Property, Casualty Insurance Underwriters [Abstract]
Schedule VI Supplemental Information for Property-Casualty Insurance Companies	SCHEDULE VI SUPPLEMENTAL INFORMATION FOR PROPERTY-CASUALTY INSURANCE COMPANIES (in millions) For the Years Ended December 31, 2018 2017 2016 Deferred acquisition costs $ 167.3 $ 160.4 $ 139.1 Reserves for losses and loss adjustment expenses $ 4,654.6 $ 4,201.0 $ 3,350.8 Unamortized discount in reserves for losses $ 16.9 $ 17.6 $ 19.4 Unearned premiums $ 1,300.9 $ 1,207.7 $ 970.0 Premiums earned $ 1,731.7 $ 1,572.3 $ 1,410.8 Net investment income $ 133.1 $ 140.0 $ 115.1 Losses and loss adjustment expenses incurred: Current year $ 1,058.8 $ 1,058.4 $ 843.4 Prior years (18.0 ) (8.2 ) (33.3 ) Losses and loss adjustment expenses incurred $ 1,040.8 $ 1,050.2 $ 810.1 Deferral of policy acquisition costs (1) $ (9.9 ) $ (11.9 ) $ (4.3 ) Paid losses and loss adjustment expenses, net of reinsurance $ 938.9 $ 889.4 $ 716.5 Gross premiums written $ 2,955.2 $ 2,697.2 $ 2,164.8 (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 Acco_2

Business and Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
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 U.S., Inc. (“Argo Group U.S.”) 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 March 5, 2018, we acquired 100% of the capital stock of Ariscom Compagnia di Assicurazioni S.p.A. (“Ariscom”) upon its release from extraordinary administration by the Italian insurance supervisory authority (“IVASS”). The acquisition is being accounted for in accordance with Accounting Standards Codification (“ASC”) 805, “Business Combinations.” See Note 2, “Acquisition of Ariscom,” for additional discussion regarding the acquisition. The Consolidated Financial Statements as of and for the year ended December 31, 2018 and the Notes to the Consolidated Financial Statements reflect the consolidated results of Argo Group and Ariscom commencing on the date of acquisition. We conduct our ongoing business through two segments. U.S. Operations is comprised of the Excess and Surplus Lines businesses focusing on the U.S.-based risks that the standard, admitted insurance market is unwilling or unable to write, and through other specialized admitted and non-admitted business distributed through retail, wholesale, and managing general brokers/agents in the specialty insurance market. Excess and Surplus Lines products are underwritten by Colony Insurance Company (“Colony”). The other U.S. specialized admitted and non-admitted businesses consist of the following operations: Argo Insurance, Rockwood Casualty Insurance Company (“Rockwood”), Argo Pro, Argo Surety, U.S. Specialty Programs, Inland Marine and Trident Insurance Services. International Operations is comprised of the Lloyd's Syndicate platform (Syndicate 1200 and Syndicate 1910), Argo Insurance Bermuda, Continental Europe and Latin America. Syndicate 1200 and Syndicate 1910 insurance and reinsurance products are underwritten by Argo Underwriting Agency Limited based in London, under the Lloyd’s of London (“Lloyd’s”) global franchise. The additional International Operations business include Argo Insurance Bermuda, ArgoGlobal SE in Malta, ArgoGlobal Assicurazioni S.p.A (formerly Ariscom) in Italy, and Argo Seguros in Brazil. These businesses provide a broad range of commercial property, casualty, professional liability and specialty coverages in a number of countries and jurisdictions outside the United States. The International Specialty businesses include Reinsurance, Argo Insurance Bermuda, ArgoGlobal SE, Argo Seguros business in Brazil and ArgoGlobal Assicurazioni in Italy. 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. Our Run-off Lines segment includes liabilities associated with other liability policies that were issued in the 1960s, 1970s and into the 1980s, as well as the former risk management business and other business no longer underwritten.
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 both Syndicate 1200 and 1910, 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. As a result of the stock dividend, 4,397,520 additional shares were issued. Cash was paid in lieu of fractional shares of our common shares. Excluding repurchased shares, all references to common shares and related per share amounts in this document and related disclosures have been adjusted to reflect the stock dividend for all periods presented. 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. Excluding repurchased shares, all references to common shares and related 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, 2018 and 2017 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 are considered available-for-sale and are carried at fair value. As available-for-sale investments, changes in the fair value fixed maturities 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, 2018 , 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. Our investments in equity securities are carried at fair value. Beginning with the adoption of Accounting Standards Update ("ASU") 2016-01, effective January 1, 2018, the changes in the fair value of equity securities are now included in "Net realized investment (losses) gains" in consolidated statements of income. See "Recently Issued Accounting Pronouncements" below for further information about ASU 2016-01 and the related impact on our consolidated financial statements. 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 $4.0 million and $3.2 million at December 31, 2018 and 2017 , 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 $1.8 million and $2.1 million at December 31, 2018 and 2017 , 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) 2018 2017 2016 Premiums receivable $ 2.1 $ 1.5 $ 1.1 Reinsurance recoverables — — — Net written off $ 2.1 $ 1.5 $ 1.1 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.
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 2018 and 2017 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. We perform our goodwill impairment test on the first day of the fourth quarter of each year, or October 1 of each year. As a result of the reviews performed on each of the entity’s reporting units for the three years ended December 31, 2018, 2017 and 2016, the Company determined that the estimated fair value substantially exceeded the respective carrying value of its reporting units for those years and goodwill was not impaired. In 2018, we recorded $15.6 million of goodwill as a result of the acquisition of Ariscom. This goodwill is included in the International Operations Segment. See Note 2, “Acquisition of Ariscom” for further discussion. Other indefinite-lived intangible assets and intangible assets with finite lives were also reviewed for impairment as of October 1, 2018. As a result of the reviews performed on each of the entity’s reporting units for the three years ended December 31, 2018, 2017 and 2016, the Company determined that the other indefinite-lived intangible assets and finite-lived intangible assets were not impaired. The following table presents our intangible assets and accumulated amortization at December 31: December 31, 2018 December 31, 2017 (in millions) Gross Carrying Accumulated Gross Carrying Accumulated Lloyd's capacity $ 89.0 n/a $ 89.0 n/a Distribution network 50.2 47.0 50.2 44.1 Additional Lloyd's capacity 4.8 4.8 4.8 4.8 Other 3.3 2.0 3.3 1.6 $ 147.3 $ 53.8 $ 147.3 $ 50.5 The remaining weighted average useful life by category at December 31, 2018 was 9.7 years for the distribution network and 6.3 years for other. As of December 31, 2018, the additional Lloyd's capacity was fully amortized. The remaining weighted average useful life for all assets that have not yet been fully amortized was 9.5 years at December 31, 2018 .
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.
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 assets” at December 31, 2018 and 2017 , respectively. The realized and unrealized gains and losses are included in “Net realized investment and other (losses) 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 three 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 income (loss).
Income Taxes	Income Taxes On December 22, 2017, the Tax Cuts and Jobs Act (“TCJA”) was enacted in the United States. 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.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements On August 28, 2018, the Financial Accounting Standards Board (“FASB”) issued ASU 2018-13, "Fair Value Measurement (Topic 820)." ASU 2018-13 eliminates, adds and modifies certain disclosure requirements on fair value measurements. The guidance is effective for fiscal years beginning after December 15, 2019, including interim periods within the year of adoption. The amendments on changes in unrealized gains and losses, the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements, and the narrative description of measurement uncertainty are applied prospectively for only the most recent interim or annual period presented in the initial fiscal year adoption. All other amendments are applied retrospectively to all periods presented upon their effective date. Early adoption is permitted. We are currently in the process of evaluating the impact that the adoption of the ASU will have on our financial disclosures. On February 14, 2018, the FASB issued ASU 2018-02, “Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income” that allows a reclassification from accumulated other comprehensive income ("AOCI") to retained earnings of the stranded tax effects in AOCI resulting from the Tax Cuts and Jobs Act (“TCJA”). Current guidance required the effect of a change in tax laws or rates on deferred tax balances to be reported in income from continuing operations in the accounting period that includes the period of enactment, even if the related income tax effects were originally charged or credited directly to AOCI. The amount of the reclassification would include the effect of the change in the U.S. federal corporate income tax rate on the gross deferred tax amounts and related valuation allowances, if any, at the date of the enactment of TCJA related to items in AOCI. The updated guidance is effective for reporting periods beginning after December 15, 2018 and is to be applied retrospectively to each period in which the effect of the TCJA related to items remaining in AOCI are recognized or at the beginning of the period of adoption. Early adoption is permitted, including adoption in any interim period for public business entities for reporting periods for which financial statements have not yet been issued. We have adopted the guidance effective January 1, 2018. The adoption of this ASU does not affect the Company’s results of operations, financial position, or liquidity. As a result of adopting this ASU, we reclassified $20.7 million of previously recognized deferred taxes from accumulated other comprehensive income into retained earnings as of January 1, 2018. 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 2017-9 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 the disclosures about modifications that are required, the entities are required to affirmatively disclose whether compensation expense has 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 adopted this ASU effective January 1, 2018. The adoption of this ASU did 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 2017-4 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 adopted this ASU effective January 1, 2018. The adoption of this ASU did not have a material impact on our financial results and disclosures. 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 is effective for fiscal years beginning after December 15, 2017, including interim periods within the year of adoption. This ASU impacts how we present the distributions received from equity method investees in our statement of cash flows. We 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 adopted retrospectively. This ASU did not have a material 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 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. In July 2018, the FASB issued Accounting Standards Update No. 2018-11 (ASU 2018-11): Leases (Topic 842): Targeted Improvements, which provides for an alternative transition method by allowing entities to initially apply the new leases standard at the adoption date (such as January 1, 2019) and recognize a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption (comparative periods presented in the financial statements will continue to be in accordance with current GAAP (Topic 840, Leases). The standard is effective for annual and interim periods beginning after December 15, 2018, with earlier application permitted. We have entered into operating leases for office space and certain other assets. We adopted the new standard on the effective date of January 1, 2019. We have elected to apply the following practical expedients: • We have elected to adopt this standard using the option transition method, which allows companies to continue applying the guidance under the lease standard in effect at that time in the comparative periods presented in the consolidated financial statements. Companies that elect this option would record a cumulative-effect adjustment to the opening balance of retained earnings on the date of adoption. • We have elected the "package of practical expedients", which permits us not to reassess under the new standard our prior conclusion about lease identification, lease classification and initial direct costs. • For certain asset classes, we have elected the practical expedient which allows us not to separate non-lease components from lease components and instead to account for each separate lease component and the non-lease components associated with that lease component as a single lease component. • Where we are the lessor, we have elected the practical expedient which permits us to not separate non-lease components from the associated lease components if the non-lease components otherwise would be accounted for in accordance with the new revenue standard. We have elected to not apply the practical expedient which allows the use of hindsight in determining the lease term and in assessing impairment of the entity’s right-of-use assets. We are continuing to evaluate the remaining practical expedients as they apply to our lease population. We have substantially completed our assessment of the standard as well as implementation of our leasing software, including data upload and test procedures. We continue to finalize our calculations, including our discount rate assumptions, related to the new standard. We are also continuing to establish new processes and internal controls that may be required to comply with the new lease accounting and disclosure requirements set by the new standard. Although we continue to assess the impact of the standard on our consolidated financial statements, we believe adoption and implementation will result in an increase to assets and liabilities, as well as additional disclosures. We expect the addition of approximately $118.0 million to $145.0 million in operating lease right of use assets and lease liabilities, using preliminary discount rates of 1.19% to 5.07% . We do not expect a material impact on our consolidated statement of income or cash flows. In January 2016, the FASB issued ASU 2016-01, “Recognition and Measurement of Financial Assets and Financial Liabilities” (Subtopic 825-10). ASU 2016-01 requires 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 also requires 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 is 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. Upon adoption of this ASU, cumulative net unrealized gains on equity securities of $117.5 million , net of deferred income taxes, were reclassified from accumulated other comprehensive income into retained earnings as of January 1, 2018. The change in the fair value of the noted investments is now included in “Net realized investment gains” in our consolidated statements of income. The standard increases the volatility of the results reported in our consolidated statements of income, resulting from recognizing the change in the fair value of our equity investments in our consolidated statements of income. 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. We adopted this ASU effective January 1, 2018. The adoption of this standard did not have a material impact on our consolidated financial results.

Business and Significant Acco_3

Business and Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
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) 2018 2017 2016 Premiums receivable $ 2.1 $ 1.5 $ 1.1 Reinsurance recoverables — — — Net written off $ 2.1 $ 1.5 $ 1.1
Intangible Assets And Accumulated Amortization	The following table presents our intangible assets and accumulated amortization at December 31: December 31, 2018 December 31, 2017 (in millions) Gross Carrying Accumulated Gross Carrying Accumulated Lloyd's capacity $ 89.0 n/a $ 89.0 n/a Distribution network 50.2 47.0 50.2 44.1 Additional Lloyd's capacity 4.8 4.8 4.8 4.8 Other 3.3 2.0 3.3 1.6 $ 147.3 $ 53.8 $ 147.3 $ 50.5
Supplemental Cash Flow Information	Interest paid and income taxes paid (recovered) were as follows: For the Years Ended December 31, (in millions) 2018 2017 2016 Senior unsecured fixed rate notes $ 9.3 $ 9.3 $ 9.3 Junior subordinated debentures 15.5 12.6 7.7 Other indebtedness 6.5 4.9 2.3 Revolving credit facility — 0.3 — Total interest paid $ 31.3 $ 27.1 $ 19.3 Income taxes paid 24.8 16.5 16.6 Income taxes recovered — (2.5 ) (0.5 ) Income taxes paid, net $ 24.8 $ 14.0 $ 16.1

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2018
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 in fixed maturity investments were as follows: December 31, 2018 (in millions) Amortized Gross Gross Fair Fixed maturities U.S. Governments $ 240.9 $ 0.2 $ 4.9 $ 236.2 Foreign Governments 224.1 0.5 7.8 216.8 Obligations of states and political subdivisions 236.7 4.3 1.2 239.8 Corporate bonds 1,808.7 7.5 58.7 1,757.5 Commercial mortgage-backed securities 205.3 0.7 3.2 202.8 Residential mortgage-backed securities 413.1 3.4 5.7 410.8 Asset-backed securities 173.6 0.4 1.2 172.8 Collateralized loan obligations 226.7 0.5 3.5 223.7 Total fixed maturities 3,529.1 17.5 86.2 3,460.4 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
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, 2018 , by contractual maturity, were as follows: (in millions) Amortized Cost Fair Value Due in one year or less $ 270.5 $ 266.3 Due after one year through five years 1,517.0 1,486.2 Due after five years through ten years 581.5 557.1 Thereafter 141.4 140.7 Structured securities 1,018.7 1,010.1 Total $ 3,529.1 $ 3,460.4
Schedule Of Carrying Value Redemption Characteristics And Unfunded Investment Commitments Of Other Invested Assets Portfolio	Details regarding the carrying value and unfunded investment commitments of the other invested assets portfolio as of December 31, 2018 and 2017 were as follows: December 31, 2018 (in millions) Carrying Value Unfunded Commitments Investment Type Hedge funds $ 120.6 $ — Private equity 211.8 120.5 Long only funds 153.0 — Other 4.4 — Total other investments $ 489.8 $ 120.5 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 4.3 — Total other invested assets $ 543.6 $ 129.9
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 is presented below: December 31, 2018 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 $ 28.2 $ 0.2 $ 173.0 $ 4.7 $ 201.2 $ 4.9 Foreign Governments 73.4 3.6 125.0 4.2 198.4 7.8 Obligations of states and political subdivisions 53.3 0.6 25.3 0.6 78.6 1.2 Corporate bonds 964.3 45.7 440.8 13.0 1,405.1 58.7 Commercial mortgage-backed securities 48.5 0.6 90.6 2.6 139.1 3.2 Residential mortgage-backed securities 63.5 0.7 176.1 5.0 239.6 5.7 Asset-backed securities 73.6 0.6 64.2 0.6 137.8 1.2 Collateralized loan obligations 209.5 3.3 10.3 0.2 219.8 3.5 Total fixed maturities 1,514.3 55.3 1,105.3 30.9 2,619.6 86.2 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 (1) 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 (1) 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) 2018 2017 2016 Other-than-temporary impairment: Obligations of states and political subdivisions $ — $ (0.1 ) $ — Corporate bonds (6.6 ) (0.7 ) (1.7 ) Equity securities — (1.7 ) (8.5 ) Other invested assets (1.0 ) — — Other-than-temporary impairment losses $ (7.6 ) $ (2.5 ) $ (10.2 )
Schedule of Investment Income and Expenses	Investment income and expenses were as follows: For the Years Ended December 31, (in millions) 2018 2017 2016 Investment income: Interest on fixed maturities $ 115.0 $ 97.1 $ 88.9 Dividends on equity securities 12.5 13.9 15.6 Income on alternative investments 19.8 49.5 29.9 Income on short-term and other investments 9.5 7.7 0.4 Investment income 156.8 168.2 134.8 Investment expenses (23.7 ) (28.2 ) (19.7 ) Net investment income $ 133.1 $ 140.0 $ 115.1
Schedule of Company's Gross Realized Investment Gains (Losses)	The following table presents our gross realized investment gains (losses): For the Years Ended December 31, (in millions) 2018 2017 2016 Realized gains on fixed maturities and other Fixed maturities $ 17.7 $ 25.7 $ 21.5 Other investments 41.4 25.7 47.5 Short-term investments 0.2 0.7 0.5 Other assets — — 1.2 59.3 52.1 70.7 Realized losses on fixed maturities and other Fixed maturities (16.0 ) (20.0 ) (35.9 ) Other investments (39.5 ) (36.2 ) (46.0 ) Short-term investments (0.5 ) (0.2 ) (0.3 ) Other Assets — — — Other-than-temporary impairment losses on fixed maturities (6.6 ) (0.8 ) (1.7 ) Other-than-temporary impairment losses on other assets (1.0 ) — — (63.6 ) (57.2 ) (83.9 ) Equity securities (1) Net realized gains on equity securities 37.4 46.1 47.8 Other-than-temporary impairment losses on equity securities — (1.7 ) (8.5 ) Change in unrealized (losses) gains on equity securities held at the end of the period (105.1 ) — — Net realized (losses) gains on equity securities (67.7 ) 44.4 39.3 Net realized investment and other (losses) gains (72.0 ) 39.3 26.1 Income tax expense (11.2 ) (12.0 ) (11.6 ) After tax $ (83.2 ) $ 27.3 $ 14.5 (1) Effective January 1, 2018, we adopted ASU 2016-1. As a result, unrealized gains (losses) at the date of adoption have been reclassified from accumulated other comprehensive income to retained earnings. Additionally, all changes in the fair value of equity securities are recognized in net realized investment gains (losses). Prior periods have not been restated to conform to the current presentation. See Note 2, "Recently Issued Accounting Pronouncements."
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) 2018 2017 2016 Change in unrealized (losses) gains Fixed maturities $ (88.1 ) $ 25.4 $ 36.5 Equity securities — 36.6 (2.6 ) Other investments 0.1 2.1 0.6 Short-term investments (0.5 ) — 0.4 Net unrealized investment (losses) gains before income taxes (88.5 ) 64.1 34.9 Income tax benefit (expense) 13.0 (14.6 ) (2.4 ) Net unrealized investment (losses) gains, net of income taxes $ (75.5 ) $ 49.5 $ 32.5
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, 2018 December 31, 2017 Operational currency exposure $ 4.4 $ (0.2 ) Asset manager investment exposure (0.3 ) (0.9 ) Total return strategy (1.5 ) 0.7 Total $ 2.6 $ (0.4 )
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) 2018 2017 2016 Realized gains Operational currency exposure 9.7 12.4 10.9 Asset manager investment exposure 5.8 1.5 9.0 Total return strategy 26.7 10.4 25.6 Gross realized investment gains 42.2 24.3 45.5 Realized losses Operational currency exposure (7.9 ) (13.8 ) (18.0 ) Asset manager investment exposure (3.0 ) (11.3 ) (4.5 ) Total return strategy (28.6 ) (7.6 ) (21.0 ) Gross realized investment losses (39.5 ) (32.7 ) (43.5 ) Net realized investment gains (losses) on foreign currency exchange forward contracts $ 2.7 $ (8.4 ) $ 2.0
Schedule of Restricted Assets	The following table presents our components of restricted assets at December 31: (in millions) December 31, 2018 December 31, 2017 Securities on deposit for regulatory and other purposes $ 172.6 $ 175.3 Securities pledged as collateral for letters of credit 120.9 78.1 Securities and cash on deposit supporting Lloyd’s business 376.8 355.5 Total restricted investments $ 670.3 $ 608.9
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 Value Measurements at Reporting Date Using (in millions) December 31, 2018 Level 1 (a) Level 2 (b) Level 3 (c) Fixed maturities U.S. Governments $ 236.2 $ 226.7 $ 9.5 $ — Foreign Governments 216.8 — 216.8 — Obligations of states and political subdivisions 239.8 — 239.8 — Corporate bonds 1,757.5 — 1,755.3 2.2 Commercial mortgage-backed securities 202.8 — 202.8 — Residential mortgage-backed securities 410.8 — 410.8 — Asset-backed securities 172.8 — 172.8 — Collateralized loan obligations 223.7 — 223.7 — Total fixed maturities 3,460.4 226.7 3,231.5 2.2 Equity securities 354.5 346.3 — 8.2 Other investments 114.4 — 114.4 — Short-term investments 482.3 453.9 28.4 — $ 4,411.6 $ 1,026.9 $ 3,374.3 $ 10.4 (a) Quoted prices in active markets for identical asset (b) Significant other observable inputs (c) Significant unobservable inputs Fair Value Measurements at Reporting Date Using (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 asset (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, 2018 $ 1.9 $ 2.3 $ 4.2 Transfers into Level 3 — — — Transfers out of Level 3 — — — Total gains or losses (realized/unrealized): Included in net income (loss) — 0.2 0.2 Included in other comprehensive income (loss) 0.3 — 0.3 Purchases, issuances, sales, and settlements: Purchases — 7.3 7.3 Issuances — — — Sales — (1.6 ) (1.6 ) Settlements — — — Ending balance, Ending balance, December 31, 2018 $ 2.2 $ 8.2 $ 10.4 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, 2018 $ — $ — $ — (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, 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 $ — $ — $ —

Reinsurance (Tables)

Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2018
Insurance [Abstract]
Schedule of Reinsurance Premiums	Premiums were as follows: For the Years Ended December 31, (in millions) 2018 2017 2016 Direct written premiums $ 2,293.8 $ 2,029.2 $ 1,792.5 Reinsurance ceded to other companies (1,189.7 ) (1,043.7 ) (724.6 ) Reinsurance assumed from other companies 661.4 668.0 372.3 Net written premiums $ 1,765.5 $ 1,653.5 $ 1,440.2 Direct earned premiums $ 2,201.9 $ 1,912.2 $ 1,722.8 Reinsurance ceded to other companies (1,137.9 ) (1,033.6 ) (675.8 ) Reinsurance assumed from other companies 667.7 693.7 363.8 Net earned premiums $ 1,731.7 $ 1,572.3 $ 1,410.8 Percentage of reinsurance assumed to net earned premiums 38.6 % 44.1 % 25.8 %

Reserves for Losses and Loss _2

Reserves for Losses and Loss Adjustment Expenses (Tables)	12 Months Ended
	Dec. 31, 2018
Insurance [Abstract]
Reserves for Losses and Loss Adjustment Expenses	The following table provides a reconciliation of reserves for losses and loss adjustment expenses (“LAE”): For the Years Ended December 31, (in millions) 2018 2017 2016 Net reserves beginning of the year $ 2,488.0 $ 2,180.2 $ 2,133.3 Net Maybrooke reserves acquired — 131.8 Net Ariscom reserves acquired 43.4 — — Add: Losses and LAE incurred during current calendar year, net of reinsurance: Current accident year 1,058.8 1,058.4 843.4 Prior accident years (18.0 ) (8.2 ) (33.3 ) Losses and LAE incurred during calendar year, net of reinsurance 1,040.8 1,050.2 810.1 Deduct: Losses and LAE payments made during current calendar year, net of reinsurance: Current accident year 273.3 289.6 178.9 Prior accident years 665.6 599.8 537.6 Losses and LAE payments made during current calendar year, net of reinsurance: 938.9 889.4 716.5 Change in participation interest (1) (25.5 ) (23.2 ) (36.3 ) Foreign exchange adjustments (44.9 ) 38.4 (10.4 ) Net reserves - end of year 2,562.9 2,488.0 2,180.2 Add: Reinsurance recoverables on unpaid losses and LAE, end of year 2,091.7 1,713.0 1,170.6 Gross reserves - end of year $ 4,654.6 $ 4,201.0 $ 3,350.8 (1) Amount represents (decrease) increase in reserves due to change in our Syndicate 1200 and Syndicate 1910 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 Years Ended December 31, (in millions) 2018 2017 2016 U.S. Operations $ (20.8 ) $ (38.7 ) $ (35.9 ) International Operations (9.5 ) 13.2 (16.0 ) Run-off Lines 12.3 17.3 18.6 Total unfavorable (favorable) prior-year development $ (18.0 ) $ (8.2 ) $ (33.3 )
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, 2018 . 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 Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ 202.9 $ 206.0 $ 205.8 $ 200.0 $ 193.5 $ 192.8 $ 189.0 $ 187.8 2012 189.6 196.0 189.7 183.6 184.4 182.1 182.3 2013 217.9 222.6 224.3 227.2 220.4 216.0 2014 213.0 215.2 213.2 211.9 212.3 2015 232.5 237.1 228.6 226.4 2016 246.4 250.6 243.1 2017 253.3 244.3 2018 278.8 Total $ 1,791.0 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ 17.6 $ 53.8 $ 91.0 $ 122.9 $ 146.6 $ 162.4 $ 170.0 $ 174.3 2012 17.2 52.8 89.1 120.8 142.4 157.5 163.4 2013 17.6 60.2 100.4 135.2 163.7 179.6 2014 15.0 52.2 95.9 131.6 154.5 2015 16.5 51.9 91.4 131.5 2016 17.4 52.8 95.5 2017 11.5 38.7 2018 15.0 Total $ 952.5 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance 54.7 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 893.2 As of December 31, 2018 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ 187.8 $ 10.0 8,454 2012 182.3 11.0 7,389 2013 216.0 21.0 7,297 2014 212.3 36.2 6,502 2015 226.4 60.3 6,000 2016 243.1 100.3 5,548 2017 244.3 153.3 5,302 2018 278.8 227.9 4,026 (1) Information presented for calendar years prior to 2018 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: Specialty Admitted Line of Business: Liability (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ 140.3 $ 155.1 $ 159.0 $ 157.5 $ 158.2 $ 154.0 $ 153.7 $ 154.0 2012 140.3 146.3 149.7 153.3 151.5 147.7 146.7 2013 126.6 133.2 136.7 133.2 131.1 130.6 2014 115.6 121.9 116.9 114.5 111.5 2015 107.3 106.7 101.7 102.3 2016 96.1 99.9 99.3 2017 121.5 129.5 2018 147.3 Total $ 1,021.2 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ 23.2 $ 57.5 $ 85.9 $ 111.3 $ 126.1 $ 135.1 $ 139.8 $ 143.1 2012 20.1 51.0 80.7 105.8 120.8 127.9 131.8 2013 18.9 49.4 74.0 93.6 102.8 109.7 2014 17.4 38.8 58.7 75.3 86.1 2015 17.2 35.0 48.8 64.2 2016 11.1 31.7 48.6 2017 16.3 44.4 2018 19.4 Total $ 647.3 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance 48.7 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 422.6 As of December 31, 2018 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ 154.0 $ 6.3 28,160 2012 146.7 8.0 23,627 2013 130.6 10.3 18,948 2014 111.5 13.9 16,316 2015 102.3 21.1 14,530 2016 99.3 22.1 11,583 2017 129.5 50.2 12,897 2018 147.3 82.8 13,351 (1) Information presented for calendar years prior to 2018 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: Specialty Admitted Line of Business: Professional (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ 35.0 $ 35.0 $ 35.0 $ 32.5 $ 28.2 $ 26.9 26.6 $ 26.0 2012 27.8 28.3 28.6 25.8 24.0 24.5 24.9 2013 20.9 21.5 21.1 19.0 19.8 19.5 2014 22.4 22.4 26.0 33.7 36.2 2015 29.9 29.5 33.2 34.0 2016 44.2 44.8 45.1 2017 60.1 61.8 2018 70.8 Total $ 318.3 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ 3.2 $ 11.8 $ 17.8 $ 22.0 $ 24.0 $ 25.4 $ 25.7 $ 25.8 2012 2.3 8.6 16.9 19.9 21.4 22.6 23.5 2013 1.9 6.3 10.9 14.2 17.6 17.5 2014 2.3 5.4 15.1 24.1 25.5 2015 1.8 8.3 15.6 20.8 2016 2.4 11.9 24.6 2017 3.5 24.9 2018 4.5 Total $ 167.1 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance 0.1 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 151.3 As of December 31, 2018 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ 26.0 $ 0.3 820 2012 24.9 0.4 640 2013 19.5 1.2 620 2014 36.2 2.8 1,043 2015 34.0 2.1 1,816 2016 45.1 13.1 3,226 2017 61.8 15.9 3,669 2018 70.8 55.0 3,603 (1) Information presented for calendar years prior to 2018 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: Specialty Admitted Line of Business: Specialty (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ 0.2 $ 3.9 $ 3.4 $ 3.4 $ 3.6 $ 2.6 $ 2.0 $ 1.7 2012 7.5 6.7 4.9 4.3 4.0 3.9 3.5 2013 10.0 8.6 4.6 2.5 1.7 0.9 2014 13.1 13.1 8.9 6.0 4.8 2015 14.8 14.3 9.5 5.5 2016 15.0 15.0 11.2 2017 16.2 16.2 2018 20.8 Total $ 64.6 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ — $ 1.6 $ 1.4 $ 1.3 $ 1.2 $ 1.7 $ 1.7 $ 1.7 2012 3.6 3.3 3.3 3.3 3.3 3.4 3.3 2013 0.4 0.9 0.9 0.9 0.9 0.9 2014 1.1 3.3 4.0 4.0 4.1 2015 0.2 0.1 0.2 0.3 2016 1.3 1.6 2.2 2017 0.3 0.1 2018 — Total $ 12.6 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance — Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 52.0 As of December 31, 2018 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ 1.7 $ — 80 2012 3.5 0.2 129 2013 0.9 — 50 2014 4.8 0.8 50 2015 5.5 2.9 24 2016 11.2 5.5 58 2017 16.2 16.2 84 2018 20.8 19.8 71 (1) Information presented for calendar years prior to 2018 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: Reinsurance Line of Business: Property (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ 116.3 $ 105.0 $ 107.1 $ 105.1 $ 104.4 $ 104.3 $ 132.3 $ 138.9 2012 47.2 51.3 50.3 51.6 46.4 62.7 66.9 2013 32.1 33.6 33.1 31.4 33.0 32.4 2014 26.5 26.5 24.2 35.3 38.1 2015 27.1 23.3 32.6 27.1 2016 43.7 175.5 169.1 2017 157.8 158.0 2018 67.6 Total $ 698.1 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ 40.9 $ 66.5 $ 87.7 $ 95.5 $ 97.8 $ 99.0 $ 127.2 $ 134.1 2012 12.4 31.1 40.6 49.6 44.0 58.7 64.6 2013 4.2 16.7 26.5 29.0 31.0 31.1 2014 2.8 12.7 18.3 36.4 36.9 2015 4.3 11.0 20.3 20.6 2016 13.6 142.3 153.0 2017 84.2 139.5 2018 24.7 Total $ 604.5 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance 3.6 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 97.2 As of December 31, 2018 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ 138.9 $ 0.3 459 2012 66.9 0.2 277 2013 32.4 0.3 219 2014 38.1 0.5 221 2015 27.1 3.2 219 2016 169.1 11.0 381 2017 158.0 (28.8 ) 774 2018 67.6 4.2 391 (1) Information presented for calendar years prior to 2018 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: Argo Insurance Bermuda Line of Business: Liability (in millions, except number of claims reported) Incurred Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ 6.6 $ 6.6 $ 6.6 $ 4.4 $ 2.2 $ 1.6 $ 1.0 $ — 2012 7.4 7.4 7.4 5.6 4.4 1.7 — 2013 8.5 8.5 8.5 8.5 4.9 2.2 2014 9.8 9.8 9.8 6.2 1.5 2015 11.3 14.3 24.8 35.4 2016 13.9 14.0 14.0 2017 17.1 17.3 2018 8.9 Total $ 79.3 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2011 $ — $ — $ — $ — $ — $ — $ — $ — 2012 — — — — — — — 2013 — — — — 2.3 2.3 2014 — — 0.1 0.1 1.2 2015 — — 16.1 20.3 2016 — — — 2017 — 3.3 2018 — Total $ 27.1 Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance — Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 52.2 As of December 31, 2018 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims Cumulative Number of Reported Claims (2) 2011 $ — $ — 1,423 2012 — — 1,381 2013 2.2 — 1,190 2014 1.5 0.2 1,329 2015 35.4 1.0 1,567 2016 14.0 13.9 1,874 2017 17.3 12.2 1,900 2018 8.9 8.8 839 (1) Information presented for calendar years prior to 2018 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 Year For the Years Ended December 31, 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2010 $ 5.7 $ 4.8 $ 4.4 $ 6.1 $ 5.9 $ 5.8 $ 5.7 $ 5.8 $ 5.7 2011 8.0 8.6 10.8 10.9 10.2 10.1 10.6 10.7 2012 8.7 10.8 14.9 14.2 13.8 14.5 15.0 2013 22.7 26.7 26.2 24.4 24.3 25.1 2014 37.1 35.9 33.6 32.5 33.8 2015 34.2 29.6 29.2 29.6 2016 25.5 26.4 26.1 2017 24.5 23.3 2018 21.4 Total $ 190.7 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2010 $ — $ 0.2 $ 0.6 $ 1.1 $ 2.0 $ 2.8 $ 3.7 $ 4.0 $ 4.3 2011 0.2 0.8 1.6 3.4 5.4 6.9 7.8 8.7 2012 0.4 1.1 2.6 5.8 8.2 10.1 11.8 2013 1.5 3.2 7.0 11.3 15.6 19.7 2014 1.9 4.5 9.8 13.6 20.2 2015 0.8 5.1 7.4 12.3 2016 1.8 5.7 10.6 2017 1.9 6.6 2018 2.2 Total $ 96.4 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 94.3 As of December 31, 2018 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims 2010 $ 5.7 $ 0.1 2011 10.7 0.2 2012 15.0 0.6 2013 25.1 2.5 2014 33.8 5.0 2015 29.6 8.2 2016 26.1 10.2 2017 23.3 15.2 2018 21.4 17.6 (1) Information presented for calendar years prior to 2018 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 Year For the Years Ended December 31, 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2010 $ 15.5 $ 12.8 $ 12.7 $ 11.2 $ 10.1 $ 9.4 $ 9.5 $ 9.5 $ 9.7 2011 19.1 21.0 18.5 15.5 14.7 14.7 15.1 15.7 2012 13.9 13.8 14.1 14.0 14.0 14.9 15.4 2013 22.5 22.4 22.6 22.2 22.3 23.1 2014 34.5 35.7 35.9 39.0 40.9 2015 37.8 37.3 38.5 38.7 2016 32.8 26.9 26.4 2017 24.5 22.0 2018 21.2 Total $ 213.1 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2010 $ 0.1 $ 0.9 $ 2.0 $ 3.0 $ 3.8 $ 5.1 $ 6.5 $ 6.8 $ 7.6 2011 1.0 2.5 4.1 6.6 8.3 10.6 11.3 12.8 2012 0.6 1.8 4.3 5.7 8.1 9.5 11.2 2013 1.7 3.7 7.0 11.6 15.4 17.6 2014 1.6 6.3 14.6 23.7 27.9 2015 2.2 8.2 14.8 19.7 2016 2.0 5.8 10.6 2017 1.2 5.1 2018 1.0 Total $ 113.5 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 99.6 As of December 31, 2018 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims 2010 $ 9.7 $ 0.1 2011 15.7 0.4 2012 15.4 0.8 2013 23.1 3.0 2014 40.9 6.5 2015 38.7 11.3 2016 26.4 10.8 2017 22.0 14.6 2018 21.2 17.6 (1) Information presented for calendar years prior to 2018 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 Year For the Years Ended December 31, 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2010 $ 50.7 $ 58.2 $ 54.6 $ 54.2 $ 51.5 $ 51.2 $ 50.7 $ 51.4 $ 51.1 2011 108.0 113.3 107.5 94.8 92.9 92.4 92.4 91.8 2012 88.7 88.5 92.8 91.9 90.9 90.8 89.9 2013 83.3 79.2 78.2 76.7 76.5 75.6 2014 69.7 64.1 65.5 66.0 65.3 2015 55.9 66.2 73.4 74.0 2016 70.7 86.6 91.8 2017 85.2 91.8 2018 62.3 Total $ 693.6 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2010 $ 1.8 $ 19.5 $ 28.0 $ 33.8 $ 35.8 $ 40.0 $ 41.5 $ 42.6 $ 43.4 2011 23.4 47.1 61.8 73.5 79.3 81.0 82.7 82.4 2012 29.4 47.6 62.8 73.9 76.3 77.5 77.7 2013 44.4 56.7 69.2 73.3 74.2 73.5 2014 29.4 51.4 57.5 60.0 59.9 2015 22.8 42.7 52.6 59.1 2016 38.8 64.1 78.8 2017 29.7 62.7 2018 30.1 Total $ 567.6 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 126.0 As of December 31, 2018 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims 2010 $ 51.1 $ — 2011 $ 91.8 $ 0.2 2012 89.9 0.2 2013 75.6 0.1 2014 65.3 0.1 2015 74.0 0.1 2016 91.8 0.8 2017 91.8 15.7 2018 62.3 31.2 (1) Information presented for calendar years prior to 2018 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 Year For the Years Ended December 31, 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2010 $ 12.8 $ 15.7 $ 13.6 $ 13.0 $ 12.1 $ 12.1 $ 12.0 $ 12.0 $ 12.0 2011 38.3 40.0 38.7 34.0 33.2 33.2 33.1 32.7 2012 52.5 56.4 60.7 59.2 59.0 58.8 58.2 2013 75.7 81.2 82.5 82.1 81.8 81.5 2014 92.1 98.4 99.7 101.1 100.6 2015 89.8 87.8 93.8 95.3 2016 86.0 84.5 87.7 2017 80.1 76.5 2018 65.0 Total $ 609.5 Cumulative Paid Losses & ALAE, Net of Reinsurance Accident Year For the Years Ended December 31, 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 (1) 2018 2010 $ 1.8 $ 8.6 $ 8.8 $ 9.2 $ 9.0 $ 9.7 $ 10.0 $ 10.0 $ 10.2 2011 11.5 19.5 23.6 27.1 28.5 29.1 29.5 29.4 2012 17.9 27.4 38.8 45.7 48.7 49.8 50.1 2013 30.9 52.3 68.7 76.2 78.2 78.6 2014 37.7 71.4 82.5 88.9 90.4 2015 30.9 54.0 66.2 74.0 2016 37.3 59.0 68.7 2017 20.1 42.6 2018 18.7 Total $ 462.7 Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance $ 146.8 As of December 31, 2018 Accident Year Incurred Losses & ALAE, Net of Reinsurance IBNR & Expected Development on Reported Claims 2010 $ 12.0 $ — 2011 32.7 — 2012 58.2 — 2013 81.5 (0.5 ) 2014 100.6 0.7 2015 95.3 3.0 2016 87.7 6.6 2017 76.5 21.8 2018 65.0 40.3 (1) Information presented for calendar years prior to 2018 is required supplementary information and is unaudited.
Summary of Reconciliation of Net Incurred and Paid Development to Liability for Unpaid Losses and LAE in Consolidated Balance Sheets	Syndicate 1200 measures claim frequency based on the number of reported claims by individual claimant at a coverage level for non-bordereau reporting, which is con
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, 2018 : 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 Year 9 U.S. Operations: Excess and Surplus Lines - Liability 7.8% 19.2% 19.0% 17.1% 11.5% 7.9% 5.3% 3.5% N/A Specialty Admitted - Liability 14.6% 21.6% 18.3% 16.0% 9.1% 6.1% 4.0% 2.7% N/A Specialty Admitted- Professional 7.2% 24.5% 29.6% 19.8% 8.4% 4.1% 2.2% 1.3% N/A Specialty Admitted - Specialty 40.5% 30.6% 11.9% 8.3% 4.2% 2.2% 1.1% 0.6% N/A International Operations: Reinsurance - Property 21.2% 33.7% 13.8% 12.9% 6.1% 4.0% 2.6% 1.8% N/A Argo Insurance Bermuda - Liability 1.3% 8.6% 17.8% 21.9% 16.6% 10.1% 6.8% 4.6% N/A Syndicate 1200 - Liability 3.3% 7.2% 9.4% 12.7% 14.6% 12.3% 10.9% 8.6% 6.4% Syndicate 1200 - Professional 4.7% 10.4% 14.5% 14.8% 11.3% 10.5% 8.1% 6.8% 5.2% Syndicate 1200 - Property 39.5% 28.0% 16.0% 9.8% 3.0% 1.7% 0.9% 0.5% 0.3% Syndicate 1200 - Specialty 38.9% 29.2% 15.6% 9.6% 3.7% 1.7% 0.8% 0.3% 0.2% (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) 2018 2017 2016 2018 2017 2016 U.S. Operations: Specialty Admitted - Liability $ 140.8 $ 126.7 $ 121.1 $ 11.9 $ 10.6 $ 10.3 Run-off Lines 163.1 175.5 184.5 5.0 7.0 9.1 Total $ 303.9 $ 302.2 $ 305.6 $ 16.9 $ 17.6 $ 19.4 Interest Accretion (1) Discount Rate For the Years Ended December 31, As of December 31, 2018 2017 2016 2018 2017 2016 U.S. Operations: Specialty Admitted - Liability $ 1.3 $ 1.9 $ 1.9 2.25% 2.25% 2.25% Run-off Lines 2.1 2.1 2.1 3.50% 3.50% 3.50% Total $ 3.4 $ 4.0 $ 4.0 (1) Interest accretion is recorded in the line item “Losses and loss adjustment expenses” in our Consolidated Statements of Income.

Run-off Lines (Tables)

Run-off Lines (Tables)	12 Months Ended
	Dec. 31, 2018
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) 2018 2017 Asbestos and Environmental: Reinsurance assumed $ 27.7 $ 29.3 Other 27.1 26.6 Total Asbestos and Environmental 54.8 55.9 Risk management 197.0 219.6 Run-off reinsurance lines 1.6 1.8 Other run-off lines 12.2 4.6 Gross reserves - Run-off Lines $ 265.6 $ 281.9
Total Gross Reserves for Asbestos Exposure	The following table represents the total gross reserves for our asbestos exposure: December 31, (in millions) 2018 2017 2016 Direct written Case reserves $ 2.7 $ 2.1 $ 2.8 Unallocated loss adjustment expense ("ULAE") 0.5 0.5 0.5 Incurred but not reported ("IBNR") 19.1 18.8 12.1 Total direct written reserves 22.3 21.4 15.4 Assumed domestic Case reserves 8.7 9.8 10.5 ULAE 0.8 0.8 0.8 IBNR 12.0 13.7 10.5 Total assumed domestic reserves 21.5 24.3 21.8 Assumed London Case reserves 1.5 2.3 3.3 ULAE — — — IBNR 1.5 0.6 1.4 Total assumed London reserves 3.0 2.9 4.7 Total asbestos reserves $ 46.8 $ 48.6 $ 41.9
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) 2018 2017 2016 Asbestos and Environmental: Reinsurance assumed $ (3.9 ) $ (8.7 ) $ (1.3 ) Other (4.1 ) (6.7 ) (9.6 ) Total Asbestos and Environmental (8.0 ) (15.4 ) (10.9 ) Risk management (2.6 ) (8.8 ) (13.1 ) Run-off reinsurance lines — (0.1 ) 0.1 Other run-off lines (5.3 ) (1.4 ) (1.2 ) Total underwriting loss - Run-off Lines $ (15.9 ) $ (25.7 ) $ (25.1 )

Junior Subordinated Debentures

Junior Subordinated Debentures (Tables)	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Schedule of Junior Subordinated Debentures	A summary of the terms of the acquired debt outstanding is presented below: December 31, 2018 (in millions) Issue Date Maturity Rate Structure Interest Rate at Principal at Carrying Value at 9/15/2007 9/15/2037 3 month LIBOR + 3.15% 5.94 % $ 91.8 $ 84.3 December 31, 2017 (in millions) Issue Date Maturity Rate Structure Interest Rate at Principal at Carrying Value at 9/15/2007 9/15/2037 3 month LIBOR + 3.15% 4.74 % $ 91.8 $ 83.9 A summary of our outstanding junior subordinated debentures is presented below: December 31, 2018 (in millions) Issue Date Trust Preferred Pools Maturity Rate Structure Interest Rate at Amount Argo Group 5/15/2003 PXRE Capital Statutory Trust II 5/15/2033 3M LIBOR + 4.10% 6.72% $ 18.1 11/6/2003 PXRE Capital Trust VI 9/30/2033 3M LIBOR + 3.90% 6.70% 10.3 Argo Group US 5/15/2003 Argonaut Group Statutory Trust I 5/15/2033 3M LIBOR + 4.10% 6.72% 15.5 12/16/2003 Argonaut Group Statutory Trust III 1/8/2034 3M LIBOR + 4.10% 6.54% 12.3 4/29/2004 Argonaut Group Statutory Trust IV 4/29/2034 3M LIBOR + 3.85% 6.47% 13.4 5/26/2004 Argonaut Group Statutory Trust V 5/24/2034 3M LIBOR + 3.85% 6.54% 12.3 5/12/2004 Argonaut Group Statutory Trust VI 5/12/2034 3M LIBOR + 3.80% 6.59% 13.4 9/17/2004 Argonaut Group Statutory Trust VII 12/15/2034 3M LIBOR + 3.60% 6.39% 15.5 9/22/2004 Argonaut Group Statutory Trust VIII 9/22/2034 3M LIBOR + 3.55% 6.37% 15.5 10/22/2004 Argonaut Group Statutory Trust IX 12/15/2034 3M LIBOR + 3.60% 6.39% 15.5 9/14/2005 Argonaut Group Statutory Trust X 9/15/2035 3M LIBOR + 3.40% 6.19% 30.9 Total Outstanding $ 172.7 December 31, 2017 (in millions) Issue Date Trust Preferred Pools Maturity Rate Structure Interest Rate at Amount Argo Group 5/15/2003 PXRE Capital Statutory Trust II 5/15/2033 3M LIBOR + 4.10% 5.52% $ 18.1 11/6/2003 PXRE Capital Trust VI 9/30/2033 3M LIBOR + 3.90% 5.59% 10.3 Argo Group US 5/15/2003 Argonaut Group Statutory Trust I 5/15/2033 3M LIBOR + 4.10% 5.52% 15.5 12/16/2003 Argonaut Group Statutory Trust III 1/8/2034 3M LIBOR + 4.10% 5.46% 12.3 4/29/2004 Argonaut Group Statutory Trust IV 4/29/2034 3M LIBOR + 3.85% 5.27% 13.4 5/26/2004 Argonaut Group Statutory Trust V 5/24/2034 3M LIBOR + 3.85% 5.30% 12.3 5/12/2004 Argonaut Group Statutory Trust VI 5/12/2034 3M LIBOR + 3.80% 5.40% 13.4 9/17/2004 Argonaut Group Statutory Trust VII 12/15/2034 3M LIBOR + 3.60% 5.19% 15.5 9/22/2004 Argonaut Group Statutory Trust VIII 9/22/2034 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 9/14/2005 Argonaut Group Statutory Trust X 9/15/2035 3M LIBOR + 3.40% 4.99% 30.9 Total Outstanding $ 172.7 A summary of the notes outstanding at December 31, 2018 and 2017 is presented below: December 31, 2018 (in millions) Currency Issue Date Maturity Rate Structure Interest Rate at Amount U.S. Dollar 12/8/2004 11/15/2034 6 month LIBOR + 4.2% 6.72% $ 6.5 U.S. Dollar 10/31/2006 1/15/2036 6 month LIBOR + 4.0% 6.52% 10.0 Total U.S. Dollar notes 16.5 Euro 9/6/2005 8/22/2035 3 month LIBOR + 4.0% 3.68% 13.8 Euro 10/31/2006 11/24/2036 3 month LIBOR + 4.0% 3.68% 12.0 Euro 6/8/2007 9/15/2037 3 month LIBOR + 3.9% 3.59% 15.5 Total Euro notes 41.3 Total notes outstanding $ 57.8 December 31, 2017 (in millions) Currency Issue Date Maturity Rate Structure Interest Rate at Amount U.S. Dollar 12/8/2004 11/15/2034 6 month LIBOR + 4.2% 5.66% $ 6.5 U.S. Dollar 10/31/2006 1/15/2036 6 month LIBOR + 4.0% 5.46% 10.0 Total U.S. Dollar notes 16.5 Euro 9/6/2005 8/22/2035 3 month LIBOR + 4.0% 3.67% 14.1 Euro 10/31/2006 11/22/2036 3 month LIBOR + 4.0% 3.67% 12.4 Euro 6/8/2007 9/15/2037 3 month LIBOR + 3.9% 3.57% 15.9 Total Euro notes 42.4 Total notes outstanding $ 58.9

Other Indebtedness (Tables)

Other Indebtedness (Tables)	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
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, 2018 December 31, 2017 Floating rate loan stock $ 57.8 $ 58.9 Term loan 125.0 125.0 Other debt 0.6 0.6 Total other indebtedness $ 183.4 $ 184.5
Schedule of Junior Subordinated Debentures	A summary of the terms of the acquired debt outstanding is presented below: December 31, 2018 (in millions) Issue Date Maturity Rate Structure Interest Rate at Principal at Carrying Value at 9/15/2007 9/15/2037 3 month LIBOR + 3.15% 5.94 % $ 91.8 $ 84.3 December 31, 2017 (in millions) Issue Date Maturity Rate Structure Interest Rate at Principal at Carrying Value at 9/15/2007 9/15/2037 3 month LIBOR + 3.15% 4.74 % $ 91.8 $ 83.9 A summary of our outstanding junior subordinated debentures is presented below: December 31, 2018 (in millions) Issue Date Trust Preferred Pools Maturity Rate Structure Interest Rate at Amount Argo Group 5/15/2003 PXRE Capital Statutory Trust II 5/15/2033 3M LIBOR + 4.10% 6.72% $ 18.1 11/6/2003 PXRE Capital Trust VI 9/30/2033 3M LIBOR + 3.90% 6.70% 10.3 Argo Group US 5/15/2003 Argonaut Group Statutory Trust I 5/15/2033 3M LIBOR + 4.10% 6.72% 15.5 12/16/2003 Argonaut Group Statutory Trust III 1/8/2034 3M LIBOR + 4.10% 6.54% 12.3 4/29/2004 Argonaut Group Statutory Trust IV 4/29/2034 3M LIBOR + 3.85% 6.47% 13.4 5/26/2004 Argonaut Group Statutory Trust V 5/24/2034 3M LIBOR + 3.85% 6.54% 12.3 5/12/2004 Argonaut Group Statutory Trust VI 5/12/2034 3M LIBOR + 3.80% 6.59% 13.4 9/17/2004 Argonaut Group Statutory Trust VII 12/15/2034 3M LIBOR + 3.60% 6.39% 15.5 9/22/2004 Argonaut Group Statutory Trust VIII 9/22/2034 3M LIBOR + 3.55% 6.37% 15.5 10/22/2004 Argonaut Group Statutory Trust IX 12/15/2034 3M LIBOR + 3.60% 6.39% 15.5 9/14/2005 Argonaut Group Statutory Trust X 9/15/2035 3M LIBOR + 3.40% 6.19% 30.9 Total Outstanding $ 172.7 December 31, 2017 (in millions) Issue Date Trust Preferred Pools Maturity Rate Structure Interest Rate at Amount Argo Group 5/15/2003 PXRE Capital Statutory Trust II 5/15/2033 3M LIBOR + 4.10% 5.52% $ 18.1 11/6/2003 PXRE Capital Trust VI 9/30/2033 3M LIBOR + 3.90% 5.59% 10.3 Argo Group US 5/15/2003 Argonaut Group Statutory Trust I 5/15/2033 3M LIBOR + 4.10% 5.52% 15.5 12/16/2003 Argonaut Group Statutory Trust III 1/8/2034 3M LIBOR + 4.10% 5.46% 12.3 4/29/2004 Argonaut Group Statutory Trust IV 4/29/2034 3M LIBOR + 3.85% 5.27% 13.4 5/26/2004 Argonaut Group Statutory Trust V 5/24/2034 3M LIBOR + 3.85% 5.30% 12.3 5/12/2004 Argonaut Group Statutory Trust VI 5/12/2034 3M LIBOR + 3.80% 5.40% 13.4 9/17/2004 Argonaut Group Statutory Trust VII 12/15/2034 3M LIBOR + 3.60% 5.19% 15.5 9/22/2004 Argonaut Group Statutory Trust VIII 9/22/2034 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 9/14/2005 Argonaut Group Statutory Trust X 9/15/2035 3M LIBOR + 3.40% 4.99% 30.9 Total Outstanding $ 172.7 A summary of the notes outstanding at December 31, 2018 and 2017 is presented below: December 31, 2018 (in millions) Currency Issue Date Maturity Rate Structure Interest Rate at Amount U.S. Dollar 12/8/2004 11/15/2034 6 month LIBOR + 4.2% 6.72% $ 6.5 U.S. Dollar 10/31/2006 1/15/2036 6 month LIBOR + 4.0% 6.52% 10.0 Total U.S. Dollar notes 16.5 Euro 9/6/2005 8/22/2035 3 month LIBOR + 4.0% 3.68% 13.8 Euro 10/31/2006 11/24/2036 3 month LIBOR + 4.0% 3.68% 12.0 Euro 6/8/2007 9/15/2037 3 month LIBOR + 3.9% 3.59% 15.5 Total Euro notes 41.3 Total notes outstanding $ 57.8 December 31, 2017 (in millions) Currency Issue Date Maturity Rate Structure Interest Rate at Amount U.S. Dollar 12/8/2004 11/15/2034 6 month LIBOR + 4.2% 5.66% $ 6.5 U.S. Dollar 10/31/2006 1/15/2036 6 month LIBOR + 4.0% 5.46% 10.0 Total U.S. Dollar notes 16.5 Euro 9/6/2005 8/22/2035 3 month LIBOR + 4.0% 3.67% 14.1 Euro 10/31/2006 11/22/2036 3 month LIBOR + 4.0% 3.67% 12.4 Euro 6/8/2007 9/15/2037 3 month LIBOR + 3.9% 3.57% 15.9 Total Euro notes 42.4 Total notes outstanding $ 58.9
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, 2018 and December 31, 2017 is presented below: December 31, 2018 (in millions) Issue Date Maturity Rate Structure Interest Rate at Amount 11/2/2018 11/2/2021 3 month LIBOR + 1.25% 3.86% $ 125.0 December 31, 2017 (in millions) Issue Date Maturity Rate Structure Interest Rate at Amount 3/3/2017 3/3/2019 2 month LIBOR + 1.5% 2.94% $ 125.0

Disclosures about Fair Value _2

Disclosures about Fair Value of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 December 31, 2017 (in millions) Carrying Amount Fair Value Carrying Amount Fair Value Junior subordinated debentures: Trust preferred debentures $ 172.7 $ 163.2 $ 172.7 $ 172.9 Subordinated debentures acquired with Maybrooke 84.3 85.0 83.9 85.0 Total junior subordinated debentures 257.0 248.2 256.6 257.9 Senior unsecured fixed rate notes 139.8 139.5 139.6 141.2 Floating rate loan stock 57.8 54.5 58.9 59.0

Shareholders' Equity (Tables)

Shareholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Schedule of Repurchase of Shares	A summary of common shares repurchased for the twelve months ended December 31, 2018 is shown below: Repurchase Type Date 2018 Purchase Period Number of Average Price Total Cost Repurchase 10b5-1 Trading Plan 12/18/2017 01/02/2018-02/16/2018 225,281 $ 59.31 $ 13.3 2016 10b5-1 Trading Plan 6/18/2018 06/22/2018- 08/07/2018 75,661 $ 59.30 4.5 2016 10b5-1 Trading Plan 9/18/2018 09/19/2018-10/24/2018 37,385 $ 59.75 2.2 2016 Open Market N/A 02/20/2018-03/02/2018 89,305 $ 59.03 5.3 2016 Open Market N/A 08/13/2018-09/17/2018 103,250 $ 62.07 6.4 2016 Total 530,882 $ 59.83 $ 31.7

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive Income (Loss) (Tables)	12 Months Ended
	Dec. 31, 2018
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 Currency Translation Adjustments Unrealized Defined Benefit Pension Plans Total Balance, January 1, 2017 $ (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 Other comprehensive income (loss) before reclassifications (3.4 ) (80.4 ) 1.0 (82.8 ) Amounts reclassified from accumulated other comprehensive income (loss) — 4.9 — 4.9 Net current-period other comprehensive income (loss) (3.4 ) (75.5 ) 1.0 (77.9 ) Cumulative effect of adoption of ASU 2016-01 — (117.5 ) — (117.5 ) Cumulative effect of adoption of ASU 2018-02 — 22.1 (1.4 ) 20.7 Balance, December 31, 2018 $ (22.4 ) $ (49.0 ) $ (6.7 ) $ (78.1 )
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) 2018 2017 2016 Unrealized gains and losses on securities: Net realized investment losses (gains) $ 5.4 $ (41.6 ) $ (23.2 ) Provision (benefit) for income taxes (0.5 ) 13.4 13.2 Net of taxes $ 4.9 $ (28.2 ) $ (10.0 )

Net Income Per Common Share (Ta

Net Income Per Common Share (Tables)	12 Months Ended
	Dec. 31, 2018
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) 2018 2017 2016 Net income $ 63.6 $ 50.3 $ 146.7 Weighted average common shares outstanding - basic 33,922,009 34,457,098 34,691,618 Effect of dilutive securities: Equity compensation awards 756,772 914,546 781,161 Weighted average common shares outstanding - diluted 34,678,781 35,371,644 35,472,779 Net income per common share: Basic $ 1.87 $ 1.46 $ 4.23 Diluted $ 1.83 $ 1.42 $ 4.13

Share-based Compensation (Table

Share-based Compensation (Tables)	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Fair Value Assumptions	The following table summarizes the assumptions we used: For the Years Ended December 31, 2018 2017 2016 Risk-free rate of return 2.61% to 2.96% 1.83% to 2.22% 1.00% to 2.02% Expected dividend yields 1.71% to 1.87% 1.63% to 1.72% 1.62% to 1.70% Expected award life (years) 4.48 to 4.49 4.48 to 4.49 4.50 to 4.61 Expected volatility 17.82% to 18.44% 18.13% to 18.70% 18.73% to 19.70%
Summary of Restricted Share Activity	A summary of restricted share activity as of December 31, 2018 and changes during the year then ended is as follows: Shares Weighted-Average Grant Date Fair Value Outstanding at January 1, 2018 767,140 $ 42.91 Granted 458,715 $ 51.84 Vested and issued (211,170 ) $ 43.35 Expired or forfeited (117,680 ) $ 46.32 Outstanding at December 31, 2018 897,005 $ 46.92
Summary of Stock-Settled SARs Activity	A summary of stock-settled SARs activity as of December 31, 2018 and changes during the year then ended is as follows: Shares Weighted-Average Exercise Price Outstanding at January 1, 2018 1,494,458 $ 30.85 Exercised (665,875 ) $ 27.00 Expired or forfeited (17,824 ) $ 36.51 Outstanding at December 31, 2018 810,759 $ 33.88 Vested or expected to vest as of end of year 781,124 $ 33.73 Exercisable at end of year 618,935 $ 32.65
Summary of Cash-Settled SARs Activity	A summary of cash-settled SARs activity as of December 31, 2018 and changes during the year then ended is as follows: Shares Weighted-Average Exercise Price Outstanding at January 1, 2018 189,568 $ 28.95 Exercised (128,572 ) $ 28.31 Expired or forfeited (2,568 ) $ 21.23 Outstanding at December 31, 2018 58,428 $ 30.71

Underwriting, Acquisition and_2

Underwriting, Acquisition and Insurance Expenses (Tables)	12 Months Ended
	Dec. 31, 2018
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) 2018 2017 2016 Commissions $ 280.4 $ 255.9 $ 234.0 General expenses 351.7 359.1 292.1 Premium taxes, boards and bureaus 32.5 32.3 25.2 664.6 647.3 551.3 Net deferral of policy acquisition costs (9.9 ) (11.9 ) (4.3 ) Total underwriting, acquisition and insurance expenses $ 654.7 $ 635.4 $ 547.0

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
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) 2018 2017 2016 Current income tax (benefit) provision related to: United States $ 13.4 $ (0.3 ) $ 30.4 United Kingdom 3.2 7.6 3.9 Other jurisdictions 0.1 0.2 2.0 Total current income tax provision 16.7 7.5 36.3 Deferred income tax (benefit) provision related to: United States (13.8 ) (0.3 ) 3.3 United Kingdom 1.1 (17.6 ) (4.4 ) Other jurisdictions 0.1 — — Total deferred income tax (benefit) provision (12.6 ) (17.9 ) (1.1 ) Income tax (benefit) provision $ 4.1 $ (10.4 ) $ 35.2
Schedule of Earned Premiums by Geographic Location	For the years ended December 31, 2018 , 2017 and 2016 , pre-tax income (loss) attributable to our operations and the operations’ effective tax rates were as follows: (in millions) 2018 2017 2016 Pre-Tax Income (Loss) Effective Tax Rate Pre-Tax Income (Loss) Effective Tax Rate Pre-Tax Income (Loss) Effective Tax Rate Bermuda $ 26.0 — % $ 30.2 — % $ 99.6 — % United States 13.9 (5.5 )% 67.5 (0.9 )% 115.2 29.2 % United Kingdom 25.0 18.8 % (53.8 ) 18.7 % (34.8 ) 1.2 % Belgium — (1) — % (3) 0.1 75.0 % (0.1 ) 23.2 % Brazil (0.5 ) — % 0.8 — % 0.6 — % United Arab Emirates 0.8 — % 0.2 — % — (1) — % Ireland (2) (0.2 ) — % (0.2 ) — % (0.2 ) 5.0 % Italy 0.9 — % — — % — — % Malta 1.7 — % 0.3 — % 1.6 0.3 % Luxembourg — (1) — % (5.2 ) — % — — % Switzerland 0.1 18.4 % — (1) 21.1 % — (1) — % Pre-tax income $ 67.7 6.1 % $ 39.9 (26.1 )% $ 181.9 19.4 % (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. (3) Not Meaningful
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) 2018 2017 2016 Income tax provision at expected rate $ 8.2 $ 12.7 $ 34.1 Tax effect of: Nontaxable investment income (1.9 ) (4.7 ) (5.5 ) Foreign exchange adjustments (0.6 ) 2.1 5.3 Withholding taxes 0.4 0.4 2.4 Change in valuation allowance (1.5 ) (0.9 ) 0.7 Impact of change in tax rate related to TCJA (1.6 ) (20.2 ) — Other 1.1 0.2 (1.8 ) Income tax provision $ 4.1 $ (10.4 ) $ 35.2
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) 2018 2017 Deferred tax assets: Losses and loss adjustment expense reserve discounting $ 19.3 $ 17.2 Unearned premiums 23.2 21.4 Net operating loss carryforwards 31.6 21.9 Investment in Limited Partnership Interests 12.0 10.4 Unrealized losses on fixed maturities and other investment securities 9.0 — Impairment of investment values 2.2 4.0 Accrued bonus 4.5 2.6 Stock option expense 1.7 2.0 United Kingdom underwriting results — 3.8 Other 8.0 9.6 Deferred tax assets, gross 111.5 92.9 Deferred tax liabilities: Unrealized gains on equity securities (13.1 ) (30.5 ) Unrealized gains on fixed maturities and other investment securities — (5.0 ) Unrealized gains on limited partnership interests (17.3 ) (14.6 ) Depreciable fixed assets (27.0 ) (22.5 ) Deferred acquisition costs (18.1 ) (16.9 ) TCJA reserve transitional liability (4.5 ) (5.5 ) United Kingdom underwriting results (0.6 ) — Other (7.2 ) (9.1 ) Deferred tax liabilities, gross (87.8 ) (104.1 ) Deferred tax (liabilities) assets, net before valuation allowance $ 23.7 $ (11.2 ) Valuation allowance (29.9 ) (20.1 ) Deferred tax liabilities, net $ (6.2 ) $ (31.3 ) Net deferred tax (liabilities) assets - Other jurisdictions $ (0.6 ) $ 1.2 Net deferred tax liabilities - United States (5.6 ) (32.5 ) Deferred tax liabilities, net $ (6.2 ) $ (31.3 )
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, 2018 Expiration Net operating loss carryforwards by jurisdiction Brazil $ 11.5 Indefinite Italy 41.0 Indefinite Malta 5.5 Indefinite United States 48.9 2025 - 2037

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2018
Leases [Abstract]
Schedule of Future Minimum Payments Under Non-Cancelable Operating Leases	At December 31, 2018 , the future minimum payments under non-cancelable operating leases are as follows: (in millions) Amount Due 2019 $ 18.7 2020 18.6 2021 17.5 2022 14.7 2023 12.3 Thereafter 80.1 Total $ 161.9

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
Revenue and Income (Loss) Before Income Taxes for Each Segment	Revenue and income before income taxes for each segment were as follows: For the Years Ended December 31, (in millions) 2018 2017 2016 Revenue: Earned premiums U.S. Operations $ 1,078.9 $ 936.6 $ 849.5 International Operations 652.5 635.8 560.9 Run-off Lines 0.3 (0.1 ) 0.4 Total earned premiums 1,731.7 1,572.3 1,410.8 Net investment income U.S. Operations 82.9 87.2 71.9 International Operations 32.9 32.7 28.7 Run-off Lines 8.1 9.3 11.3 Corporate and Other 9.2 10.8 3.2 Total net investment income 133.1 140.0 115.1 Fee and other income 9.0 22.5 24.5 Net realized investment (losses) gains (72.0 ) 39.3 26.1 Total revenue $ 1,801.8 $ 1,774.1 $ 1,576.5 For the Years Ended December 31, (in millions) 2018 2017 2016 Income before income taxes U.S. Operations $ 161.4 $ 169.4 $ 174.4 International Operations 32.9 (86.7 ) 51.6 Run-off Lines (9.3 ) (17.9 ) (15.2 ) Total segment income before taxes 185.0 64.8 210.8 Corporate and Other (45.3 ) (64.2 ) (55.0 ) Net realized investment (losses) gains (72.0 ) 39.3 26.1 Total income before income taxes $ 67.7 $ 39.9 $ 181.9
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) 2018 2017 2016 United States $ 1,073.1 $ 930.8 $ 850.0 United Kingdom 455.8 489.3 406.9 Bermuda 85.4 92.1 112.6 Malta 61.0 11.1 2.1 All other jurisdictions 56.4 49.0 39.2 Total earned premiums $ 1,731.7 $ 1,572.3 $ 1,410.8
Identifiable Assets	The following table represents identifiable assets: December 31, (in millions) 2018 2017 U.S. Operations $ 4,707.8 $ 4,298.4 International Operations 3,984.7 3,553.8 Run-off Lines 444.8 449.6 Corporate and Other 420.9 462.2 Total $ 9,558.2 $ 8,764.0
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 (in millions) 2018 2017 2018 2017 U.S. Operations $ 123.5 $ 123.5 $ 0.5 $ 1.5 International Operations 53.5 37.9 93.0 95.3 Total $ 177.0 $ 161.4 $ 93.5 $ 96.8

Statutory Accounting Principl_2

Statutory Accounting Principles (Tables)	12 Months Ended
	Dec. 31, 2018
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) 2018 2017 Bermuda $ 1,451.3 $ 1,489.1 United Kingdom (2) 357.3 330.3 United States 1,003.8 992.8 (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) 2018 2017 2016 Bermuda $ 47.4 $ 55.1 $ 158.2 United Kingdom (2) (9.5 ) (95.5 ) 1.8 United States 110.8 57.0 99.0 (1) Such amounts include ownership interests in affiliate insurance and reinsurance subsidiaries. (2) In U.S. Dollars

Unaudited Quarterly Financial_2

Unaudited Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data	The following tables represent unaudited quarterly financial data for the years ended December 31, 2018 and 2017 . In the opinion of management, all adjustments necessary to present fairly the results of operations for such periods have been made. (in millions, except per share amounts) 1st Quarter 2nd Quarter 3rd Quarter 4th Quarter Year 2018 Earned premiums $ 414.7 $ 417.7 $ 446.9 $ 452.4 $ 1,731.7 Losses and loss adjustment expenses 237.2 245.5 277.5 280.6 1,040.8 Underwriting, acquisition and insurance expenses 160.2 156.8 168.0 169.7 654.7 Underwriting income 17.3 15.4 1.4 2.1 36.2 Net income (loss) before income taxes 25.0 57.1 45.3 (59.7 ) 67.7 Net income (loss) 24.8 41.8 40.6 (43.6 ) 63.6 Net income (loss) per common share: Basic (1) $ 0.73 $ 1.23 $ 1.20 $ (1.29 ) $ 1.87 Diluted (1) $ 0.71 $ 1.20 $ 1.17 $ (1.29 ) $ 1.83 Dividends per common share $ 0.27 $ 0.27 $ 0.27 $ 0.27 $ 1.08 (in millions, except per share amounts) 1st Quarter 2nd Quarter 3rd Quarter 4th Quarter Year 2017 Earned premiums $ 379.4 $ 399.1 $ 389.3 $ 404.5 $ 1,572.3 Losses and loss adjustment expenses 222.5 230.6 326.4 270.7 1,050.2 Underwriting, acquisition and insurance expenses 153.6 154.7 166.1 161.0 635.4 Underwriting income (loss) 3.3 13.8 (103.2 ) (27.2 ) (113.3 ) 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 (2) : Basic (1) $ 1.06 $ 1.32 $ (1.78 ) $ 0.85 $ 1.46 Diluted (1) $ 1.03 $ 1.29 $ (1.78 ) $ 0.83 $ 1.42 Dividends per common share (2) $ 0.23 $ 0.23 $ 0.24 $ 0.24 $ 0.94 (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. (2) On February 20, 2018, our Board of Directors declared at 15% stock dividend, payable on March 21, 2018, to shareholders of record at the close of business on March 7, 2018. The net income (loss) per common share and dividends declared per share have been adjusted to reflect the effect of the stock dividend

Senior Unsecured Fixed Rate N_2

Senior Unsecured Fixed Rate Notes (Tables)	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Schedule Of Unamortized Debt Issuance Costs Deducted From Carrying Value Of Debt Liability	At December 31, 2018 and 2017 , the Notes consisted of the following: (in millions) December 31, 2018 December 31, 2017 Senior unsecured fixed rate notes Principal $ 143.8 $ 143.8 Less: unamortized debt issuance costs (4.0 ) (4.2 ) Senior unsecured fixed rate notes, less unamortized debt issuance costs $ 139.8 $ 139.6
Condensed Consolidating Balance Sheet	CONDENSED CONSOLIDATING BALANCE SHEET DECEMBER 31, 2018 (in millions) Argo Group Argo Group US, Inc. Other Subsidiaries Eliminations (1) Consolidating (2) Total Assets Investments $ 3.8 $ 3,175.9 $ 1,607.3 $ — $ 4,787.0 Cash 1.7 31.7 105.8 — 139.2 Accrued investment income — 20.3 6.9 — 27.2 Premiums receivable — 229.5 420.4 — 649.9 Reinsurance recoverables — 1,635.2 1,053.1 — 2,688.3 Goodwill and other intangible assets, net 41.9 123.8 104.8 — 270.5 Current income taxes receivable, net — 9.1 (0.9 ) — 8.2 Deferred acquisition costs, net — 86.2 81.1 — 167.3 Ceded unearned premiums — 250.4 207.3 — 457.7 Other assets 15.7 165.3 181.9 — 362.9 Intercompany note receivable — 53.7 (53.7 ) — — Investments in subsidiaries 1,852.7 — — (1,852.7 ) — Total assets $ 1,915.8 $ 5,781.1 $ 3,714.0 $ (1,852.7 ) $ 9,558.2 Liabilities and Shareholders' Equity Reserves for losses and loss adjustment expenses $ — $ 2,771.4 $ 1,883.2 $ — $ 4,654.6 Unearned premiums — 797.4 503.5 — 1,300.9 Funds held and ceded reinsurance payable, net — 739.3 268.4 — 1,007.7 Debt 153.4 284.7 142.1 — 580.2 Deferred tax liabilities, net — 5.6 0.6 — 6.2 Accrued underwriting expenses and other liabilities 7.2 112.4 142.3 — 261.9 Due to (from) affiliates 8.5 2.0 (2.0 ) (8.5 ) — Intercompany note payable — 19.1 (19.1 ) — — Total liabilities 169.1 4,731.9 2,919.0 (8.5 ) 7,811.5 Total shareholders' equity 1,746.7 1,049.2 795.0 (1,844.2 ) 1,746.7 Total liabilities and shareholders' equity $ 1,915.8 $ 5,781.1 $ 3,714.0 $ (1,852.7 ) $ 9,558.2 (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 BALANCE SHEET DECEMBER 31, 2017 (in millions) Argo Group Guarantor) Argo Group US, Inc. Other Subsidiaries Eliminations (1) Consolidating (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 (from) 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 STATEMENT OF INCOME FOR THE YEAR ENDED DECEMBER 31, 2018 (in millions) Argo Group Guarantor) Argo Group US, Inc. Other Subsidiaries Eliminations (1) Consolidating (2) Total Premiums and other revenue: Earned premiums $ — $ 861.9 $ 869.8 $ — $ 1,731.7 Net investment income (expense) 33.8 79.4 56.4 (36.5 ) 133.1 Fee and other income — 4.4 4.6 — 9.0 Net realized investment gains (losses) 2.5 (51.3 ) (20.5 ) (2.7 ) (72.0 ) Total revenue 36.3 894.4 910.3 (39.2 ) 1,801.8 Expenses: Losses and loss adjustment expenses — 523.7 517.1 — 1,040.8 Underwriting, acquisition and insurance expenses 11.3 333.6 309.8 — 654.7 Interest expense 6.2 18.2 7.2 — 31.6 Fee and other expense — 5.3 1.8 — 7.1 Foreign currency exchange losses (gains) — 0.2 (0.3 ) — (0.1 ) Total expenses 17.5 881.0 835.6 — 1,734.1 Income (loss)before income taxes 18.8 13.4 74.7 (39.2 ) 67.7 Provision for income taxes — (0.8 ) 4.9 — 4.1 Net income (loss) before equity in earnings of subsidiaries 18.8 14.2 69.8 (39.2 ) 63.6 Equity in undistributed earnings of subsidiaries 44.8 — — (44.8 ) — Net income $ 63.6 $ 14.2 $ 69.8 $ (84.0 ) $ 63.6 (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 Income	CONDENSED CONSOLIDATING STATEMENT OF INCOME FOR THE YEAR ENDED DECEMBER 31, 2017 (in millions) Argo Group Guarantor) Argo Group US, Inc. Other Subsidiaries Eliminations (1) Consolidating (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 gains (losses) 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 losses 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 (benefit) 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 Guarantor) Argo Group US, Inc. Other Subsidiaries Eliminations (1) Consolidating (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 gains (losses) 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 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 losses (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 Cash Flows	CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS FOR YEAR ENDED DECEMBER 31, 2018 (in millions) Argo Group Guarantor) Argo Group US, Inc. Other Subsidiaries Eliminations (1) Consolidating (2) Total Net cash flows from operating activities $ 72.4 $ 182.4 $ 46.5 $ — $ 301.3 Cash flows from investing activities: Proceeds from sales of investments — 1,067.7 532.1 — 1,599.8 Maturities and mandatory calls of fixed maturity investments — 344.9 73.7 — 418.6 Purchases of investments — (1,508.3 ) (640.8 ) — (2,149.1 ) Change in short-term investments and foreign regulatory deposits (3.4 ) (105.0 ) (10.8 ) — (119.2 ) Settlements of foreign currency exchange forward contracts (0.5 ) 2.2 (3.2 ) — (1.5 ) Cash included in acquisition of Ariscom — — 15.6 — 15.6 Purchases of fixed assets and other, net (0.1 ) (19.0 ) (13.4 ) — (32.5 ) Cash used in investing activities (4.0 ) (217.5 ) (46.8 ) — (268.3 ) Cash flows from financing activities: Borrowing under the intercompany note — 19.0 (19.0 ) — — Activity under stock incentive plans 1.6 — — — 1.6 Repurchase of Company's common shares (31.7 ) — — — (31.7 ) Payment of cash dividend to common shareholders (37.5 ) — — — (37.5 ) Cash used in financing activities (67.6 ) 19.0 (19.0 ) — (67.6 ) Effect of exchange rate changes on cash — — (2.8 ) — (2.8 ) Change in cash 0.8 (16.1 ) (22.1 ) — (37.4 ) Cash, beginning of year 0.9 47.8 127.9 — 176.6 Cash, end of period $ 1.7 $ 31.7 $ 105.8 $ — $ 139.2 (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 Guarantor) Argo Group US, Inc. Other Subsidiaries Eliminations (1) Consolidating (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 intercompany note, net 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 period $ 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 Guarantor) Argo Group US, Inc. Other Subsidiaries Eliminations (1) Consolidating ( 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 provided by (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.

Business and Significant Acco_4

Business and Significant Accounting Policies - Additional Information (Detail) $ in Millions	Mar. 21, 2018shares	Feb. 20, 2018shares	Jun. 15, 2016shares	May 03, 2016shares	Dec. 31, 2018USD ($)segmentfoundationtrust	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Jan. 01, 2019USD ($)	Mar. 05, 2018USD ($)	Jan. 01, 2018USD ($)
Business And Significant Accounting Policies [Line Items]
Number of reportable segments | segment					2
Percentage of contractual participation					100.00%
Number of statutory trusts | trust					12
Number of charitable foundations | foundation					2
Stock dividend declared, percent				10.00%	15.00%		10.00%
Short-term investments, maturity term					1 year
Percentage of investment portfolio in fair value from pricing service or broker (less than)					1.00%
Allowance for doubtful accounts					$ 4	$ 3.2
Reinsurance recoverables, allowance for doubtful accounts					1.8	2.1
Estimated liability for return of premiums					6.9	7.9
Estimated amount of premiums receivables due					0.4	0.1
Goodwill					$ 177	161.4
Weighted average useful life					9 years 6 months
Amortization expense					$ 3.3	5.9	$ 5.5
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					0.2
Accumulated depreciation for property and equipment					146	124.1
Net book value of property and equipment					154.8	151
Depreciation expense					24.5	24.1	$ 20.9
Foreign currency translation adjustment					$ 22.4	19
US federal tax rate					21.00%
Accounting Standards Update 2018-02 [Member] | Retained Earnings [Member]
Business And Significant Accounting Policies [Line Items]
Cumulative effect of adoption of ASU, net of taxes						(20.7)
Accounting Standards Update 2018-02 [Member] | AOCI Attributable to Parent [Member]
Business And Significant Accounting Policies [Line Items]
Cumulative effect of adoption of ASU, net of taxes						20.7				$ 20.7
Accounting Standards Update 2016-02 [Member] | Subsequent Event [Member]
Business And Significant Accounting Policies [Line Items]
Right-of-use asset								$ 118
Operating lease, liability								$ 145
Accounting Standards Update 2016-01 [Member] | Retained Earnings [Member]
Business And Significant Accounting Policies [Line Items]
Cumulative effect of adoption of ASU, net of taxes						117.5
Accounting Standards Update 2016-01 [Member] | AOCI Attributable to Parent [Member]
Business And Significant Accounting Policies [Line Items]
Cumulative effect of adoption of ASU, net of taxes						$ (117.5)				$ (117.5)
Minimum [Member]
Business And Significant Accounting Policies [Line Items]
Useful life					3 years
Minimum [Member] | Accounting Standards Update 2016-02 [Member] | Subsequent Event [Member]
Business And Significant Accounting Policies [Line Items]
Lessee, operating lease, discount rate								1.19%
Maximum [Member]
Business And Significant Accounting Policies [Line Items]
Useful life					39 years
Maximum [Member] | Accounting Standards Update 2016-02 [Member] | Subsequent Event [Member]
Business And Significant Accounting Policies [Line Items]
Lessee, operating lease, discount rate								5.07%
Distribution Network [Member]
Business And Significant Accounting Policies [Line Items]
Weighted average useful life					9 years 8 months 12 days
Other [Member]
Business And Significant Accounting Policies [Line Items]
Weighted average useful life					6 years 3 months 18 days
Stock Dividend [Member]
Business And Significant Accounting Policies [Line Items]
Stock dividend declared, percent		15.00%		10.00%
Additional stock issued as dividend (in shares) | shares	4,397,520	4,397,520	2,735,542	2,735,542
Ariscom Compagnia Di Assicurazioni S P A [Member]
Business And Significant Accounting Policies [Line Items]
Percentage of voting interests acquired									100.00%
Goodwill					$ 15.6				$ 15.6

Business and Significant Acco_5

Business and Significant Accounting Policies - Summary of Receivables (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Premiums receivable	$ 2.1	$ 1.5	$ 1.1
Reinsurance recoverables	0	0	0
Net written off	$ 2.1	$ 1.5	$ 1.1

Business and Significant Acco_6

Business and Significant Accounting Policies - Intangible Assets and Accumulated Amortization (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 147.3	$ 147.3
Accumulated Amortization	53.8	50.5
Lloyd's Capacity [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	89	89
Distribution Network [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	50.2	50.2
Accumulated Amortization	47	44.1
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	3.3
Accumulated Amortization	$ 2	$ 1.6

Business and Significant Acco_7

Business and Significant Accounting Policies - Schedule of Interest Paid (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure - Business and Significant Accounting Policies - Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) [Line Items]
Total interest paid	$ 31.3	$ 27.1	$ 19.3
Income taxes paid	24.8	16.5	16.6
Income taxes recovered	0	(2.5)	(0.5)
Income taxes paid, net	24.8	14	16.1
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	0.3	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	15.5	12.6	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	$ 6.5	$ 4.9	$ 2.3

Acquisition of Ariscom - Additi

Acquisition of Ariscom - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Mar. 05, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill	$ 177		$ 161.4
Ariscom Compagnia Di Assicurazioni S P A [Member]
Business Acquisition [Line Items]
Percentage of voting interests acquired		100.00%
Goodwill	$ 15.6	$ 15.6

Investments - Additional Inform

Investments - Additional Information (Detail) $ in Millions	Dec. 31, 2018USD ($)Securitysecurity	Dec. 31, 2017USD ($)
Amortized Cost And Fair Value Debt Securities [Abstract]
Fair value of investments, assets managed on behalf of the trade capital providers | $	$ 133.4	$ 130.8
Number of securities in an unrealized loss position, total	9,241
Number of securities in an unrealized loss position for less than one year | security	3,113
Number of securities in an unrealized loss position for a period of one year or greater	1,267

Investments - Schedule of Amort

Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Schedule of Invested Assets [Line Items]
Amortized Cost	$ 3,529.1	$ 3,320.6
Fair Value	3,460.4	3,343.4
Fixed Maturities [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	3,529.1	3,320.6
Gross Unrealized Gains	17.5	50.1
Gross Unrealized Losses	86.2	27.3
Fair Value	3,460.4	3,343.4
Fixed Maturities [Member] | U.S. Governments [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	240.9	419.9
Gross Unrealized Gains	0.2	0.2
Gross Unrealized Losses	4.9	5
Fair Value	236.2	415.1
Fixed Maturities [Member] | Foreign Governments [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	224.1	229
Gross Unrealized Gains	0.5	6.7
Gross Unrealized Losses	7.8	2.5
Fair Value	216.8	233.2
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	236.7	327.7
Gross Unrealized Gains	4.3	9.3
Gross Unrealized Losses	1.2	1.1
Fair Value	239.8	335.9
Fixed Maturities [Member] | Corporate Bonds [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	1,808.7	1,514.5
Gross Unrealized Gains	7.5	24.4
Gross Unrealized Losses	58.7	13.2
Fair Value	1,757.5	1,525.7
Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	205.3	136.3
Gross Unrealized Gains	0.7	0.1
Gross Unrealized Losses	3.2	1.5
Fair Value	202.8	134.9
Fixed Maturities [Member] | Residential Mortgage Backed Securities [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	413.1	309.3
Gross Unrealized Gains	3.4	2.8
Gross Unrealized Losses	5.7	2.7
Fair Value	410.8	309.4
Fixed Maturities [Member] | Asset-backed Securities [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	173.6	161.3
Gross Unrealized Gains	0.4	0.7
Gross Unrealized Losses	1.2	0.8
Fair Value	172.8	161.2
Fixed Maturities [Member] | Collateralized Debt Obligations [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	226.7	222.6
Gross Unrealized Gains	0.5	5.9
Gross Unrealized Losses	3.5	0.5
Fair Value	$ 223.7	$ 228

Investments - Schedule of Amo_2

Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) $ in Millions	Dec. 31, 2018USD ($)
Amortized Cost
Due in one year or less	$ 270.5
Due after one year through five years	1,517
Due after five years through ten years	581.5
Thereafter	141.4
Structured securities	1,018.7
Total	3,529.1
Fair Value
Due in one year or less	266.3
Due after one year through five years	1,486.2
Due after five years through ten years	557.1
Thereafter	140.7
Structured securities	1,010.1
Total	$ 3,460.4

Investments - Schedule of Carry

Investments - Schedule of Carrying Value and Unfunded Investment Commitments of Other Invested Assets Portfolio (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Carrying Value	$ 4,787	$ 4,742.9
Hedge funds [Member]
Schedule of Investments [Line Items]
Carrying Value	120.6	163.6
Unfunded Commitments	0	0
Private equity [Member]
Schedule of Investments [Line Items]
Carrying Value	211.8	179.2
Unfunded Commitments	120.5	129.9
Long only funds [Member]
Schedule of Investments [Line Items]
Carrying Value	153	196.5
Unfunded Commitments	0	0
Other Investments [Member]
Schedule of Investments [Line Items]
Carrying Value	4.4	4.3
Unfunded Commitments	0	0
Other Invested Assets [Member]
Schedule of Investments [Line Items]
Carrying Value	489.8	543.6
Unfunded Commitments	$ 120.5	$ 129.9

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, 2018	Dec. 31, 2017
Unrealized Losses
Unrealized losses one year or greater	$ 0.1
Fixed Maturities [Member]
Fair Value
Less Than One Year	1,514.3	$ 1,538.9
One Year or Greater	1,105.3	342.7
Total	2,619.6	1,881.6
Unrealized Losses
Less Than One Year	55.3	17.2
One Year or Greater	30.9	10.1
Total	86.2	27.3
Fixed Maturities [Member] | U.S. Governments [Member]
Fair Value
Less Than One Year	28.2	313.7
One Year or Greater	173	83.7
Total	201.2	397.4
Unrealized Losses
Less Than One Year	0.2	1.9
One Year or Greater	4.7	3.1
Total	4.9	5
Fixed Maturities [Member] | Foreign Governments [Member]
Fair Value
Less Than One Year	73.4	175.2
One Year or Greater	125	35.9
Total	198.4	211.1
Unrealized Losses
Less Than One Year	3.6	2
One Year or Greater	4.2	0.5
Total	7.8	2.5
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value
Less Than One Year	53.3	33.3
One Year or Greater	25.3	22.4
Total	78.6	55.7
Unrealized Losses
Less Than One Year	0.6	0.5
One Year or Greater	0.6	0.6
Total	1.2	1.1
Fixed Maturities [Member] | Corporate Bonds [Member]
Fair Value
Less Than One Year	964.3	674.1
One Year or Greater	440.8	77.7
Total	1,405.1	751.8
Unrealized Losses
Less Than One Year	45.7	9.9
One Year or Greater	13	3.3
Total	58.7	13.2
Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value
Less Than One Year	48.5	58.2
One Year or Greater	90.6	37.8
Total	139.1	96
Unrealized Losses
Less Than One Year	0.6	0.4
One Year or Greater	2.6	1.1
Total	3.2	1.5
Fixed Maturities [Member] | Residential Mortgage Backed Securities [Member]
Fair Value
Less Than One Year	63.5	164.4
One Year or Greater	176.1	52.4
Total	239.6	216.8
Unrealized Losses
Less Than One Year	0.7	1.6
One Year or Greater	5	1.1
Total	5.7	2.7
Fixed Maturities [Member] | Asset-backed Securities [Member]
Fair Value
Less Than One Year	73.6	85.4
One Year or Greater	64.2	31.9
Total	137.8	117.3
Unrealized Losses
Less Than One Year	0.6	0.4
One Year or Greater	0.6	0.4
Total	1.2	0.8
Fixed Maturities [Member] | Collateralized Debt Obligations [Member]
Fair Value
Less Than One Year	209.5	34.6
One Year or Greater	10.3	0.9
Total	219.8	35.5
Unrealized Losses
Less Than One Year	3.3	0.5
One Year or Greater	0.2	0
Total	$ 3.5	$ 0.5

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Other-than-temporary impairment:
Other-than-temporary impairment losses	$ (7.6)	$ (2.5)	$ (10.2)
Equity Securities [Member]
Other-than-temporary impairment:
Other-than-temporary impairment losses	0	(1.7)	(8.5)
Other Invested Assets [Member]
Other-than-temporary impairment:
Other-than-temporary impairment losses	(1)	0	0
Obligations of States and Political Subdivisions [Member]
Other-than-temporary impairment:
Other-than-temporary impairment losses	0	(0.1)	0
Corporate Bonds [Member]
Other-than-temporary impairment:
Other-than-temporary impairment losses	$ (6.6)	$ (0.7)	$ (1.7)

Investments - Schedule of Inves

Investments - Schedule of Investment Income and Expenses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment income:
Interest on fixed maturities	$ 115	$ 97.1	$ 88.9
Dividends on equity securities	12.5	13.9	15.6
Income on alternative investments	19.8	49.5	29.9
Income on short-term and other investments	9.5	7.7	0.4
Investment income	156.8	168.2	134.8
Investment expenses	(23.7)	(28.2)	(19.7)
Net investment income	$ 133.1	$ 140	$ 115.1

Investments - Schedule of Compa

Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	$ 59.3	$ 52.1	$ 70.7
Realized losses on fixed maturities and other	(63.6)	(57.2)	(83.9)
Net realized gains on equity securities	37.4	46.1	47.8
Other-than-temporary impairment losses on equity securities	0	(1.7)	(8.5)
Change in unrealized (losses) gains on equity securities held at the end of the period	(105.1)	0	0
Net realized (losses) gains on equity securities	(67.7)	44.4	39.3
Net realized investment and other (losses) gains	(72)	39.3	26.1
Income tax expense	(11.2)	(12)	(11.6)
After tax	(83.2)	27.3	14.5
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	17.7	25.7	21.5
Realized losses on fixed maturities and other	(16)	(20)	(35.9)
Other Investments [Member]
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	41.4	25.7	47.5
Realized losses on fixed maturities and other	(39.5)	(36.2)	(46)
Short-Term Investments [Member]
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	0.2	0.7	0.5
Realized losses on fixed maturities and other	(0.5)	(0.2)	(0.3)
Other Assets [Member]
Schedule of Investments [Line Items]
Realized gains on fixed maturities and other	0	0	1.2
Realized losses on fixed maturities and other	0	0	0
Other-Than-Temporary Impairment Losses on Fixed Maturities [Member]
Schedule of Investments [Line Items]
Realized losses on fixed maturities and other	(6.6)	(0.8)	(1.7)
Other-Than-Temporary Impairment Losses on Equity Securities [Member]
Schedule of Investments [Line Items]
Realized losses on fixed maturities and other	$ (1)	$ 0	$ 0

Investments - Schedule of Chang

Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	$ (88.5)	$ 64.1	$ 34.9
Income tax benefit (expense)	13	(14.6)	(2.4)
Net unrealized investment (losses) gains, net of income taxes	(75.5)	49.5	32.5
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	(88.1)	25.4	36.5
Equity Securities [Member]
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	0	36.6	(2.6)
Other Investments [Member]
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	0.1	2.1	0.6
Short-Term Investments [Member]
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	$ (0.5)	$ 0	$ 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, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 2.6	$ (0.4)
Operational Currency Exposure [Member]
Schedule of Investments [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	4.4	(0.2)
Asset Manager Investment Exposure [Member]
Schedule of Investments [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	(0.3)	(0.9)
Total return strategy [Member]
Schedule of Investments [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ (1.5)	$ 0.7

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Gross realized investment gains	$ 42.2	$ 24.3	$ 45.5
Gross realized investment losses	(39.5)	(32.7)	(43.5)
Net realized investment gains (losses) on foreign currency exchange forward contracts	2.7	(8.4)	2
Operational Currency Exposure [Member]
Schedule of Investments [Line Items]
Gross realized investment gains	9.7	12.4	10.9
Gross realized investment losses	(7.9)	(13.8)	(18)
Asset Manager Investment Exposure [Member]
Schedule of Investments [Line Items]
Gross realized investment gains	5.8	1.5	9
Gross realized investment losses	(3)	(11.3)	(4.5)
Total return strategy [Member]
Schedule of Investments [Line Items]
Gross realized investment gains	26.7	10.4	25.6
Gross realized investment losses	$ (28.6)	$ (7.6)	$ (21)

Investments - Components of Res

Investments - Components of Restricted Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2017
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities pledged as collateral for letters of credit	$ 120.9	$ 78.1
Securities and cash on deposit supporting Lloyd’s business	376.8	355.5
Total restricted investments	670.3	608.9
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	$ 172.6	$ 175.3

Investments - Financial Assets

Investments - Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	$ 4,787	$ 4,742.9
Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	489.8	543.6
Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	4,411.6	4,308.1
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	1,026.9	1,227.3
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	3,374.3	3,076.6
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	10.4	4.2
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	3,460.4	3,343.4
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	226.7	410.6
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	3,231.5	2,930.9
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	2.2	1.9
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	223.7	228
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Collateralized Loan Obligations [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0	0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Collateralized Loan Obligations [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	223.7	228
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Collateralized Loan Obligations [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0	0
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	410.8	309.4
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Residential 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	0	0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Residential Mortgage Backed Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	410.8	309.4
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Residential Mortgage Backed Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	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	236.2	415.1
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	226.7	410.6
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	9.5	4.5
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	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	216.8	233.2
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	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	216.8	233.2
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	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	239.8	335.9
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	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	239.8	335.9
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	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	1,757.5	1,525.7
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	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	1,755.3	1,523.8
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	2.2	1.9
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	202.8	134.9
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	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	202.8	134.9
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	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	172.8	161.2
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	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	172.8	161.2
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	0	0
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	354.5	487.4
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	346.3	483
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	0	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	8.2	2.3
Fair Value, Measurements, Recurring [Member] | Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	114.4	108.8
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	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	114.4	108.8
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	0	0
Fair Value, Measurements, Recurring [Member] | Short-Term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	482.3	368.5
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	453.9	333.7
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	28.4	34.8
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	$ 0	$ 0

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, 2018	Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 4.2	$ 2.4
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Total gains or losses (realized/unrealized):
Included in net income (loss)	0.2	0
Included in other comprehensive income (loss)	0.3	0.1
Purchases, issuances, sales, and settlements:
Purchases	7.3	1.7
Issuances	0	0
Sales	(1.6)	0
Settlements	0	0
Ending balance	10.4	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 end of period	0	0
Credit-Financial [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	1.9	2
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Total gains or losses (realized/unrealized):
Included in net income (loss)	0	0
Included in other comprehensive income (loss)	0.3	(0.1)
Purchases, issuances, sales, and settlements:
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Ending balance	2.2	1.9
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, Calculation [Roll Forward]
Beginning balance	2.3	0.4
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Total gains or losses (realized/unrealized):
Included in net income (loss)	0.2	0
Included in other comprehensive income (loss)	0	0.2
Purchases, issuances, sales, and settlements:
Purchases	7.3	1.7
Issuances	0	0
Sales	(1.6)	0
Settlements	0	0
Ending balance	8.2	2.3
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	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Insurance [Abstract]
Reinsurance recoverables, allowance for doubtful accounts	$ 1.8				$ 2.1				$ 1.8	$ 2.1
Amount of collateral under reinsurance agreement	944				907.7				944	907.7
Paid loss recoverables in insurance recoverables	596.6				380.3				596.6	380.3
Losses and loss adjustment expenses	$ 280.6	$ 277.5	$ 245.5	$ 237.2	$ 270.7	$ 326.4	$ 230.6	$ 222.5	1,040.8	1,050.2	$ 810.1
Net of amounts ceded to reinsurers									$ 888.9	$ 992.6	$ 419.3

Reinsurance - Schedule of Reins

Reinsurance - Schedule of Reinsurance Premium (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Insurance [Abstract]
Direct written premiums									$ 2,293.8	$ 2,029.2	$ 1,792.5
Reinsurance ceded to other companies									(1,189.7)	(1,043.7)	(724.6)
Reinsurance assumed from other companies									661.4	668	372.3
Net written premiums									1,765.5	1,653.5	1,440.2
Direct earned premiums									2,201.9	1,912.2	1,722.8
Reinsurance ceded to other companies									(1,137.9)	(1,033.6)	(675.8)
Reinsurance assumed from other companies									667.7	693.7	363.8
Net earned premiums	$ 452.4	$ 446.9	$ 417.7	$ 414.7	$ 404.5	$ 389.3	$ 399.1	$ 379.4	$ 1,731.7	$ 1,572.3	$ 1,410.8
Percentage of reinsurance assumed to net earned premiums									38.60%	44.10%	25.80%

Reserves for Losses and Loss _3

Reserves for Losses and Loss Adjustment Expenses - Reserves for Losses and Loss Adjustment Expenses (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves beginning of the year				$ 2,488				$ 2,180.2	$ 2,488	$ 2,180.2	$ 2,133.3
Losses and LAE incurred during current calendar year, net of reinsurance:
Current accident year									1,058.8	1,058.4	843.4
Prior accident years									(18)	(8.2)	(33.3)
Losses and LAE incurred during calendar year, net of reinsurance	$ 280.6	$ 277.5	$ 245.5	$ 237.2	$ 270.7	$ 326.4	$ 230.6	$ 222.5	1,040.8	1,050.2	810.1
Losses and LAE payments made during current calendar year, net of reinsurance:
Current accident year									273.3	289.6	178.9
Prior accident years									665.6	599.8	537.6
Losses and LAE payments made during current calendar year, net of reinsurance:									938.9	889.4	716.5
Change in participation interest									(25.5)	(23.2)	(36.3)
Foreign exchange adjustments									(44.9)	38.4	(10.4)
Net reserves - end of year	2,562.9				2,488				2,562.9	2,488	2,180.2
Reinsurance recoverables on unpaid losses and LAE, end of year	2,091.7				1,713				2,091.7	1,713	1,170.6
Reserves for losses and loss adjustment expenses	$ 4,654.6				$ 4,201				4,654.6	4,201	3,350.8
Maybrooke Holdings S A [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net Maybrooke reserves acquired									0	131.8
Ariscom Compagnia Di Assicurazioni S P A [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net Maybrooke reserves acquired									$ 43.4	$ 0	$ 0

Reserves for Losses and Loss _4

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	$ (18)	$ (8.2)	$ (33.3)
U.S. Operations [Member]
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	(20.8)	(38.7)	(35.9)
International Operations [Member]
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	(9.5)	13.2	(16)
Run Off Lines [Member]
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	$ 12.3	$ 17.3	$ 18.6

Reserves for Losses and Loss _5

Reserves for Losses and Loss Adjustment Expenses - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018segment
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Number of reportable segments	2
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	6.00%

Reserves for Losses and Loss _6

Reserves for Losses and Loss Adjustment Expenses - Summary of Information about Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance (Detail) $ in Millions	Dec. 31, 2018USD ($)claim	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2012USD ($)	Dec. 31, 2011USD ($)	Dec. 31, 2010USD ($)
Claims Development [Line Items]
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	$ 2,516.2
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	1,791
Cumulative Paid Losses & ALAE, Net of Reinsurance	952.5
Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance	54.7
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	893.2
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	187.8	$ 189	$ 192.8	$ 193.5	$ 200	$ 205.8	$ 206	$ 202.9
Cumulative Paid Losses & ALAE, Net of Reinsurance	174.3	170	162.4	146.6	122.9	91	53.8	17.6
IBNR & Expected Development on Reported Claims	$ 10
Cumulative Number of Reported Claims | claim	8,454
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 182.3	182.1	184.4	183.6	189.7	196	189.6
Cumulative Paid Losses & ALAE, Net of Reinsurance	163.4	157.5	142.4	120.8	89.1	52.8	17.2
IBNR & Expected Development on Reported Claims	$ 11
Cumulative Number of Reported Claims | claim	7,389
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 216	220.4	227.2	224.3	222.6	217.9
Cumulative Paid Losses & ALAE, Net of Reinsurance	179.6	163.7	135.2	100.4	60.2	17.6
IBNR & Expected Development on Reported Claims	$ 21
Cumulative Number of Reported Claims | claim	7,297
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 212.3	211.9	213.2	215.2	213
Cumulative Paid Losses & ALAE, Net of Reinsurance	154.5	131.6	95.9	52.2	15
IBNR & Expected Development on Reported Claims	$ 36.2
Cumulative Number of Reported Claims | claim	6,502
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 226.4	228.6	237.1	232.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	131.5	91.4	51.9	16.5
IBNR & Expected Development on Reported Claims	$ 60.3
Cumulative Number of Reported Claims | claim	6,000
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 243.1	250.6	246.4
Cumulative Paid Losses & ALAE, Net of Reinsurance	95.5	52.8	17.4
IBNR & Expected Development on Reported Claims	$ 100.3
Cumulative Number of Reported Claims | claim	5,548
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 244.3	253.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	38.7	11.5
IBNR & Expected Development on Reported Claims	$ 153.3
Cumulative Number of Reported Claims | claim	5,302
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 278.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	15
IBNR & Expected Development on Reported Claims	$ 227.9
Cumulative Number of Reported Claims | claim	4,026
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 1,021.2
Cumulative Paid Losses & ALAE, Net of Reinsurance	647.3
Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance	48.7
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	422.6
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	154	153.7	154	158.2	157.5	159	155.1	140.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	143.1	139.8	135.1	126.1	111.3	85.9	57.5	23.2
IBNR & Expected Development on Reported Claims	$ 6.3
Cumulative Number of Reported Claims | claim	28,160
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 146.7	147.7	151.5	153.3	149.7	146.3	140.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	131.8	127.9	120.8	105.8	80.7	51	20.1
IBNR & Expected Development on Reported Claims	$ 8
Cumulative Number of Reported Claims | claim	23,627
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 130.6	131.1	133.2	136.7	133.2	126.6
Cumulative Paid Losses & ALAE, Net of Reinsurance	109.7	102.8	93.6	74	49.4	18.9
IBNR & Expected Development on Reported Claims	$ 10.3
Cumulative Number of Reported Claims | claim	18,948
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 111.5	114.5	116.9	121.9	115.6
Cumulative Paid Losses & ALAE, Net of Reinsurance	86.1	75.3	58.7	38.8	17.4
IBNR & Expected Development on Reported Claims	$ 13.9
Cumulative Number of Reported Claims | claim	16,316
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 102.3	101.7	106.7	107.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	64.2	48.8	35	17.2
IBNR & Expected Development on Reported Claims	$ 21.1
Cumulative Number of Reported Claims | claim	14,530
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 99.3	99.9	96.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	48.6	31.7	11.1
IBNR & Expected Development on Reported Claims	$ 22.1
Cumulative Number of Reported Claims | claim	11,583
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 129.5	121.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	44.4	16.3
IBNR & Expected Development on Reported Claims	$ 50.2
Cumulative Number of Reported Claims | claim	12,897
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 147.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	19.4
IBNR & Expected Development on Reported Claims	$ 82.8
Cumulative Number of Reported Claims | claim	13,351
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 318.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	167.1
Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance	0.1
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	151.3
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	26	26.6	26.9	28.2	32.5	35	35	35
Cumulative Paid Losses & ALAE, Net of Reinsurance	25.8	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	820
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 24.9	24.5	24	25.8	28.6	28.3	27.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	23.5	22.6	21.4	19.9	16.9	8.6	2.3
IBNR & Expected Development on Reported Claims	$ 0.4
Cumulative Number of Reported Claims | claim	640
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 19.5	19.8	19	21.1	21.5	20.9
Cumulative Paid Losses & ALAE, Net of Reinsurance	17.5	17.6	14.2	10.9	6.3	1.9
IBNR & Expected Development on Reported Claims	$ 1.2
Cumulative Number of Reported Claims | claim	620
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 36.2	33.7	26	22.4	22.4
Cumulative Paid Losses & ALAE, Net of Reinsurance	25.5	24.1	15.1	5.4	2.3
IBNR & Expected Development on Reported Claims	$ 2.8
Cumulative Number of Reported Claims | claim	1,043
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 34	33.2	29.5	29.9
Cumulative Paid Losses & ALAE, Net of Reinsurance	20.8	15.6	8.3	1.8
IBNR & Expected Development on Reported Claims	$ 2.1
Cumulative Number of Reported Claims | claim	1,816
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 45.1	44.8	44.2
Cumulative Paid Losses & ALAE, Net of Reinsurance	24.6	11.9	2.4
IBNR & Expected Development on Reported Claims	$ 13.1
Cumulative Number of Reported Claims | claim	3,226
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 61.8	60.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	24.9	3.5
IBNR & Expected Development on Reported Claims	$ 15.9
Cumulative Number of Reported Claims | claim	3,669
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 70.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	4.5
IBNR & Expected Development on Reported Claims	$ 55
Cumulative Number of Reported Claims | claim	3,603
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 64.6
Cumulative Paid Losses & ALAE, Net of Reinsurance	12.6
Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance	0
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	52
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	1.7	2	2.6	3.6	3.4	3.4	3.9	0.2
Cumulative Paid Losses & ALAE, Net of Reinsurance	1.7	1.7	1.7	1.2	1.3	1.4	1.6	0
IBNR & Expected Development on Reported Claims	$ 0
Cumulative Number of Reported Claims | claim	80
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 3.5	3.9	4	4.3	4.9	6.7	7.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	3.3	3.4	3.3	3.3	3.3	3.3	3.6
IBNR & Expected Development on Reported Claims	$ 0.2
Cumulative Number of Reported Claims | claim	129
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 0.9	1.7	2.5	4.6	8.6	10
Cumulative Paid Losses & ALAE, Net of Reinsurance	0.9	0.9	0.9	0.9	0.9	0.4
IBNR & Expected Development on Reported Claims	$ 0
Cumulative Number of Reported Claims | claim	50
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 4.8	6	8.9	13.1	13.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	4.1	4	4	3.3	1.1
IBNR & Expected Development on Reported Claims	$ 0.8
Cumulative Number of Reported Claims | claim	50
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 5.5	9.5	14.3	14.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	0.3	0.2	0.1	0.2
IBNR & Expected Development on Reported Claims	$ 2.9
Cumulative Number of Reported Claims | claim	24
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 11.2	15	15
Cumulative Paid Losses & ALAE, Net of Reinsurance	2.2	1.6	1.3
IBNR & Expected Development on Reported Claims	$ 5.5
Cumulative Number of Reported Claims | claim	58
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 16.2	16.2
Cumulative Paid Losses & ALAE, Net of Reinsurance	0.1	0.3
IBNR & Expected Development on Reported Claims	$ 16.2
Cumulative Number of Reported Claims | claim	84
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 20.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	0
IBNR & Expected Development on Reported Claims	$ 19.8
Cumulative Number of Reported Claims | claim	71
International Operations [Member] | Reinsurance Division [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 698.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	604.5
Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance	3.6
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	97.2
International Operations [Member] | Reinsurance Division [Member] | Property [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	138.9	132.3	104.3	104.4	105.1	107.1	105	116.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	134.1	127.2	99	97.8	95.5	87.7	66.5	40.9
IBNR & Expected Development on Reported Claims	$ 0.3
Cumulative Number of Reported Claims | claim	459
International Operations [Member] | Reinsurance Division [Member] | Property [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 66.9	62.7	46.4	51.6	50.3	51.3	47.2
Cumulative Paid Losses & ALAE, Net of Reinsurance	64.6	58.7	44	49.6	40.6	31.1	12.4
IBNR & Expected Development on Reported Claims	$ 0.2
Cumulative Number of Reported Claims | claim	277
International Operations [Member] | Reinsurance Division [Member] | Property [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 32.4	33	31.4	33.1	33.6	32.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	31.1	31	29	26.5	16.7	4.2
IBNR & Expected Development on Reported Claims	$ 0.3
Cumulative Number of Reported Claims | claim	219
International Operations [Member] | Reinsurance Division [Member] | Property [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 38.1	35.3	24.2	26.5	26.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	36.9	36.4	18.3	12.7	2.8
IBNR & Expected Development on Reported Claims	$ 0.5
Cumulative Number of Reported Claims | claim	221
International Operations [Member] | Reinsurance Division [Member] | Property [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 27.1	32.6	23.3	27.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	20.6	20.3	11	4.3
IBNR & Expected Development on Reported Claims	$ 3.2
Cumulative Number of Reported Claims | claim	219
International Operations [Member] | Reinsurance Division [Member] | Property [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 169.1	175.5	43.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	153	142.3	13.6
IBNR & Expected Development on Reported Claims	$ 11
Cumulative Number of Reported Claims | claim	381
International Operations [Member] | Reinsurance Division [Member] | Property [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 158	157.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	139.5	84.2
IBNR & Expected Development on Reported Claims	$ (28.8)
Cumulative Number of Reported Claims | claim	774
International Operations [Member] | Reinsurance Division [Member] | Property [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 67.6
Cumulative Paid Losses & ALAE, Net of Reinsurance	24.7
IBNR & Expected Development on Reported Claims	$ 4.2
Cumulative Number of Reported Claims | claim	391
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 79.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	27.1
Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance	0
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	52.2
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	0	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	0	0
IBNR & Expected Development on Reported Claims	$ 0
Cumulative Number of Reported Claims | claim	1,423
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 0	1.7	4.4	5.6	7.4	7.4	7.4
Cumulative Paid Losses & ALAE, Net of Reinsurance	0	0	0	0	0	0	0
IBNR & Expected Development on Reported Claims	$ 0
Cumulative Number of Reported Claims | claim	1,381
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 2.2	4.9	8.5	8.5	8.5	8.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	2.3	2.3	0	0	0	0
IBNR & Expected Development on Reported Claims	$ 0
Cumulative Number of Reported Claims | claim	1,190
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 1.5	6.2	9.8	9.8	9.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	1.2	0.1	0.1	0	0
IBNR & Expected Development on Reported Claims	$ 0.2
Cumulative Number of Reported Claims | claim	1,329
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 35.4	24.8	14.3	11.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	20.3	16.1	0	0
IBNR & Expected Development on Reported Claims	$ 1
Cumulative Number of Reported Claims | claim	1,567
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 14	14	13.9
Cumulative Paid Losses & ALAE, Net of Reinsurance	0	0	0
IBNR & Expected Development on Reported Claims	$ 13.9
Cumulative Number of Reported Claims | claim	1,874
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 17.3	17.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	3.3	0
IBNR & Expected Development on Reported Claims	$ 12.2
Cumulative Number of Reported Claims | claim	1,900
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 8.9
Cumulative Paid Losses & ALAE, Net of Reinsurance	0
IBNR & Expected Development on Reported Claims	$ 8.8
Cumulative Number of Reported Claims | claim	839
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 190.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	96.4
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	94.3
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member] | Accident Year 2010 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	5.7	5.8	5.7	5.8	5.9	6.1	4.4	4.8	$ 5.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	4.3	4	3.7	2.8	2	1.1	0.6	0.2	0
IBNR & Expected Development on Reported Claims	0.1
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	10.7	10.6	10.1	10.2	10.9	10.8	8.6	8
Cumulative Paid Losses & ALAE, Net of Reinsurance	8.7	7.8	6.9	5.4	3.4	1.6	0.8	0.2
IBNR & Expected Development on Reported Claims	0.2
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	15	14.5	13.8	14.2	14.9	10.8	8.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	11.8	10.1	8.2	5.8	2.6	1.1	0.4
IBNR & Expected Development on Reported Claims	0.6
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	25.1	24.3	24.4	26.2	26.7	22.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	19.7	15.6	11.3	7	3.2	1.5
IBNR & Expected Development on Reported Claims	2.5
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	33.8	32.5	33.6	35.9	37.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	20.2	13.6	9.8	4.5	1.9
IBNR & Expected Development on Reported Claims	5
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	29.6	29.2	29.6	34.2
Cumulative Paid Losses & ALAE, Net of Reinsurance	12.3	7.4	5.1	0.8
IBNR & Expected Development on Reported Claims	8.2
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	26.1	26.4	25.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	10.6	5.7	1.8
IBNR & Expected Development on Reported Claims	10.2
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	23.3	24.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	6.6	1.9
IBNR & Expected Development on Reported Claims	15.2
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	21.4
Cumulative Paid Losses & ALAE, Net of Reinsurance	2.2
IBNR & Expected Development on Reported Claims	17.6
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	213.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	113.5
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	99.6
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2010 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	9.7	9.5	9.5	9.4	10.1	11.2	12.7	12.8	15.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	7.6	6.8	6.5	5.1	3.8	3	2	0.9	0.1
IBNR & Expected Development on Reported Claims	0.1
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	15.7	15.1	14.7	14.7	15.5	18.5	21	19.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	12.8	11.3	10.6	8.3	6.6	4.1	2.5	1
IBNR & Expected Development on Reported Claims	0.4
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	15.4	14.9	14	14	14.1	13.8	13.9
Cumulative Paid Losses & ALAE, Net of Reinsurance	11.2	9.5	8.1	5.7	4.3	1.8	0.6
IBNR & Expected Development on Reported Claims	0.8
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	23.1	22.3	22.2	22.6	22.4	22.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	17.6	15.4	11.6	7	3.7	1.7
IBNR & Expected Development on Reported Claims	3
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	40.9	39	35.9	35.7	34.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	27.9	23.7	14.6	6.3	1.6
IBNR & Expected Development on Reported Claims	6.5
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	38.7	38.5	37.3	37.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	19.7	14.8	8.2	2.2
IBNR & Expected Development on Reported Claims	11.3
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	26.4	26.9	32.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	10.6	5.8	2
IBNR & Expected Development on Reported Claims	10.8
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	22	24.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	5.1	1.2
IBNR & Expected Development on Reported Claims	14.6
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	21.2
Cumulative Paid Losses & ALAE, Net of Reinsurance	1
IBNR & Expected Development on Reported Claims	17.6
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	609.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	462.7
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	146.8
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2010 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	12	12	12	12.1	12.1	13	13.6	15.7	12.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	10.2	10	10	9.7	9	9.2	8.8	8.6	1.8
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	32.7	33.1	33.2	33.2	34	38.7	40	38.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	29.4	29.5	29.1	28.5	27.1	23.6	19.5	11.5
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	58.2	58.8	59	59.2	60.7	56.4	52.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	50.1	49.8	48.7	45.7	38.8	27.4	17.9
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	81.5	81.8	82.1	82.5	81.2	75.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	78.6	78.2	76.2	68.7	52.3	30.9
IBNR & Expected Development on Reported Claims	(0.5)
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	100.6	101.1	99.7	98.4	92.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	90.4	88.9	82.5	71.4	37.7
IBNR & Expected Development on Reported Claims	0.7
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	95.3	93.8	87.8	89.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	74	66.2	54	30.9
IBNR & Expected Development on Reported Claims	3
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	87.7	84.5	86
Cumulative Paid Losses & ALAE, Net of Reinsurance	68.7	59	37.3
IBNR & Expected Development on Reported Claims	6.6
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	76.5	80.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	42.6	20.1
IBNR & Expected Development on Reported Claims	21.8
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	65
Cumulative Paid Losses & ALAE, Net of Reinsurance	18.7
IBNR & Expected Development on Reported Claims	40.3
International Operations [Member] | Syndicate 1200 [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	693.6
Cumulative Paid Losses & ALAE, Net of Reinsurance	567.6
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	126
International Operations [Member] | Syndicate 1200 [Member] | Property [Member] | Accident Year 2010 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	51.1	51.4	50.7	51.2	51.5	54.2	54.6	58.2	50.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	43.4	42.6	41.5	40	35.8	33.8	28	19.5	$ 1.8
IBNR & Expected Development on Reported Claims	0
International Operations [Member] | Syndicate 1200 [Member] | Property [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	91.8	92.4	92.4	92.9	94.8	107.5	113.3	108
Cumulative Paid Losses & ALAE, Net of Reinsurance	82.4	82.7	81	79.3	73.5	61.8	47.1	$ 23.4
IBNR & Expected Development on Reported Claims	0.2
International Operations [Member] | Syndicate 1200 [Member] | Property [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	89.9	90.8	90.9	91.9	92.8	88.5	88.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	77.7	77.5	76.3	73.9	62.8	47.6	$ 29.4
IBNR & Expected Development on Reported Claims	0.2
International Operations [Member] | Syndicate 1200 [Member] | Property [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	75.6	76.5	76.7	78.2	79.2	83.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	73.5	74.2	73.3	69.2	56.7	$ 44.4
IBNR & Expected Development on Reported Claims	0.1
International Operations [Member] | Syndicate 1200 [Member] | Property [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	65.3	66	65.5	64.1	69.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	59.9	60	57.5	51.4	$ 29.4
IBNR & Expected Development on Reported Claims	0.1
International Operations [Member] | Syndicate 1200 [Member] | Property [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	74	73.4	66.2	55.9
Cumulative Paid Losses & ALAE, Net of Reinsurance	59.1	52.6	42.7	$ 22.8
IBNR & Expected Development on Reported Claims	0.1
International Operations [Member] | Syndicate 1200 [Member] | Property [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	91.8	86.6	70.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	78.8	64.1	$ 38.8
IBNR & Expected Development on Reported Claims	0.8
International Operations [Member] | Syndicate 1200 [Member] | Property [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	91.8	85.2
Cumulative Paid Losses & ALAE, Net of Reinsurance	62.7	$ 29.7
IBNR & Expected Development on Reported Claims	15.7
International Operations [Member] | Syndicate 1200 [Member] | Property [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	62.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	30.1
IBNR & Expected Development on Reported Claims	$ 31.2

Reserves for Losses and Loss _7

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	$ 2,516.2
Total reinsurance recoverables on unpaid losses and LAE	2,091.7
Unallocated loss adjustment expenses	63.6
Unamortized reserve discount	(16.9)	$ (17.6)	$ (19.4)
Reserves for losses and loss adjustment expenses	4,654.6	4,201	3,350.8
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	893.2
Total reinsurance recoverables on unpaid losses and LAE	296.3
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	422.6
Total reinsurance recoverables on unpaid losses and LAE	303.3
Unamortized reserve discount	(11.9)	(10.6)	(10.3)
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	151.3
Total reinsurance recoverables on unpaid losses and LAE	93.7
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	52
Total reinsurance recoverables on unpaid losses and LAE	25.8
International Operations [Member] | Reinsurance Division [Member] | Property [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	97.2
Total reinsurance recoverables on unpaid losses and LAE	404.5
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	52.2
Total reinsurance recoverables on unpaid losses and LAE	100.6
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	94.3
Total reinsurance recoverables on unpaid losses and LAE	59.4
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	99.6
Total reinsurance recoverables on unpaid losses and LAE	70.8
International Operations [Member] | Syndicate 1200 [Member] | Property [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	126
Total reinsurance recoverables on unpaid losses and LAE	116.4
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	146.8
Total reinsurance recoverables on unpaid losses and LAE	109.8
Run Off Lines [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	182.5
Total reinsurance recoverables on unpaid losses and LAE	88.1
Unamortized reserve discount	(5)	$ (7)	$ (9.1)
Other Lines [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	198.5
Total reinsurance recoverables on unpaid losses and LAE	$ 423

Reserves for Losses and Loss _8

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, 2018
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	7.80%
Year 2	19.20%
Year 3	19.00%
Year 4	17.10%
Year 5	11.50%
Year 6	7.90%
Year 7	5.30%
Year 8	3.50%
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	14.60%
Year 2	21.60%
Year 3	18.30%
Year 4	16.00%
Year 5	9.10%
Year 6	6.10%
Year 7	4.00%
Year 8	2.70%
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	7.20%
Year 2	24.50%
Year 3	29.60%
Year 4	19.80%
Year 5	8.40%
Year 6	4.10%
Year 7	2.20%
Year 8	1.30%
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	40.50%
Year 2	30.60%
Year 3	11.90%
Year 4	8.30%
Year 5	4.20%
Year 6	2.20%
Year 7	1.10%
Year 8	0.60%
International Operations [Member] | Reinsurance Division [Member] | Property [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	21.20%
Year 2	33.70%
Year 3	13.80%
Year 4	12.90%
Year 5	6.10%
Year 6	4.00%
Year 7	2.60%
Year 8	1.80%
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	1.30%
Year 2	8.60%
Year 3	17.80%
Year 4	21.90%
Year 5	16.60%
Year 6	10.10%
Year 7	6.80%
Year 8	4.60%
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	3.30%
Year 2	7.20%
Year 3	9.40%
Year 4	12.70%
Year 5	14.60%
Year 6	12.30%
Year 7	10.90%
Year 8	8.60%
Year 9	6.40%
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	4.70%
Year 2	10.40%
Year 3	14.50%
Year 4	14.80%
Year 5	11.30%
Year 6	10.50%
Year 7	8.10%
Year 8	6.80%
Year 9	5.20%
International Operations [Member] | Syndicate 1200 [Member] | Property [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	39.50%
Year 2	28.00%
Year 3	16.00%
Year 4	9.80%
Year 5	3.00%
Year 6	1.70%
Year 7	0.90%
Year 8	0.50%
Year 9	0.30%
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	38.90%
Year 2	29.20%
Year 3	15.60%
Year 4	9.60%
Year 5	3.70%
Year 6	1.70%
Year 7	0.80%
Year 8	0.30%
Year 9	0.20%

Reserves for Losses and Loss _9

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-duration Insurance Contracts, Discounted Liabilities [Line Items]
Carrying Amount of Reserves for Losses & LAE	$ 303.9	$ 302.2	$ 305.6
Aggregate Amount of Discount	16.9	17.6	19.4
Interest Accretion	3.4	4	4
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member]
Short-duration Insurance Contracts, Discounted Liabilities [Line Items]
Carrying Amount of Reserves for Losses & LAE	140.8	126.7	121.1
Aggregate Amount of Discount	11.9	10.6	10.3
Interest Accretion	$ 1.3	$ 1.9	$ 1.9
Discount Rate	2.25%	2.25%	2.25%
Run Off Lines [Member]
Short-duration Insurance Contracts, Discounted Liabilities [Line Items]
Carrying Amount of Reserves for Losses & LAE	$ 163.1	$ 175.5	$ 184.5
Aggregate Amount of Discount	5	7	9.1
Interest Accretion	$ 2.1	$ 2.1	$ 2.1
Discount Rate	3.50%	3.50%	3.50%

Run-off Lines - Gross Reserves

Run-off Lines - Gross Reserves for Run-Off Lines (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	$ 54.8	$ 55.9
Gross reserves - Run-off Lines	265.6	281.9
Reinsurance Assumed [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	27.7	29.3
Other [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	27.1	26.6
Risk Management [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross reserves - Run-off Lines	197	219.6
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	1.6	1.8
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	$ 12.2	$ 4.6

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total direct written reserves	$ 22.3	$ 21.4	$ 15.4
Total asbestos reserves	46.8	48.6	41.9
Case Reserves [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total direct written reserves	2.7	2.1	2.8
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	19.1	18.8	12.1
Assumed domestic [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total assumed reserves	21.5	24.3	21.8
Assumed domestic [Member] | Case Reserves [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total assumed reserves	8.7	9.8	10.5
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	12	13.7	10.5
Assumed London [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total assumed reserves	3	2.9	4.7
Assumed London [Member] | Case Reserves [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total assumed reserves	1.5	2.3	3.3
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	$ 1.5	$ 0.6	$ 1.4

Run-off Lines - Underwriting Lo

Run-off Lines - Underwriting Losses for Run-Off Lines (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	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	$ (8)	$ (15.4)	$ (10.9)
Total underwriting loss - Run-off Lines	(15.9)	(25.7)	(25.1)
Reinsurance Assumed [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	(3.9)	(8.7)	(1.3)
Other [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	(4.1)	(6.7)	(9.6)
Risk Management [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total underwriting loss - Run-off Lines	(2.6)	(8.8)	(13.1)
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	(0.1)	0.1
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	$ (5.3)	$ (1.4)	$ (1.2)

Junior Subordinated Debenture_2

Junior Subordinated Debentures - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Junior subordinated debentures	$ 257,000,000	$ 256,600,000
Junior Subordinated Debentures [Member]
Debt Instrument [Line Items]
Redemption price, percentage	100.00%	100.00%
Junior Subordinated Debentures, Maybrooke [Member]
Debt Instrument [Line Items]
Junior subordinated debentures	$ 91,800,000
Unsecured Debt	$ 84,300,000	$ 83,900,000

Junior Subordinated Debenture_3

Junior Subordinated Debentures - Schedule of Junior Subordinated Debentures (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Junior subordinated debentures, Amount	$ 257	$ 256.6
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] | Argo Group [Member] | PXRE Capital Statutory Trust II [Member]
Debt Instrument [Line Items]
Interest Rate	6.72%	5.52%
Junior subordinated debentures, Amount	$ 18.1	$ 18.1
Trust Preferred Debentures [Member] | Argo Group [Member] | PXRE Capital Trust VI [Member]
Debt Instrument [Line Items]
Interest Rate	6.70%	5.59%
Junior subordinated debentures, Amount	$ 10.3	$ 10.3
Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust I [Member]
Debt Instrument [Line Items]
Interest Rate	6.72%	5.52%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust III [Member]
Debt Instrument [Line Items]
Interest Rate	6.54%	5.46%
Junior subordinated debentures, Amount	$ 12.3	$ 12.3
Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust IV [Member]
Debt Instrument [Line Items]
Interest Rate	6.47%	5.27%
Junior subordinated debentures, Amount	$ 13.4	$ 13.4
Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust V [Member]
Debt Instrument [Line Items]
Interest Rate	6.54%	5.30%
Junior subordinated debentures, Amount	$ 12.3	$ 12.3
Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust VI [Member]
Debt Instrument [Line Items]
Interest Rate	6.59%	5.40%
Junior subordinated debentures, Amount	$ 13.4	$ 13.4
Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust VII [Member]
Debt Instrument [Line Items]
Interest Rate	6.39%	5.19%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust VIII [Member]
Debt Instrument [Line Items]
Interest Rate	6.37%	5.21%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust IX [Member]
Debt Instrument [Line Items]
Interest Rate	6.39%	5.19%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust X [Member]
Debt Instrument [Line Items]
Interest Rate	6.19%	4.99%
Junior subordinated debentures, Amount	$ 30.9	$ 30.9
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group [Member] | PXRE Capital Statutory Trust II [Member]
Debt Instrument [Line Items]
Interest rate basis spread	4.10%	4.10%
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group [Member] | PXRE Capital Trust VI [Member]
Debt Instrument [Line Items]
Interest rate basis spread	3.90%	3.90%
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust I [Member]
Debt Instrument [Line Items]
Interest rate basis spread	4.10%	4.10%
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust III [Member]
Debt Instrument [Line Items]
Interest rate basis spread	4.10%	4.10%
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust IV [Member]
Debt Instrument [Line Items]
Interest rate basis spread	3.85%	3.85%
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust V [Member]
Debt Instrument [Line Items]
Interest rate basis spread	3.85%	3.85%
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust VI [Member]
Debt Instrument [Line Items]
Interest rate basis spread	3.80%	3.80%
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust VII [Member]
Debt Instrument [Line Items]
Interest rate basis spread	3.60%	3.60%
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust VIII [Member]
Debt Instrument [Line Items]
Interest rate basis spread	3.55%	3.55%
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust IX [Member]
Debt Instrument [Line Items]
Interest rate basis spread	3.60%	3.60%
London Interbank Offered Rate (LIBOR) [Member] | Trust Preferred Debentures [Member] | Argo Group US [Member] | Argonaut Group Statutory Trust X [Member]
Debt Instrument [Line Items]
Interest rate basis spread	3.40%	3.40%

Junior Subordinated Debenture_4

Junior Subordinated Debentures - Schedule of Maybrooke Junior Subordinated Debentures (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Principal	$ 143,800,000	$ 143,800,000
Junior Subordinated Debentures, Maybrooke [Member]
Debt Instrument [Line Items]
Interest Rate	5.94%	4.74%
Principal	$ 91,800,000	$ 91,800,000
Carrying Value	$ 84,300,000	$ 83,900,000
London Interbank Offered Rate (LIBOR) [Member] | Junior Subordinated Debentures, Maybrooke [Member]
Debt Instrument [Line Items]
Interest rate basis spread	3.15%	3.15%

Other Indebtedness - Schedule o

Other Indebtedness - Schedule of Terms and Principal Amounts of Each Debt Instrument (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Other indebtedness	$ 183.4	$ 184.5
Term Loan [Member]
Debt Instrument [Line Items]
Other indebtedness	125	125
Floating Rate Loan Stock [Member]
Debt Instrument [Line Items]
Other indebtedness	57.8	58.9
Other Debt [Member]
Debt Instrument [Line Items]
Other indebtedness	$ 0.6	$ 0.6

Other Indebtedness - Additional

Other Indebtedness - Additional Information (Detail) £ in Millions	Nov. 02, 2018USD ($)	Dec. 31, 2018USD ($)debt_instrument	Dec. 31, 2017USD ($)	Dec. 31, 2018GBP (£)debt_instrument	Mar. 02, 2017USD ($)
Debt Instrument [Line Items]
Floating rate loan stock denominated in Euros		$ 41,300,000	$ 42,400,000
Amount of collateral under reinsurance agreement		944,000,000	$ 907,700,000
New Credit Agreement [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing amount	$ 325,000,000
Letter of credit facility amount outstanding		$ 500,000
New Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Variable interest rate of note payable, above 30-day LIBOR		1.25%	1.50%
New Credit Agreement [Member] | Term Loan [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing amount	125,000,000
New Credit Agreement [Member] | Revolving credit facility [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing amount	200,000,000
Letter of credit facility amount outstanding	500,000	$ 0	$ 0
Prior Agreement [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing amount					$ 325,000,000
Letter of credit facility amount outstanding			500,000
Prior Agreement [Member] | Term Loan [Member]
Debt Instrument [Line Items]
Repayments of borrowings	$ 125,000,000
Reinsurance Credit Facilities [Member] | Letter of Credit [Member]
Debt Instrument [Line Items]
Letter of credit facility amount outstanding		$ 99,700,000
Number of letters of credit | debt_instrument		2		2
Debt instrument, term		1 year
Amount of collateral under reinsurance agreement		$ 120,900,000
Reinsurance Credit Facilities, Secured Bilateral Line of Credit Facilities [Member] | Letter of Credit [Member]
Debt Instrument [Line Items]
Letter of credit facility amount outstanding		80,700,000
Reinsurance Credit Facilities, Issued by Banks [Member] | Letter of Credit [Member]
Debt Instrument [Line Items]
Letter of credit facility amount outstanding		$ 19,000,000
Funds at Lloyd's Instrument [Member] | Letter of Credit [Member]
Debt Instrument [Line Items]
Letter of credit facility amount outstanding | £				£ 23.3
Debt instrument, term		1 year
Unsecured Debt [Member]
Debt Instrument [Line Items]
Redemption price, percentage		100.00%
Other Debt - Note Payable [Member]
Debt Instrument [Line Items]
Note payable as part of ARIS acquisition		$ 600,000	$ 600,000
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 - Schedule_2

Other Indebtedness - Schedule of Floating Rate Loan Stock, Notes Outstanding (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Amount	$ 57.8	$ 58.9
Floating Rate Loan Stock, U.S. Dollar, Start Year 2004 [Member]
Debt Instrument [Line Items]
Interest Rate	6.72%	5.66%
Amount	$ 6.5	$ 6.5
Floating Rate Loan Stock, U.S. Dollar, Start Year 2006 [Member]
Debt Instrument [Line Items]
Interest Rate	6.52%	5.46%
Amount	$ 10	$ 10
Floating Rate Loan Stock, U.S. Dollar [Member]
Debt Instrument [Line Items]
Amount	$ 16.5	$ 16.5
Floating Rate Loan Stock, Euro, Start Year 2005 [Member]
Debt Instrument [Line Items]
Interest Rate	3.68%	3.67%
Amount	$ 13.8	$ 14.1
Floating Rate Loan Stock, Euro, Start Year 2006 [Member]
Debt Instrument [Line Items]
Interest Rate	3.68%	3.67%
Amount	$ 12	$ 12.4
Floating Rate Loan Stock, Euro, Start Year 2007 [Member]
Debt Instrument [Line Items]
Interest Rate	3.59%	3.57%
Amount	$ 15.5	$ 15.9
Floating Rate Loan Stock, Euro [Member]
Debt Instrument [Line Items]
Amount	$ 41.3	$ 42.4
London Interbank Offered Rate (LIBOR) [Member] | Floating Rate Loan Stock, U.S. Dollar, Start Year 2004 [Member]
Debt Instrument [Line Items]
Interest rate basis spread	4.20%	4.20%
London Interbank Offered Rate (LIBOR) [Member] | Floating Rate Loan Stock, U.S. Dollar, Start Year 2006 [Member]
Debt Instrument [Line Items]
Interest rate basis spread	4.00%	4.00%
London Interbank Offered Rate (LIBOR) [Member] | Floating Rate Loan Stock, Euro, Start Year 2005 [Member]
Debt Instrument [Line Items]
Interest rate basis spread	4.00%	4.00%
London Interbank Offered Rate (LIBOR) [Member] | Floating Rate Loan Stock, Euro, Start Year 2006 [Member]
Debt Instrument [Line Items]
Interest rate basis spread	4.00%	4.00%
London Interbank Offered Rate (LIBOR) [Member] | Floating Rate Loan Stock, Euro, Start Year 2007 [Member]
Debt Instrument [Line Items]
Interest rate basis spread	3.90%	3.90%

Other Indebtedness - Summary of

Other Indebtedness - Summary of Terms of Outstanding Debt (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Other indebtedness	$ 183.4	$ 184.5
New Credit Agreement [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage	3.86%	2.94%
Other indebtedness	$ 125	$ 125
London Interbank Offered Rate (LIBOR) [Member] | New Credit Agreement [Member]
Debt Instrument [Line Items]
Interest rate basis spread	1.25%	1.50%

Disclosures about Fair Value _3

Disclosures about Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Junior Subordinated Debentures [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	$ 257	$ 256.6
Junior Subordinated Debentures [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	248.2	257.9
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	163.2	172.9
Junior Subordinated Debentures [Member] | Junior Subordinated Debentures, Maybrooke [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	84.3	83.9
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	85
Senior Unsecured Fixed Rate Notes [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	139.8	139.6
Senior Unsecured Fixed Rate Notes [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	139.5	141.2
Floating Rate Loan Stock [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	57.8	58.9
Floating Rate Loan Stock [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	$ 54.5	$ 59

Shareholders' Equity - Addition

Shareholders' Equity - Additional Information (Detail) - USD ($)	Mar. 21, 2018	Feb. 20, 2018	Jun. 15, 2016	May 03, 2016	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Stock dividend declared, percent				10.00%	15.00%								15.00%		10.00%
Dividend declared per common share (in dollars per share)					$ 0.27	$ 0.27	$ 0.27	$ 0.27	$ 0.24	$ 0.24	$ 0.23	$ 0.23	$ 1.08	$ 0.94	$ 0.75
Cash dividends paid - common shares, total													$ 37,500,000	$ 33,200,000	$ 26,600,000
Preferred shares, authorized (in shares)					30,000,000								30,000,000
Preferred shares, par value (in dollars per share)					$ 1								$ 1
Preferred shares, issued (in shares)					0				0				0	0
Preferred shares, outstanding (in shares)					0				0				0	0
Common shares repurchased on open market (in shares)													530,882	756,252	847,111
Common shares repurchased on open market, total cost													$ 31,700,000	$ 45,200,000	$ 47,100,000
2016 Repurchase Authorization [Member]
Class of Stock [Line Items]
Remaining number of shares available under repurchase authorization, Value					$ 53,300,000								$ 53,300,000
Common shares repurchased on open market (in shares)													530,882	756,252
Common shares repurchased on open market, total cost													$ 31,700,000	$ 45,200,000
Maximum [Member] | 2016 Repurchase Authorization [Member]
Class of Stock [Line Items]
Total number of shares authorized for purchase, value				$ 150,000,000
Stock Dividend [Member]
Class of Stock [Line Items]
Stock dividend declared, percent		15.00%		10.00%
Additional stock issued as dividend (in shares)	4,397,520	4,397,520	2,735,542	2,735,542
Before 2016 Adjustment Stock Dividend [Member]
Class of Stock [Line Items]
Dividend declared per common share (in dollars per share)															$ 0.75

Shareholders' Equity - Schedule

Shareholders' Equity - Schedule of Repurchase of Shares (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity, Class of Treasury Stock [Line Items]
Common shares repurchased on open market (in shares)	530,882	756,252	847,111
Average Price of Shares Repurchased (in dollars per share)	$ 59.83	$ 59.76	$ 55.64
Total Cost (in millions)	$ 31.7	$ 45.2	$ 47.1
10b5-1 Trading Plan Initiated December 2017 [Member]
Equity, Class of Treasury Stock [Line Items]
Common shares repurchased on open market (in shares)	225,281
Average Price of Shares Repurchased (in dollars per share)	$ 59.31
Total Cost (in millions)	$ 13.3
10b5-1 Trading Plan Initiated June 2018 [Member]
Equity, Class of Treasury Stock [Line Items]
Common shares repurchased on open market (in shares)	75,661
Average Price of Shares Repurchased (in dollars per share)	$ 59.30
Total Cost (in millions)	$ 4.5
10b5-1 Trading Plan Initiated September 2018 [Member]
Equity, Class of Treasury Stock [Line Items]
Common shares repurchased on open market (in shares)	37,385
Average Price of Shares Repurchased (in dollars per share)	$ 59.75
Total Cost (in millions)	$ 2.2
Open Market Repurchase Period 1 [Member]
Equity, Class of Treasury Stock [Line Items]
Common shares repurchased on open market (in shares)	89,305
Average Price of Shares Repurchased (in dollars per share)	$ 59.03
Total Cost (in millions)	$ 5.3
Open Market Repurchase Period 2 [Member]
Equity, Class of Treasury Stock [Line Items]
Common shares repurchased on open market (in shares)	103,250
Average Price of Shares Repurchased (in dollars per share)	$ 62.07
Total Cost (in millions)	$ 6.4

Accumulated Other Comprehensi_3

Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 1,819.7	$ 1,792.7	$ 1,668.1
Other comprehensive income (loss) before reclassifications	(82.8)	77.1
Amounts reclassified from accumulated other comprehensive income (loss)	4.9	(28.2)
Net current-period other comprehensive income (loss)	(77.9)	48.9
Ending Balance	1,746.7	1,819.7	1,792.7
Foreign Currency Translation Adjustments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(19)	(17.6)
Other comprehensive income (loss) before reclassifications	(3.4)	(1.4)
Amounts reclassified from accumulated other comprehensive income (loss)	0	0
Net current-period other comprehensive income (loss)	(3.4)	(1.4)
Ending Balance	(22.4)	(19)	(17.6)
Foreign Currency Translation Adjustments [Member] | Accounting Standards Update 2016-01 [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of adoption of ASU, net of taxes				$ 0
Foreign Currency Translation Adjustments [Member] | Accounting Standards Update 2018-02 [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of adoption of ASU, net of taxes				0
Unrealized Holding Gains on Securities [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	121.9	72.4
Other comprehensive income (loss) before reclassifications	(80.4)	77.7
Amounts reclassified from accumulated other comprehensive income (loss)	4.9	(28.2)	(10)
Net current-period other comprehensive income (loss)	(75.5)	49.5
Ending Balance	(49)	121.9	72.4
Unrealized Holding Gains on Securities [Member] | Accounting Standards Update 2016-01 [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of adoption of ASU, net of taxes				(117.5)
Unrealized Holding Gains on Securities [Member] | Accounting Standards Update 2018-02 [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of adoption of ASU, net of taxes				22.1
Defined Benefit Pension Plans [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(6.3)	(7.1)
Other comprehensive income (loss) before reclassifications	1	0.8
Amounts reclassified from accumulated other comprehensive income (loss)	0	0
Net current-period other comprehensive income (loss)	1	0.8
Ending Balance	(6.7)	(6.3)	(7.1)
Defined Benefit Pension Plans [Member] | Accounting Standards Update 2016-01 [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of adoption of ASU, net of taxes				0
Defined Benefit Pension Plans [Member] | Accounting Standards Update 2018-02 [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of adoption of ASU, net of taxes				(1.4)
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	96.6	47.7	11.5
Ending Balance	$ (78.1)	96.6	$ 47.7
AOCI Attributable to Parent [Member] | Accounting Standards Update 2016-01 [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of adoption of ASU, net of taxes		(117.5)		(117.5)
AOCI Attributable to Parent [Member] | Accounting Standards Update 2018-02 [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of adoption of ASU, net of taxes		$ 20.7		$ 20.7

Accumulated Other Comprehensi_4

Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Unrealized gains and losses on securities:
Net of taxes	$ 4.9	$ (28.2)
Unrealized Gains and Losses on Securities [Member]
Unrealized gains and losses on securities:
Net realized investment losses (gains)	5.4	(41.6)	$ (23.2)
Provision (benefit) for income taxes	(0.5)	13.4	13.2
Net of taxes	$ 4.9	$ (28.2)	$ (10)

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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income	$ (43.6)	$ 40.6	$ 41.8	$ 24.8	$ 28.9	$ (61.3)	$ 46	$ 36.7	$ 63.6	$ 50.3	$ 146.7
Weighted average common shares outstanding - basic (in shares)									33,922,009	34,457,098	34,691,618
Effect of dilutive securities:
Equity compensation awards (in shares)									756,772	914,546	781,161
Weighted average common shares outstanding - diluted (in shares)									34,678,781	35,371,644	35,472,779
Net income per common share:
Basic (in dollars per share)	$ (1.29)	$ 1.20	$ 1.23	$ 0.73	$ 0.85	$ (1.78)	$ 1.32	$ 1.06	$ 1.87	$ 1.46	$ 4.23
Diluted (in dollars per share)	$ (1.29)	$ 1.17	$ 1.20	$ 0.71	$ 0.83	$ (1.78)	$ 1.29	$ 1.03	$ 1.83	$ 1.42	$ 4.13

Share-based Compensation - Addi

Share-based Compensation - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	May 31, 2014	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	May 03, 2016	Nov. 30, 2007
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, award vesting period, years		5 years
Stock dividend declared, percent		15.00%		10.00%	10.00%
Percentage increase in number of awards outstanding due to stock dividend		15.00%		10.00%
Reduction in exercise price of awards outstanding due to stock dividend		13.04%		9.09%
Share-based payment expense		$ 18.3	$ 12.3	$ 19.8
Share-based payments expense, net of tax		$ 16.5	10.3	15.5
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period		7 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			$ 27.8
Employee service share-based compensation, unrecognized compensation costs weighted-average period, years		1 year 9 months 18 days
Share-based compensation arrangement by share-based payment award, total fair value of shares vested		$ 9.2
Options outstanding (in shares)		897,005	767,140
Restricted Stock Units (RSUs) [Member] | Performance Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options outstanding (in shares)		307,335
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		$ 0.3
Employee service share-based compensation, unrecognized compensation costs weighted-average period, years		1 year 6 months
Remaining contractual term		2 years 5 months 23 days
Exercised (in shares)		665,875
Share based compensation arrangement by share based payment award, shares issued (in shares)		372,705
Share based compensation arrangement by share based payment award options exercises, intrinsic value		$ 27.1
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 (in shares)		128,572
Share based compensation arrangement by share based payment award options exercises, intrinsic value		$ 2.1
Employee service share-based compensation, cash paid to exercise of stock options		4.3
Liability for stock awards		2.1	$ 5.7
Stock Compensation Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense		$ 0.4	$ 0.4	$ 0.3
Two Thousand Seven Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)						4,500,000
Number of shares available for grant (in shares)	1,500,000
Two Thousand Fourteen Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares authorized (in shares)	2,800,000
Count of restricted shares, settle in common shares (in shares)		2.75
Two Thousand Seven Employee Share Purchase Plan [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		3 months
Discount from market price		85.00%
Earn Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Discount from market price		85.00%
Earn Plan [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		3 years
Earn Plan [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		5 years

Net Income Per Common Share - A

Net Income Per Common Share - Additional Information (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Feb. 20, 2018	May 03, 2016
Earnings Per Share [Line Items]
Treasury shares (in shares)	11,315,889	10,785,007	10,028,755
Anti-dilutive securities excluded from computation of diluted net income per common share (in shares)	0	0	0
Stock dividend declared, percent	15.00%		10.00%		10.00%
Stock Dividend [Member]
Earnings Per Share [Line Items]
Stock dividend declared, percent				15.00%	10.00%

Share-based Compensation - Fair

Share-based Compensation - Fair Value Assumptions (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free rate of return, minimum	2.61%	1.83%	1.00%
Risk-free rate of return, maximum	2.96%	2.22%	2.02%
Expected volatility, minimum	17.82%	18.13%	18.73%
Expected volatility, maximum	18.44%	18.70%	19.70%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yields	1.71%	1.63%	1.62%
Expected award life (years)	4 years 5 months 23 days	4 years 5 months 23 days	4 years 6 months
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yields	1.87%	1.72%	1.70%
Expected award life (years)	4 years 5 months 27 days	4 years 5 months 27 days	4 years 7 months 10 days

Share-based Compensation - Summ

Share-based Compensation - Summary of Restricted Share Activity (Detail) - Restricted Stock Units (RSUs) [Member] - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Shares
Outstanding, beginning balance (in shares)	767,140
Granted (in shares)	458,715
Vested and issued (in shares)	(211,170)
Expired or forfeited (in shares)	(117,680)
Outstanding, ending balance (in shares)	897,005
Weighted-Average Grant Date Fair Value
Outstanding, Weighted-Average Grant Date Fair Value (in dollars per share)	$ 46.92	$ 42.91
Granted, Weighted-Average Grant Date Fair Value (in dollars per share)	51.84
Vested and issued, Weighted-Average Grant Date Fair Value (in dollars per share)	43.35
Expired or forfeited, Weighted-Average Grant Date Fair Value (in dollars per share)	$ 46.32

Share-based Compensation - Su_2

Share-based Compensation - Summary of Stock-Settled SARs Activity (Detail) - Stock-Settled SARs Activity [Member] - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Shares
Outstanding, beginning balance (in shares)	1,494,458
Exercised (in shares)	(665,875)
Expired or forfeited (in shares)	(17,824)
Outstanding, ending balance (in shares)	810,759
Vested or expected to vest as of year end (in shares)	781,124
Exercisable at end of year (in shares)	618,935
Weighted-Average Exercise Price
Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 33.88	$ 30.85
Exercised, Weighted-Average Exercise Price (in dollars per share)	27
Expired or forfeited, Weighted-Average Exercise Price (in dollars per share)	36.51
Vested or expected to vest as of year end, Weighted-Average Exercise Price (in dollars per share)	33.73
Exercisable at end of year, Weighted-Average Exercise Price (in dollars per share)	$ 32.65

Share-based Compensation - Su_3

Share-based Compensation - Summary of Cash-Settled SARs Activity (Detail) - Cash-Settled SARs Activity [Member] - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Shares
Outstanding, beginning balance (in shares)	189,568
Exercised (in shares)	(128,572)
Expired or forfeited (in shares)	(2,568)
Outstanding, ending balance (in shares)	58,428
Weighted-Average Exercise Price
Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 30.71	$ 28.95
Exercised, Weighted-Average Exercise Price (in dollars per share)	28.31
Expired or forfeited, Weighted-Average Exercise Price (in dollars per share)	$ 21.23

Underwriting, Acquisition and_3

Underwriting, Acquisition and Insurance Expenses (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Underwriting Acquisition And Insurance Expenses [Abstract]
Commissions									$ 280.4	$ 255.9	$ 234
General expenses									351.7	359.1	292.1
Premium taxes, boards and bureaus									32.5	32.3	25.2
Underwriting, acquisition and insurance expenses, total									664.6	647.3	551.3
Net deferral of policy acquisition costs									(9.9)	(11.9)	(4.3)
Total underwriting, acquisition and insurance expenses	$ 169.7	$ 168	$ 156.8	$ 160.2	$ 161	$ 166.1	$ 154.7	$ 153.6	$ 654.7	$ 635.4	$ 547

Income Taxes - Income Tax (Bene

Income Taxes - Income Tax (Benefit) Provision Expense (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current income tax (benefit) provision related to:
Total current income tax (benefit) provision	$ 16.7	$ 7.5	$ 36.3
Deferred income tax (benefit) provision related to:
Deferred income tax (benefit) provision, net	(12.6)	(17.9)	(1.1)
Income tax (benefit) provision	4.1	(10.4)	35.2
United States [Member]
Current income tax (benefit) provision related to:
Total current income tax (benefit) provision	13.4	(0.3)	30.4
Deferred income tax (benefit) provision related to:
Deferred income tax (benefit) provision, net	(13.8)	(0.3)	3.3
United Kingdom [Member]
Current income tax (benefit) provision related to:
Total current income tax (benefit) provision	3.2	7.6	3.9
Deferred income tax (benefit) provision related to:
Deferred income tax (benefit) provision, net	1.1	(17.6)	(4.4)
Other jurisdictions [Member]
Current income tax (benefit) provision related to:
Total current income tax (benefit) provision	0.1	0.2	2
Deferred income tax (benefit) provision related to:
Deferred income tax (benefit) provision, net	$ 0.1	$ 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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$ (59.7)	$ 45.3	$ 57.1	$ 25	$ 12.3	$ (65.9)	$ 50.8	$ 42.7	$ 67.7	$ 39.9	$ 181.9
Effective Tax Rate									6.10%	(26.10%)	19.40%
Bermuda [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ 26	$ 30.2	$ 99.6
Effective Tax Rate									0.00%	0.00%	0.00%
United States [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ 13.9	$ 67.5	$ 115.2
Effective Tax Rate									(5.50%)	(0.90%)	29.20%
United Kingdom [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ 25	$ (53.8)	$ (34.8)
Effective Tax Rate									18.80%	18.70%	1.20%
Belgium [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ 0	$ 0.1	$ (0.1)
Effective Tax Rate									0.00%	75.00%	23.20%
Belgium [Member] | Maximum [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ 0.1
Brazil [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ (0.5)	$ 0.8	$ 0.6
Effective Tax Rate									0.00%	0.00%	0.00%
United Arab Emirates [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ 0.8	$ 0.2	$ 0
Effective Tax Rate									0.00%	0.00%	0.00%
Ireland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ (0.2)	$ (0.2)	$ (0.2)
Effective Tax Rate									0.00%	0.00%	5.00%
Effective Income Tax Rate Reconciliation, Deduction, Dividends, Amount											$ 40
Italy [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ 0.9	$ 0	$ 0
Effective Tax Rate									0.00%	0.00%	0.00%
Malta [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ 1.7	$ 0.3	$ 1.6
Effective Tax Rate									0.00%	0.00%	0.30%
Luxembourg [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ 0	$ (5.2)	$ 0
Effective Tax Rate									0.00%	0.00%	0.00%
Switzerland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)									$ 0.1	$ 0	$ 0
Effective Tax Rate									18.40%	21.10%	0.00%

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes Disclosure [Line Items]
Net operating losses carryback period	2 years
Capital losses carryback period	3 years
Net change in valuation allowance, for deferred tax asset	$ 9.8
Net operating loss carryforward	6.6
Loss carryforwards per year	2.8
Valuation allowance	29.9	$ 20.1
Unrecognized tax benefits	0	$ 0	$ 0
Ariel Re [Member]
Income Taxes Disclosure [Line Items]
Valuation allowance	1.4
Ariscom Compagnia Di Assicurazioni S P A [Member]
Income Taxes Disclosure [Line Items]
Valuation allowance	12.9
Brazil [Member]
Income Taxes Disclosure [Line Items]
Valuation allowance	6.5
Malta [Member]
Income Taxes Disclosure [Line Items]
Valuation allowance	1.9
ARIS [Member]
Income Taxes Disclosure [Line Items]
Net operating loss carryforward	$ 0.6

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income tax provision at expected rate	$ 8.2	$ 12.7	$ 34.1
Tax effect of:
Nontaxable investment income	(1.9)	(4.7)	(5.5)
Foreign exchange adjustments	(0.6)	2.1	5.3
Withholding taxes	0.4	0.4	2.4
Change in valuation allowance	(1.5)	(0.9)	0.7
Impact of change in tax rate related to TCJA	(1.6)	(20.2)	0
Other	1.1	0.2	(1.8)
Income tax (benefit) provision	$ 4.1	$ (10.4)	$ 35.2

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, 2018	Dec. 31, 2017
Deferred tax assets:
Losses and loss adjustment expense reserve discounting	$ 19.3	$ 17.2
Unearned premiums	23.2	21.4
Net operating loss carryforwards	31.6	21.9
Investment in Limited Partnership Interests	12	10.4
Unrealized losses on fixed maturities and other investment securities	9	0
Impairment of investment values	2.2	4
Accrued bonus	4.5	2.6
Stock option expense	1.7	2
United Kingdom underwriting results	0	3.8
Other	8	9.6
Deferred tax assets, gross	111.5	92.9
Deferred tax liabilities:
Unrealized gains on equity securities	(13.1)	(30.5)
Unrealized gains on fixed maturities and other investment securities	0	(5)
Unrealized gains on limited partnership interests	(17.3)	(14.6)
Depreciable fixed assets	(27)	(22.5)
Deferred acquisition costs	(18.1)	(16.9)
TCJA reserve transitional liability	(4.5)	(5.5)
United Kingdom underwriting results	(0.6)	0
Other	(7.2)	(9.1)
Deferred tax liabilities, gross	(87.8)	(104.1)
Deferred tax (liabilities) assets, net before valuation allowance	23.7	(11.2)
Valuation allowance	(29.9)	(20.1)
Deferred tax liabilities, net	(6.2)	(31.3)
Foreign Tax Authority [Member]
Deferred tax liabilities:
Deferred tax liabilities, net	(0.6)
Net deferred tax (liabilities) assets - Other jurisdictions		1.2
Domestic Tax Authority [Member]
Deferred tax liabilities:
Deferred tax liabilities, net	$ (5.6)	$ (32.5)

Income Taxes - Schedule of Ne_2

Income Taxes - Schedule of Net Operating Loss Carryforwards Amounts by Jurisdiction and Year of Expiration (Detail) $ in Millions	Dec. 31, 2018USD ($)
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards by jurisdiction	$ 6.6
Brazil [Member] | Indefinite Expiration [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards by jurisdiction	11.5
Italy [Member] | Indefinite Expiration [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards by jurisdiction	41
Malta [Member] | Indefinite Expiration [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards by jurisdiction	5.5
United States [Member] | Expiration Year From 2025 - 2037 [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards by jurisdiction	$ 48.9

Pension Benefits and Savings _2

Pension Benefits and Savings Plans (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		$ 0.2	$ 0.1
Employee Savings Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributions by employer towards savings plan	$ 7.9	7	$ 6
Qualified Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Amount of pension related to plan	3.5	2.7
Non- Qualified Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Amount of pension related to plan	$ 2	$ 2.2

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - Maximum [Member] $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)
Commitments And Contingencies [Line Items]
Contractual commitments related to its limited partnership investments	$ 120.5
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, 2018USD ($)
Leases [Abstract]
2,019	$ 18.7
2,020	18.6
2,021	17.5
2,022	14.7
2,023	12.3
Thereafter	80.1
Total	$ 161.9

Leases - Additional Information

Leases - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Leases [Abstract]
Operating leases expense	$ 21.6	$ 21	$ 12.2

Segment Information - Additiona

Segment Information - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)segment	Dec. 31, 2017USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment	2
Assets associated with trade capital providers | $	$ 880.4	$ 731.8

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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Earned premiums	$ 452.4	$ 446.9	$ 417.7	$ 414.7	$ 404.5	$ 389.3	$ 399.1	$ 379.4	$ 1,731.7	$ 1,572.3	$ 1,410.8
Net investment income									133.1	140	115.1
Fee and other income									9	22.5	24.5
Net realized investment gains (loss)									(72)	39.3	26.1
Total revenue									1,801.8	1,774.1	1,576.5
Net income (loss) before income taxes	$ (59.7)	$ 45.3	$ 57.1	$ 25	$ 12.3	$ (65.9)	$ 50.8	$ 42.7	67.7	39.9	181.9
U.S. Operations [Member]
Segment Reporting Information [Line Items]
Earned premiums									1,078.9	936.6	849.5
Net investment income									82.9	87.2	71.9
International Operations [Member]
Segment Reporting Information [Line Items]
Earned premiums									652.5	635.8	560.9
Net investment income									32.9	32.7	28.7
Run Off Lines [Member]
Segment Reporting Information [Line Items]
Earned premiums									0.3	(0.1)	0.4
Net investment income									8.1	9.3	11.3
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net income (loss) before income taxes									185	64.8	210.8
Operating Segments [Member] | U.S. Operations [Member]
Segment Reporting Information [Line Items]
Earned premiums									1,078.9	936.6	849.5
Net investment income									82.9	87.2	71.9
Net income (loss) before income taxes									161.4	169.4	174.4
Operating Segments [Member] | International Operations [Member]
Segment Reporting Information [Line Items]
Earned premiums									652.5	635.8	560.9
Net investment income									32.9	32.7	28.7
Net income (loss) before income taxes									32.9	(86.7)	51.6
Operating Segments [Member] | Run Off Lines [Member]
Segment Reporting Information [Line Items]
Earned premiums									0.3	(0.1)	0.4
Net investment income									8.1	9.3	11.3
Net income (loss) before income taxes									(9.3)	(17.9)	(15.2)
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Net investment income									9.2	10.8	3.2
Net income (loss) before income taxes									$ (45.3)	$ (64.2)	$ (55)

Segment Information - Schedule

Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 452.4	$ 446.9	$ 417.7	$ 414.7	$ 404.5	$ 389.3	$ 399.1	$ 379.4	$ 1,731.7	$ 1,572.3	$ 1,410.8
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums									1,073.1	930.8	850
United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums									455.8	489.3	406.9
Bermuda [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums									85.4	92.1	112.6
Malta [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums									61	11.1	2.1
Other Jurisdiction [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums									$ 56.4	$ 49	$ 39.2

Segment Information - Identifia

Segment Information - Identifiable Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 9,558.2	$ 8,764
Operating Segments [Member] | U.S. Operations [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	4,707.8	4,298.4
Operating Segments [Member] | International Operations [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	3,984.7	3,553.8
Operating Segments [Member] | Run Off Lines [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	444.8	449.6
Corporate and Other [Member] | Corporate and Other [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 420.9	$ 462.2

Segment Information - Schedul_2

Segment Information - Schedule of Goodwill and Intangible Assets Net of Accumulated Amortization (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Goodwill	$ 177	$ 161.4
Intangible assets, net of accumulated amortization	93.5	96.8
U.S. Operations [Member]
Segment Reporting Information [Line Items]
Goodwill	123.5	123.5
Intangible assets, net of accumulated amortization	0.5	1.5
International Operations [Member]
Segment Reporting Information [Line Items]
Goodwill	53.5	37.9
Intangible assets, net of accumulated amortization	$ 93	$ 95.3

Statutory Accounting Principl_3

Statutory Accounting Principles - Statutory Capital and Surplus for Principal Operating Subsidiaries (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Bermuda [Member]
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	$ 1,451.3	$ 1,489.1
United Kingdom [Member]
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	357.3	330.3
United States [Member]
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	$ 1,003.8	$ 992.8

Statutory Accounting Principl_4

Statutory Accounting Principles - Statutory Net Income (Loss) for Principal Operating Subsidiaries (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Bermuda [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	$ 47.4	$ 55.1	$ 158.2
United Kingdom [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	(9.5)	(95.5)	1.8
United States [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	$ 110.8	$ 57	$ 99

Statutory Accounting Principl_5

Statutory Accounting Principles - Additional Information (Detail) - USD ($)	1 Months Ended			12 Months Ended
	Dec. 31, 2018	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statutory Accounting Practices [Line Items]
Cash dividends				$ 36,500,000		$ 41,000,000
Argo Re [Member]
Statutory Accounting Practices [Line Items]
Maximum permissible limit for dividend	25.00%			25.00%
Reduction in total statutory capital				15.00%
Cash dividends				$ 36,500,000	$ 0	$ 41,000,000
Maximum permitted amount of dividends				362,800,000
Rockwood Casualty Insurance Company [Member]
Statutory Accounting Practices [Line Items]
Cash dividends	$ 20,000,000	$ 18,100,000
Maximum permitted amount of dividends				12,600,000
Argonaut Insurance Company [Member]
Statutory Accounting Practices [Line Items]
Cash dividends		$ 41,600,000
Maximum permitted amount of dividends				89,600,000
Colony Insurance Company [Member]
Statutory Accounting Practices [Line Items]
Cash dividends			$ 19,900,000
Ordinary dividends			35,000,000
Dividends received, securities			$ 15,100,000
Minimum [Member]
Statutory Accounting Practices [Line Items]
Minimum statutory capital and surplus balance	$ 320,400,000			$ 320,400,000	$ 403,800,000

Insurance Assessments (Detail)

Insurance Assessments (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Insurance [Abstract]
Insurance assessments on current insolvencies	$ 6.4	$ 4.8

Transactions with Related Par_2

Transactions with Related Parties Transactions with Related Parties (Detail) - Kinetica [Member] $ in Millions	Dec. 31, 2013USD ($)
Schedule Of Other Related Party Transactions [Line Items]
Surety bonds issued	$ 13
Percentage of investment ownership	10.00%

Unaudited Quarterly Financial_3

Unaudited Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Feb. 20, 2018	May 03, 2016
Class of Stock [Line Items]
Earned premiums	$ 452.4	$ 446.9	$ 417.7	$ 414.7	$ 404.5	$ 389.3	$ 399.1	$ 379.4	$ 1,731.7	$ 1,572.3	$ 1,410.8
Losses and loss adjustment expenses	280.6	277.5	245.5	237.2	270.7	326.4	230.6	222.5	1,040.8	1,050.2	810.1
Underwriting, acquisition and insurance expenses	169.7	168	156.8	160.2	161	166.1	154.7	153.6	654.7	635.4	547
Underwriting income	2.1	1.4	15.4	17.3	(27.2)	(103.2)	13.8	3.3	36.2	(113.3)
Net income (loss) before income taxes	(59.7)	45.3	57.1	25	12.3	(65.9)	50.8	42.7	67.7	39.9	181.9
Net income (loss)	$ (43.6)	$ 40.6	$ 41.8	$ 24.8	$ 28.9	$ (61.3)	$ 46	$ 36.7	$ 63.6	$ 50.3	$ 146.7
Net income (loss) per common share:
Basic (in dollars per share)	$ (1.29)	$ 1.20	$ 1.23	$ 0.73	$ 0.85	$ (1.78)	$ 1.32	$ 1.06	$ 1.87	$ 1.46	$ 4.23
Diluted (in dollars per share)	(1.29)	1.17	1.20	0.71	0.83	(1.78)	1.29	1.03	1.83	1.42	4.13
Dividends per common share (in dollars per shares)	$ 0.27	$ 0.27	$ 0.27	$ 0.27	$ 0.24	$ 0.24	$ 0.23	$ 0.23	$ 1.08	$ 0.94	$ 0.75
Stock dividend declared, percent	15.00%								15.00%		10.00%		10.00%
Stock Dividend [Member]
Net income (loss) per common share:
Stock dividend declared, percent												15.00%	10.00%

Senior Unsecured Fixed Rate N_3

Senior Unsecured Fixed Rate Notes - Additional Information (Detail) - USD ($)	1 Months Ended
	Sep. 30, 2012	Dec. 31, 2018	Dec. 31, 2017
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%
Redemption price, percentage	100.00%

Senior Unsecured Fixed Rate N_4

Senior Unsecured Fixed Rate Notes - Schedule Of Unamortized Debt Issuance Costs Deducted From Carrying Value Of Debt Liability (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Senior unsecured fixed rate notes
Principal	$ 143.8	$ 143.8
Less: unamortized debt issuance costs	(4)	(4.2)
Senior unsecured fixed rate notes, less unamortized debt issuance costs	$ 139.8	$ 139.6

Senior Unsecured Fixed Rate N_5

Senior Unsecured Fixed Rate Notes - Condensed Consolidating Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Investments	$ 4,787	$ 4,742.9
Cash	139.2	176.6	$ 86	$ 121.7
Accrued investment income	27.2	23.5
Premiums receivable	649.9	598.6
Reinsurance recoverables	2,688.3	2,093.3
Goodwill and other intangible assets, net	270.5	258.2
Current income taxes receivable, net	8.2	1.4
Deferred acquisition costs, net	167.3	160.4
Ceded unearned premiums	457.7	399.5
Other assets	362.9	309.6
Intercompany note receivable	0	0
Investment in subsidiaries	0	0
Total assets	9,558.2	8,764
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	4,654.6	4,201	3,350.8
Unearned premiums	1,300.9	1,207.7
Funds held and ceded reinsurance payable, net	1,007.7	776.7
Debt	580.2	580.7
Deferred tax liabilities, net	6.2	31.3
Accrued underwriting expenses and other liabilities	261.9	146.9
Due to (from) affiliates	0	0
Intercompany note payable	0
Total liabilities	7,811.5	6,944.3
Total shareholders' equity	1,746.7	1,819.7	1,792.7	1,668.1
Total liabilities and shareholders' equity	9,558.2	8,764
Consolidating Adjustments [Member]
Assets
Investments	0	0
Cash	0	0	0	0
Accrued investment income	0	0
Premiums receivable	0	0
Reinsurance recoverables	0	0
Goodwill and other intangible assets, net	0	0
Current income taxes receivable, net	0	0
Deferred acquisition costs, net	0	0
Ceded unearned premiums	0	0
Other assets	0	0
Intercompany note receivable	0	0
Investment in subsidiaries	(1,852.7)	(1,940)
Total assets	(1,852.7)	(1,940)
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
Debt	0	0
Deferred tax liabilities, net	0	0
Accrued underwriting expenses and other liabilities	0	0
Due to (from) affiliates	(8.5)	(11.7)
Intercompany note payable	0
Total liabilities	(8.5)	(11.7)
Total shareholders' equity	(1,844.2)	(1,928.3)
Total liabilities and shareholders' equity	(1,852.7)	(1,940)
Other Subsidiaries and Eliminations [Member]
Assets
Investments	1,607.3	1,663.5
Cash	105.8	127.9	32.3	32.9
Accrued investment income	6.9	5.5
Premiums receivable	420.4	382.1
Reinsurance recoverables	1,053.1	606
Goodwill and other intangible assets, net	104.8	90.1
Current income taxes receivable, net	(0.9)	(1)
Deferred acquisition costs, net	81.1	79.7
Ceded unearned premiums	207.3	201
Other assets	181.9	128.9
Intercompany note receivable	(53.7)	(50.9)
Investment in subsidiaries	0	0
Total assets	3,714	3,232.8
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	1,883.2	1,717.1
Unearned premiums	503.5	503.7
Funds held and ceded reinsurance payable, net	268.4	(22.7)
Debt	142.1	142.8
Deferred tax liabilities, net	0.6	(1.2)
Accrued underwriting expenses and other liabilities	142.3	43
Due to (from) affiliates	(2)	0.4
Intercompany note payable	(19.1)
Total liabilities	2,919	2,383.1
Total shareholders' equity	795	849.7
Total liabilities and shareholders' equity	3,714	3,232.8
Argo Group [Member]
Assets
Investments	3.8	0.4
Cash	1.7	0.9	0	0
Accrued investment income	0	0
Premiums receivable	0	0
Reinsurance recoverables	0	0
Goodwill and other intangible assets, net	41.9	43.2
Current income taxes receivable, net	0	0
Deferred acquisition costs, net	0	0
Ceded unearned premiums	0	0
Other assets	15.7	9.2
Intercompany note receivable	0	0
Investment in subsidiaries	1,852.7	1,940
Total assets	1,915.8	1,993.7
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
Debt	153.4	153.4
Deferred tax liabilities, net	0	0
Accrued underwriting expenses and other liabilities	7.2	8.9
Due to (from) affiliates	8.5	11.7
Intercompany note payable	0
Total liabilities	169.1	174
Total shareholders' equity	1,746.7	1,819.7
Total liabilities and shareholders' equity	1,915.8	1,993.7
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Assets
Investments	3,175.9	3,079
Cash	31.7	47.8	$ 53.7	$ 88.8
Accrued investment income	20.3	18
Premiums receivable	229.5	216.5
Reinsurance recoverables	1,635.2	1,487.3
Goodwill and other intangible assets, net	123.8	124.9
Current income taxes receivable, net	9.1	2.4
Deferred acquisition costs, net	86.2	80.7
Ceded unearned premiums	250.4	198.5
Other assets	165.3	171.5
Intercompany note receivable	53.7	50.9
Investment in subsidiaries	0	0
Total assets	5,781.1	5,477.5
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	2,771.4	2,483.9
Unearned premiums	797.4	704
Funds held and ceded reinsurance payable, net	739.3	799.4
Debt	284.7	284.5
Deferred tax liabilities, net	5.6	32.5
Accrued underwriting expenses and other liabilities	112.4	95
Due to (from) affiliates	2	(0.4)
Intercompany note payable	19.1
Total liabilities	4,731.9	4,398.9
Total shareholders' equity	1,049.2	1,078.6
Total liabilities and shareholders' equity	$ 5,781.1	$ 5,477.5

Senior Unsecured Fixed Rate N_6

Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Income (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Premiums and other revenue:
Earned premiums	$ 452.4	$ 446.9	$ 417.7	$ 414.7	$ 404.5	$ 389.3	$ 399.1	$ 379.4	$ 1,731.7	$ 1,572.3	$ 1,410.8
Net investment income									133.1	140	115.1
Fee and other income									9	22.5	24.5
Net realized investment gains (loss)									(72)	39.3	26.1
Total revenue									1,801.8	1,774.1	1,576.5
Expenses:
Losses and loss adjustment expenses	280.6	277.5	245.5	237.2	270.7	326.4	230.6	222.5	1,040.8	1,050.2	810.1
Underwriting, acquisition and insurance expenses	169.7	168	156.8	160.2	161	166.1	154.7	153.6	654.7	635.4	547
Interest expense									31.6	27.7	19.6
Fee and other expense									7.1	14.6	22.4
Foreign currency exchange (gains) losses									(0.1)	6.3	(4.5)
Total expenses									1,734.1	1,734.2	1,394.6
Income before income taxes	(59.7)	45.3	57.1	25	12.3	(65.9)	50.8	42.7	67.7	39.9	181.9
Provision for income taxes									4.1	(10.4)	35.2
Net income (loss) before equity in earnings of subsidiaries									63.6	50.3	146.7
Equity in undistributed earnings of subsidiaries									0	0	0
Net income	$ (43.6)	$ 40.6	$ 41.8	$ 24.8	$ 28.9	$ (61.3)	$ 46	$ 36.7	63.6	50.3	146.7
Consolidating Adjustments [Member]
Premiums and other revenue:
Earned premiums									0	0	0
Net investment income									(36.5)	0	(41)
Fee and other income									0	0	0
Net realized investment gains (loss)									(2.7)	0	0
Total revenue									(39.2)	0	(41)
Expenses:
Losses and loss adjustment expenses									0	0	0
Underwriting, acquisition and insurance expenses									0	0	0
Interest expense									0	0	0
Fee and other expense									0	0	0
Foreign currency exchange (gains) losses									0	0	0
Total expenses									0	0	0
Income before income taxes									(39.2)	0	(41)
Provision for income taxes									0	0	0
Net income (loss) before equity in earnings of subsidiaries									(39.2)	0	(41)
Equity in undistributed earnings of subsidiaries									(44.8)	(73.1)	(123.1)
Net income									(84)	(73.1)	(164.1)
Other Subsidiaries and Eliminations [Member]
Premiums and other revenue:
Earned premiums									869.8	1,016.4	913.5
Net investment income									56.4	57	40
Fee and other income									4.6	3.7	3.1
Net realized investment gains (loss)									(20.5)	(1.9)	(24.5)
Total revenue									910.3	1,075.2	932.1
Expenses:
Losses and loss adjustment expenses									517.1	712.3	519.7
Underwriting, acquisition and insurance expenses									309.8	354.5	329.8
Interest expense									7.2	6.2	2.4
Fee and other expense									1.8	2.2	0.7
Foreign currency exchange (gains) losses									(0.3)	6.1	(4.7)
Total expenses									835.6	1,081.3	847.9
Income before income taxes									74.7	(6.1)	84.2
Provision for income taxes									4.9	(9.8)	1.6
Net income (loss) before equity in earnings of subsidiaries									69.8	3.7	82.6
Equity in undistributed earnings of subsidiaries									0	0	0
Net income									69.8	3.7	82.6
Argo Group [Member]
Premiums and other revenue:
Earned premiums										0	0
Net investment income									33.8	(4.5)	37.2
Fee and other income										0	0
Net realized investment gains (loss)									2.5	0.4	0.6
Total revenue									36.3	(4.1)	37.8
Expenses:
Losses and loss adjustment expenses										0	0
Underwriting, acquisition and insurance expenses									11.3	14.3	12.8
Interest expense									6.2	4.3	1.4
Fee and other expense										0	0
Foreign currency exchange (gains) losses									0	0.1	0
Total expenses										18.7	14.2
Income before income taxes										(22.8)	23.6
Provision for income taxes										0	0
Net income (loss) before equity in earnings of subsidiaries									18.8	(22.8)	23.6
Equity in undistributed earnings of subsidiaries									44.8	73.1	123.1
Net income									63.6	50.3	146.7
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Premiums and other revenue:
Earned premiums									861.9	555.9	497.3
Net investment income									79.4	87.5	78.9
Fee and other income									4.4	18.8	21.4
Net realized investment gains (loss)									(51.3)	40.8	50
Total revenue									894.4	703	647.6
Expenses:
Losses and loss adjustment expenses									523.7	337.9	290.4
Underwriting, acquisition and insurance expenses									333.6	266.6	204.4
Interest expense									18.2	17.2	15.8
Fee and other expense									5.3	12.4	21.7
Foreign currency exchange (gains) losses									0.2	0.1	0.2
Total expenses									881	634.2	532.5
Income before income taxes									13.4	68.8	115.1
Provision for income taxes									(0.8)	(0.6)	33.6
Net income (loss) before equity in earnings of subsidiaries									14.2	69.4	81.5
Equity in undistributed earnings of subsidiaries									0	0	0
Net income									$ 14.2	$ 69.4	$ 81.5

Senior Unsecured Fixed Rate N_7

Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Cash Flows (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	$ 301.3	$ 165	$ 182
Cash flows from investing activities:
Proceeds from sales of investments	1,599.8	1,729.9	1,443.5
Maturities and mandatory calls of fixed maturity investments	418.6	678.3	1,002.7
Purchases of investments	(2,149.1)	(2,660.8)	(2,380.5)
Change in short-term investments and foreign regulatory deposits	(119.2)	299.5	(195.2)
Settlements of foreign currency exchange forward contracts	(1.5)	(2.9)	(5.4)
Cash acquired with acquisition of Ariscom	15.6
Acquisition of subsidiaries, net of cash		(105.2)
Issuance of intercompany note, net		0
Purchases of fixed assets and other, net	(32.5)	(60.1)	(10.2)
Cash used in investing activities	(268.3)	(121.3)	(145.1)
Cash flows from financing activities:
Additional long-term borrowings	0	125	0
Borrowing under intercompany note, net	0	0
Activity under stock incentive plans	1.6	1.4	1
Repurchase of Company's common shares	(31.7)	(45.2)	(47.1)
Payment of cash dividends to common shareholders	(37.5)	(33.2)	(26.6)
Intercompany cash dividend			0
Cash (used in) provided by financing activities	(67.6)	48	(72.7)
Effect of exchange rate changes on cash	(2.8)	(1.1)	0.1
Change in cash	(37.4)	90.6	(35.7)
Cash, beginning of year	176.6	86	121.7
Cash, end of period	139.2	176.6	86
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	0	0	0
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	0	0	0
Settlements of foreign currency exchange forward contracts	0	0	0
Cash acquired with acquisition of Ariscom	0
Acquisition of subsidiaries, net of cash		0
Issuance of intercompany note, net		120
Purchases of fixed assets and other, net	0	0	0
Cash used in investing activities	0	120	0
Cash flows from financing activities:
Additional long-term borrowings		0
Borrowing under intercompany note, net	0	(120)
Activity under stock incentive plans	0	0	0
Repurchase of Company's common shares	0	0	0
Payment of cash dividends to common shareholders	0	0	0
Intercompany cash dividend			0
Cash (used in) provided by financing activities	0	(120)	0
Effect of exchange rate changes on cash	0	0	0
Change in cash	0	0	0
Cash, beginning of year	0	0	0
Cash, end of period	0	0	0
Other Subsidiaries and Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	46.5	(5.3)	83.2
Cash flows from investing activities:
Proceeds from sales of investments	532.1	920.4	407.6
Maturities and mandatory calls of fixed maturity investments	73.7	195.2	459.5
Purchases of investments	(640.8)	(1,165.2)	(930.5)
Change in short-term investments and foreign regulatory deposits	(10.8)	230.9	(56.2)
Settlements of foreign currency exchange forward contracts	(3.2)	4.4	(5.4)
Cash acquired with acquisition of Ariscom	15.6
Acquisition of subsidiaries, net of cash		130.1
Issuance of intercompany note, net		(120)
Purchases of fixed assets and other, net	(13.4)	(33.8)	1.1
Cash used in investing activities	(46.8)	162	(123.9)
Cash flows from financing activities:
Additional long-term borrowings		0
Borrowing under intercompany note, net	(19)	(60)
Activity under stock incentive plans	0	0	0
Repurchase of Company's common shares	0	0	0
Payment of cash dividends to common shareholders	0	0	0
Intercompany cash dividend			40
Cash (used in) provided by financing activities	(19)	(60)	40
Effect of exchange rate changes on cash	(2.8)	(1.1)	0.1
Change in cash	(22.1)	95.6	(0.6)
Cash, beginning of year	127.9	32.3	32.9
Cash, end of period	105.8	127.9	32.3
Argo Group [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	72.4	20.8	26.5
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	(3.4)	1.5	(0.9)
Settlements of foreign currency exchange forward contracts	(0.5)	0.9	0
Cash acquired with acquisition of Ariscom	0
Acquisition of subsidiaries, net of cash	0	(235.3)	0
Issuance of intercompany note, net		0
Purchases of fixed assets and other, net	(0.1)	(0.1)	0
Cash used in investing activities	(4)	(233)	(0.9)
Cash flows from financing activities:
Additional long-term borrowings		125
Borrowing under intercompany note, net	0	120
Activity under stock incentive plans	1.6	1.4	1
Repurchase of Company's common shares	(31.7)	(0.1)	0
Payment of cash dividends to common shareholders	(37.5)	(33.2)	(26.6)
Intercompany cash dividend			0
Cash (used in) provided by financing activities	(67.6)	213.1	(25.6)
Effect of exchange rate changes on cash	0	0	0
Change in cash	0.8	0.9	0
Cash, beginning of year	0.9	0	0
Cash, end of period	1.7	0.9	0
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	182.4	149.5	72.3
Cash flows from investing activities:
Proceeds from sales of investments	1,067.7	809.5	1,035.9
Maturities and mandatory calls of fixed maturity investments	344.9	483.1	543.2
Purchases of investments	(1,508.3)	(1,495.6)	(1,450)
Change in short-term investments and foreign regulatory deposits	(105)	67.1	(138.1)
Settlements of foreign currency exchange forward contracts	2.2	(8.2)	0
Cash acquired with acquisition of Ariscom	0
Acquisition of subsidiaries, net of cash		0
Issuance of intercompany note, net		0
Purchases of fixed assets and other, net	(19)	(26.2)	(11.3)
Cash used in investing activities	(217.5)	(170.3)	(20.3)
Cash flows from financing activities:
Additional long-term borrowings		0
Borrowing under intercompany note, net	19	60
Activity under stock incentive plans	0	0	0
Repurchase of Company's common shares	0	(45.1)	(47.1)
Payment of cash dividends to common shareholders	0	0	0
Intercompany cash dividend			(40)
Cash (used in) provided by financing activities	19	14.9	(87.1)
Effect of exchange rate changes on cash	0	0	0
Change in cash	(16.1)	(5.9)	(35.1)
Cash, beginning of year	47.8	53.7	88.8
Cash, end of period	$ 31.7	$ 47.8	$ 53.7

Schedule II Condensed Financi_2

Schedule II Condensed Financial Information of Registrant - Schedule of Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Short-term investments	$ 482.3	$ 368.5
Investment in subsidiaries	0	0
Cash	139.2	176.6	$ 86	$ 121.7
Goodwill and other intangible assets, net	270.5	258.2
Other assets	362.9	309.6
Total assets	9,558.2	8,764
Liabilities and Shareholders' Equity
Junior subordinated debentures	257	256.6
Other indebtedness	183.4	184.5
Accrued underwriting expenses and other liabilities	261.9	146.9
Total liabilities	7,811.5	6,944.3
Total shareholders' equity	1,746.7	1,819.7	1,792.7	1,668.1
Total liabilities and shareholders' equity	9,558.2	8,764
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Assets
Short-term investments	3.8	0.4
Investment in subsidiaries	1,852.7	1,940
Cash	1.7	0.9	$ 0	$ 0
Goodwill and other intangible assets, net	41.9	43.2
Other assets	15.7	9.2
Total assets	1,915.8	1,993.7
Liabilities and Shareholders' Equity
Junior subordinated debentures	28.4	28.4
Other indebtedness	125	125
Accrued underwriting expenses and other liabilities	7.2	8.9
Due to subsidiaries	8.5	11.7
Total liabilities	169.1	174
Total shareholders' equity	1,746.7	1,819.7
Total liabilities and shareholders' equity	$ 1,915.8	$ 1,993.7

Schedule II Condensed Financi_3

Schedule II Condensed Financial Information of Registrant - Schedule of Statements of Income (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue:
Net investment (expense) income									$ 133.1	$ 140	$ 115.1
Net realized investment gains (loss)									(72)	39.3	26.1
Total revenue									1,801.8	1,774.1	1,576.5
Expenses:
Interest expense									31.6	27.7	19.6
Other expenses	$ 169.7	$ 168	$ 156.8	$ 160.2	$ 161	$ 166.1	$ 154.7	$ 153.6	654.7	635.4	547
Foreign currency exchange loss									(0.1)	6.3	(4.5)
Net income (loss) before equity in earnings of subsidiaries									63.6	50.3	146.7
Equity in undistributed earnings of subsidiaries									0	0	0
Net income	$ (43.6)	$ 40.6	$ 41.8	$ 24.8	$ 28.9	$ (61.3)	$ 46	$ 36.7	63.6	50.3	146.7
Cash dividends									36.5		41
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Revenue:
Net investment (expense) income									33.8	(4.5)	37.2
Net realized investment gains (loss)									2.5	0.4	0.6
Total revenue									36.3	(4.1)	37.8
Expenses:
Interest expense									6.2	4.3	1.4
Other expenses									11.3	14.3	12.8
Foreign currency exchange loss									0	0.1	0
Total expenses									17.5	18.7	14.2
Net income (loss) before equity in earnings of subsidiaries									18.8	(22.8)	23.6
Equity in undistributed earnings of subsidiaries									44.8	73.1	123.1
Net income									$ 63.6	$ 50.3	$ 146.7

Schedule II Condensed Financi_4

Schedule II Condensed Financial Information of Registrant - Schedule of Statements of Cash Flows (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net income	$ (43.6)	$ 40.6	$ 41.8	$ 24.8	$ 28.9	$ (61.3)	$ 46	$ 36.7	$ 63.6	$ 50.3	$ 146.7
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Amortization and depreciation									31.9	33.8	35.4
Share-based payments expense									18.3	12.3	19.8
Undistributed earnings of subsidiaries									0	0	0
Change in:
Accrued underwriting expenses									15.2	(24.4)	(18.9)
Other, net									113.2	(0.6)	(48.9)
Cash provided by operating activities									301.3	165	182
Cash flows from investing activities:
Change in short-term investments									(132.2)	306.7	(193.9)
Settlements of foreign currency exchange forward contracts									(1.5)	(2.9)	(5.4)
Acquisition of subsidiaries, net of cash										(105.2)
Purchases of fixed assets and other, net									(32.5)	(60.1)	(10.2)
Cash used in investing activities									(268.3)	(121.3)	(145.1)
Cash flows from financing activities:
Activity under stock incentive plans									1.6	1.4	1
Repurchase of Company's common shares									(31.7)	(45.2)	(47.1)
Payment of cash dividends to common shareholders									(37.5)	(33.2)	(26.6)
Cash (used in) provided by financing activities									(67.6)	48	(72.7)
Change in cash									(37.4)	90.6	(35.7)
Cash, beginning of year				176.6				86	176.6	86	121.7
Cash, end of period	139.2				176.6				139.2	176.6	86
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Cash flows from operating activities:
Net income									63.6	50.3	146.7
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Amortization and depreciation									1.4	1.3	0.2
Share-based payments expense									7	3.3	4.2
Net realized investment and other gains									(2.5)	(0.4)	(0.6)
Undistributed earnings of subsidiaries									(44.8)	(73.1)	(123.1)
Change in:
Prepaid assets									(2.3)	(0.3)	(0.2)
Accrued underwriting expenses									(4.4)	(2.3)	0.9
Due to subsidiaries									57.6	48.1	(0.6)
Other, net									(3.2)	(6.1)	(1)
Cash provided by operating activities									72.4	20.8	26.5
Cash flows from investing activities:
Change in short-term investments									(3.4)	1.5	(0.9)
Settlements of foreign currency exchange forward contracts									(0.5)	0.9	0
Acquisition of subsidiaries, net of cash									0	(235.3)	0
Purchases of fixed assets and other, net									(0.1)	(0.1)	0
Cash used in investing activities									(4)	(233)	(0.9)
Cash flows from financing activities:
Additional borrowings									0	125	0
Borrowings under intercompany note payable, net									0	120	0
Activity under stock incentive plans									1.6	1.4	1
Repurchase of Company's common shares									(31.7)	(0.1)	0
Payment of cash dividends to common shareholders									(37.5)	(33.2)	(26.6)
Cash (used in) provided by financing activities									(67.6)	213.1	(25.6)
Change in cash									0.8	0.9	0
Cash, beginning of year				$ 0.9				$ 0	0.9	0	0
Cash, end of period	$ 1.7				$ 0.9				$ 1.7	$ 0.9	$ 0

Schedule III Supplemental Ins_2

Schedule III Supplemental Insurance Information (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplementary Insurance Information, by Segment [Line Items]
DAC	$ 167.3				$ 160.4				$ 167.3	$ 160.4	$ 139.1
Reserves for Losses and Loss Adjustment Expenses	4,654.6				4,201				4,654.6	4,201	3,350.8
UPR	1,300.9				1,207.7				1,300.9	1,207.7	970
Premium Revenue	452.4	$ 446.9	$ 417.7	$ 414.7	404.5	$ 389.3	$ 399.1	$ 379.4	1,731.7	1,572.3	1,410.8
Net investment income									133.1	140	115.1
Loss & LAE									1,040.8	1,050.2	810.1
Net deferral of policy acquisition costs									(9.9)	(11.9)	(4.3)
Other Operating Expenses	664.6				647.3				664.6	647.3	551.3
Net Premiums Written									1,765.5	1,653.5	1,440.2
U.S. Operations [Member]
Supplementary Insurance Information, by Segment [Line Items]
DAC	87.2				80.8				87.2	80.8	63.5
Reserves for Losses and Loss Adjustment Expenses	2,498.9				2,196.1				2,498.9	2,196.1	2,028.4
UPR	793.3				695.1				793.3	695.1	575.4
Premium Revenue									1,078.9	936.6	849.5
Net investment income									82.9	87.2	71.9
Loss & LAE									628.2	528.1	467.5
Net deferral of policy acquisition costs									(6.4)	(17.3)	(5.1)
Other Operating Expenses	361.2				336.4				361.2	336.4	275.6
Net Premiums Written									1,125.7	1,031.8	883.5
International Operations [Member]
Supplementary Insurance Information, by Segment [Line Items]
DAC	80.1				79.6				80.1	79.6	75.6
Reserves for Losses and Loss Adjustment Expenses	1,890.1				1,723				1,890.1	1,723	1,031.5
UPR	507.6				512.6				507.6	512.6	394.6
Premium Revenue									652.5	635.8	560.9
Net investment income									32.9	32.7	28.7
Loss & LAE									400.3	504.8	324
Net deferral of policy acquisition costs									(3.5)	5.4	0.8
Other Operating Expenses	249.3				236.8				249.3	236.8	210.3
Net Premiums Written									639.5	621.7	556.4
Run Off Lines [Member]
Supplementary Insurance Information, by Segment [Line Items]
DAC	0				0				0	0	0
Reserves for Losses and Loss Adjustment Expenses	265.6				281.9				265.6	281.9	290.9
UPR	0				0				0	0	0
Premium Revenue									0.3	(0.1)	0.4
Net investment income									8.1	9.3	11.3
Loss & LAE									12.3	17.3	18.6
Net deferral of policy acquisition costs									0	0	0
Other Operating Expenses	3.9				8.3				3.9	8.3	6.9
Net Premiums Written									0.3	0	0.3
Corporate and Other [Member]
Supplementary Insurance Information, by Segment [Line Items]
DAC	0				0				0	0	0
Reserves for Losses and Loss Adjustment Expenses	0				0				0	0	0
UPR	0				0				0	0	0
Premium Revenue									0	0	0
Net investment income									9.2	10.8	3.2
Loss & LAE									0	0	0
Net deferral of policy acquisition costs									0	0	0
Other Operating Expenses	$ 50.2				$ 65.8				50.2	65.8	58.5
Net Premiums Written									$ 0	$ 0	$ 0

Schedule V Valuation and Qual_2

Schedule V Valuation and Qualifying Accounts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	$ 20.1	$ 23.5	$ 22.8
Charged to Cost and Expense	(1.5)	(6.2)	(1)
Capital Loss Carryforward	0	0	0
Net Operating Loss Carryforward	11.3	2.7	0
Charged to Other Accounts	0	0.1	1.7
Deductions	0	0	0
Balance at End of Year	$ 29.9	$ 20.1	$ 23.5

Schedule VI Supplemental Info_2

Schedule VI Supplemental Information for Property-Casualty Insurance Companies (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplemental Information for Property, Casualty Insurance Underwriters [Abstract]
Deferred acquisition costs	$ 167.3	$ 160.4	$ 139.1
Reserves for losses and loss adjustment expenses	4,654.6	4,201	3,350.8
Unamortized discount in reserves for losses	16.9	17.6	19.4
Unearned premiums	1,300.9	1,207.7	970
Premiums earned	1,731.7	1,572.3	1,410.8
Net investment income	133.1	140	115.1
Current year	1,058.8	1,058.4	843.4
Prior years	(18)	(8.2)	(33.3)
Losses and loss adjustment expenses incurred	1,040.8	1,050.2	810.1
Net deferral of policy acquisition costs	(9.9)	(11.9)	(4.3)
Paid losses and loss adjustment expenses, net of reinsurance	938.9	889.4	716.5
Gross premiums written	$ 2,955.2	$ 2,697.2	$ 2,164.8