Argo Group International (ARGO-PA) 10-Q 9 May 18 2018 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2018	May 03, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q1
Trading Symbol	ARGO
Entity Registrant Name	Argo Group International Holdings, Ltd.
Entity Central Index Key	1,091,748
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		33,889,661

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	[1]
Assets
Fixed maturities available-for-sale, at fair value (cost: 2018 - $3,306.7; 2017 - $3,320.6)	$ 3,291.5	$ 3,343.4
Equity securities, at fair value (cost: 2018 - $358.3; 2017 - $338.2)	476.5	487.4
Other investments (cost: 2018 - $513.7; 2017 - $534.1)	523.3	543.6
Short-term investments, at fair value (cost: 2018 - $542.4; 2017 - $368.5)	542.2	368.5
Total investments	4,833.5	4,742.9
Cash	173.7	176.6
Accrued investment income	24.3	23.5
Premiums receivable	699.6	598.6
Reinsurance recoverables	2,087.5	2,093.3
Goodwill	161.4	161.4
Intangible assets, net of accumulated amortization	107.3	96.8
Current income taxes receivable, net	0	1.4
Deferred acquisition costs, net	163.9	160.4
Ceded unearned premiums	501.9	399.5
Other assets	388.2	309.6
Total assets	9,141.3	8,764
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	4,283.6	4,201
Unearned premiums	1,270.1	1,207.7
Accrued underwriting expenses	113.5	115.3
Ceded reinsurance payable, net	865.2	734
Funds held	47.7	42.7
Senior unsecured fixed rate notes	139.7	139.6
Other indebtedness	186.4	184.5
Junior subordinated debentures	256.6	256.6
Current income taxes payable, net	2.2	0
Deferred tax liabilities, net	21.3	31.3
Other liabilities	167.6	31.6
Total liabilities	7,353.9	6,944.3
Commitments and contingencies (Note 16)	0	0
Shareholders' equity:
Common shares - $1.00 par, 500,000,000 shares authorized; 44,911,805 and 40,385,309 shares issued at March 31, 2018 and December 31, 2017, respectively	44.9	40.4
Additional paid-in capital	1,364.7	1,129.1
Treasury shares (11,099,593 and 10,785,007 shares at March 31, 2018 and December 31, 2017, respectively)	(442)	(423.4)
Retained earnings	851.8	977
Accumulated other comprehensive (loss) income, net of taxes	(32)	96.6
Total shareholders' equity	1,787.4	1,819.7
Total liabilities and shareholders' equity	$ 9,141.3	$ 8,764
[1]	Derived from audited consolidated financial statements.

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	[1]
Statement Of Financial Position [Abstract]
Available-for-sale cost	$ 3,306.7	$ 3,320.6
Equity securities cost	358.3	338.2
Other investments cost	513.7	534.1
Short-term investments, Cost	$ 542.4	$ 368.5
Common shares, par value	$ 1	$ 1
Common shares, shares authorized	500,000,000	500,000,000
Common shares, shares issued	44,911,805	40,385,309
Treasury shares, shares	11,099,593	10,785,007
[1]	Derived from audited consolidated financial statements.

Condensed Consolidated Statemen

Condensed Consolidated Statements of Income - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Premiums and other revenue:
Earned premiums		$ 414.7	$ 379.4
Net investment income		36	30.5
Fee and other income		2	3.6
Net realized investment (losses) gains:
Net realized investment gains		15.2	14.6
Change in fair value of equity securities	[1]	(30.9)	0
Net realized investment (losses) gains		(15.7)	14.6
Total revenue		437	428.1
Expenses:
Losses and loss adjustment expenses		237.2	222.5
Underwriting, acquisition and insurance expenses		160.2	153.6
Interest expense		7.7	5.9
Fee and other expense		2	4.1
Foreign currency exchange losses (gains)		4.9	(0.7)
Total expenses		412	385.4
Income before income taxes		25	42.7
Income tax provision		0.2	6
Net income		$ 24.8	$ 36.7
Net income per common share:
Basic		$ 0.73	$ 1.06
Diluted		0.71	1.03
Dividend declared per common share		$ 0.27	$ 0.23
Weighted average common shares:
Basic		33,868,749	34,554,341
Diluted		34,740,343	35,582,816
Net realized investment (losses) gains before other-than-temporary impairment losses		$ (14.7)	$ 15
Other-than-temporary impairment losses recognized in earnings:
Other-than-temporary impairment losses on fixed maturities		(1)	0
Other-than-temporary impairment losses on equity securities		0	(0.4)
Impairment losses recognized in earnings		(1)	(0.4)
Net realized investment (losses) gains		$ (15.7)	$ 14.6
[1]	Effective January 1, 2018, we adopted ASU 2016-01. 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.

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Comprehensive (Loss) Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 24.8	$ 36.7
Other comprehensive income:
Foreign currency translation adjustments	(1.1)	0.6
Unrealized (losses) gains on securities:
(Losses) gains arising during the year	(24.7)	34.1
Reclassification adjustment for gains included in net income	(12.5)	(16.2)
Other comprehensive (loss) income before tax	(38.3)	18.5
Unrealized (losses) gains on securities:
(Losses) gains arising during the year	(4)	9.8
Reclassification adjustment for gains included in net income	(2.3)	(5.2)
Income tax (benefit) provision related to other comprehensive income	(6.3)	4.6
Other comprehensive (loss) income, net of tax	(32)	13.9
Comprehensive (loss) income	$ (7.2)	$ 50.6

Condensed Consolidated Stateme6

Condensed 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, 2016	$ 1,792.7		$ 40	$ 1,123.3	$ (378.2)	$ 959.9	$ 47.7
Net income	36.7		0	0	0	36.7	0
Other comprehensive income (loss), net of tax	13.9		0	0	0	0	13.9
Activity under stock incentive plans	3.5		0.2	3.3	0	0	0
Retirement of common shares (tax payments on equity compensation)	(4.2)		0	(4.2)	0	0	0
Deferred tax - share-based payments	0		0	0	0	0	0
Employee stock purchase plan	0.3		0	0.3	0	0	0
Cash dividends declared - common shares	(8.3)		0	0	0	(8.3)	0
Ending Balance at Mar. 31, 2017	1,834.6		40.2	1,122.7	(378.2)	988.3	61.6
Beginning Balance at Dec. 31, 2017	1,819.7	[1]	40.4	1,129.1	(423.4)	977	96.6
Net income	24.8		0	0	0	24.8	0
Other comprehensive income (loss), net of tax	(32)		0	0	0	0	(32)
Repurchase of common shares (314,586 at a weighted average price of $59.23)	(18.6)		0	0	(18.6)	0	0
Activity under stock incentive plans	4.6		0.2	4.4	0	0	0
Retirement of common shares (tax payments on equity compensation)	(2.2)		(0.1)	(2.1)	0	0	0
Employee stock purchase plan	0.4		0	0.4	0	0	0
15% Stock Dividend	0		4.4	232.9	0	(237.3)	0
Cash dividends declared - common shares	(9.3)		0	0	0	(9.3)	0
Cumulative effect of adoption, net of taxes	20.9
Cumulative effect of adoption, net of taxes | ASU 2016-01 [Member]	0		0	0	0	117.5	(117.5)
Cumulative effect of adoption, net of taxes	117.5
Cumulative effect of adoption, net of taxes | ASU 2018-02 [Member]	0		0	0	0	(20.9)	20.9
Ending Balance at Mar. 31, 2018	$ 1,787.4		$ 44.9	$ 1,364.7	$ (442)	$ 851.8	$ (32)
[1]	Derived from audited consolidated financial statements.

Condensed Consolidated Stateme7

Condensed Consolidated Statements of Shareholders' Equity (Parenthetical) - $ / shares	Feb. 20, 2018	Feb. 21, 2017	Mar. 31, 2018	Mar. 31, 2017
Statement Of Stockholders Equity [Abstract]
Common shares repurchased			314,586
Repurchase of common shares, weighted average price			$ 59.23
Stock dividend declared			15.00%
Dividend declared per common share	$ 0.27	$ 0.23	$ 0.27	$ 0.23

Condensed Consolidated Stateme8

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018		Mar. 31, 2017
Cash flows from operating activities:
Net income	$ 24.8		$ 36.7
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Amortization and depreciation	6.1		8.3
Share-based payments expense	5.2		4.9
Deferred income tax (benefit) provision, net	(3.7)		6
Net realized investment losses (gains)	15.7		(14.6)
Undistributed earnings from alternative investment portfolio	(8.7)		(8.3)
Loss on disposals of fixed assets, net	0		1.8
Change in:
Accrued investment income	(0.7)		(0.9)
Receivables	0.9		22.8
Deferred acquisition costs	(2.5)		(6.1)
Ceded unearned premiums	(93.8)		(43.1)
Reserves for losses and loss adjustment expenses	(42.7)		23.3
Unearned premiums	47.7		7.3
Ceded reinsurance payable and funds held	133.9		(36.6)
Income taxes	3.6		0.5
Accrued underwriting expenses	(3.8)		(19.2)
Other, net	71.6		(19.7)
Cash provided by (used in) operating activities	153.6		(36.9)
Cash flows from investing activities:
Sales of fixed maturity investments	469.2		383.6
Maturities and mandatory calls of fixed maturity investments	144.8		182.1
Sales of equity securities	51.7		53.4
Sales of other investments	34.5		17
Purchases of fixed maturity investments	(616.5)		(601.6)
Purchases of equity securities	(60.1)		(33.5)
Purchases of other investments	(10.9)		(9.3)
Change in foreign regulatory deposits and voluntary pools	5.1		(5.2)
Change in short-term investments	(166.1)		125.5
Settlements of foreign currency exchange forward contracts	1.7		(2.8)
Acquisition of Maybrooke, net of cash acquired			(83.1)
Purchases of fixed assets	(6.4)		(4.9)
Other, net	8.9		3.6
Cash (used in) provided by investing activities	(128.5)		24.8
Cash flows from financing activities:
Additional long-term borrowings	0		125
Activity under stock incentive plans	0.2		0.2
Repurchase of Company's common shares	(18.6)		0
Payment of cash dividends to common shareholders	(9.3)		(8.3)
Cash (used in) provided by financing activities	(27.7)		116.9
Effect of exchange rate changes on cash	(0.3)		(0.1)
Change in cash	(2.9)		104.7
Cash, beginning of year	176.6	[1]	86
Cash, end of period	173.7		190.7
Maybrooke [Member]
Cash flows from investing activities:
Acquisition of Maybrooke, net of cash acquired	0		(83.1)
Ariscom [Member]
Cash flows from investing activities:
Cash acquired with acquisition of Ariscom	$ 15.6		$ 0
[1]	Derived from audited consolidated financial statements.

Basis of Presentation

Basis of Presentation	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	1. Basis of Presentation The accompanying consolidated financial statements of Argo Group International Holdings, Ltd. (“Argo Group,” “we” or the “Company”) and its subsidiaries have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial information and with the instructions for Form 10-Q and Article 10 of Regulation S-X. The preparation of interim 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 intangibles and our deferred tax asset valuation allowance. Actual results could differ from those estimates. Certain financial information that normally is included in annual financial statements, including certain financial statement footnotes, prepared in accordance with GAAP, is not required for interim reporting purposes and has been condensed or omitted. These statements should be read in conjunction with the consolidated financial statements and notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2017, filed with the Securities and Exchange Commission on February 27, 2018. 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 3, “Acquisition of Ariscom,” for additional discussion regarding the acquisition. The Consolidated Financial Statements as of and for the three months ended March 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. The interim financial information as of, and for the three months ended March 31, 2018 and 2017 is unaudited. However, in the opinion of management, the interim information includes all adjustments, consisting of normal recurring accruals, necessary for a fair presentation of the results presented for the interim periods. The operating results for the interim periods are not necessarily indicative of the results to be expected for the full year. All significant intercompany amounts have been eliminated in consolidation. 15% Stock Dividend 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. 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.

Recently Issued Accounting Pron

Recently Issued Accounting Pronouncements	3 Months Ended
	Mar. 31, 2018
Accounting Changes And Error Corrections [Abstract]
Recently Issued Accounting Pronouncements	2. Recently Issued Accounting Pronouncements On February 14, 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update ("ASU") 2018-02, “Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income” that allows a reclassification from accumulated other comprehensive income 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 US 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 $20.9 million of previously recognized deferred taxes from accumulated other comprehensive income into retained earnings as of January 1, 2018. In May 2017, the FASB issued ASU 2017-09, “Compensation – Stock Compensation” (Topic 718): Scope of Modification Accounting. ASU 2017-09 clarifies when changes to the terms or conditions of a share-based payment award must be accounted for as modifications. The guidance requires entities to apply the modification accounting guidance if the value, vesting conditions or classification of the award changes. In addition to all the disclosures about modifications that are required today, the entities are required to affirmatively disclose when compensation expense has not changed. The ASU is applied prospectively and is effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2017. We have adopted this ASU effective January 1, 2018. The adoption of this ASU 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 2017-04 eliminates the requirement to calculate the implied fair value of goodwill that is done in Step 2 of the current goodwill impairment test to measure a goodwill impairment loss. Instead, entities will record an impairment loss based on the excess of a reporting unit’s carrying amount over its fair value. The guidance will be applied prospectively and is effective for annual and interim impairment tests performed in periods beginning after December 15, 2019. We do not anticipate that this ASU will have a material impact on our financial results or disclosures. In January 2017, the FASB issued ASU 2017-01, “Business Combinations” (Topic 805). ASU 2017-01 clarifies the definition of a business to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The guidance specifies the minimum inputs and processes required to meet the definition of a business. The guidance is effective for annual periods beginning after December 15, 2017, and interim periods within the year of adoption. We have adopted this ASU effective January 1, 2018. The adoption of this ASU did not have a material impact on our financial results and disclosures. In October 2016, the FASB issued ASU 2016-16, “Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory.” This ASU 2016-16 simplifies the income tax accounting of intra-entity transfers of an asset other than inventory by requiring an entity to recognize the income tax effect when the transfer occurs. The guidance is effective for annual reporting periods beginning after December 15, 2017, including interim reporting periods within those annual reporting periods and early adoption is permitted. We early adopted this ASU in 2017, and the adoption resulted in intercompany transfers being tax effected. In August 2016, the FASB issued ASU 2016-15, “Classification of Certain Cash Receipts and Cash Payments (Topic 230)”. ASU 2016-15 will reduce diversity in practice on how eight specific cash receipts and payments are classified on the statement of cash flows. The ASU will be effective for fiscal years beginning after December 15, 2017, including interim periods within the year of adoption. This ASU will have an impact on how we present the distributions received from equity method investees in our statement of cash flows. We have adopted this ASU effective January 1, 2018. This ASU did not have a material impact on the classification of specific cash receipts and payments within the cash flow 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. The ASU is effective for interim and annual reporting periods beginning on or after January 1, 2019, with early adoption permitted. We do not anticipate that this ASU will have a material impact on our results of operations, but we anticipate an increase to the value of our assets and liabilities related to leases, with no material impact to equity. In January 2016, the FASB issued ASU 2016-01, “Recognition and Measurement of Financial Assets and Financial Liabilities” (Subtopic 825-10). ASU 2016-01 will require equity investments that are not consolidated or accounted for under the equity method of accounting to be measured at fair value with changes in fair value recognized in net income. This ASU will also require us to assess the ability to realize our deferred tax assets related to an available-for-sale debt security in combination with our other deferred tax assets. The ASU will be effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. We have adopted this ASU effective January 1, 2018. Upon adoption of this ASU, cumulative net unrealized gain 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 the remeasurement of our equity investments. In May 2014, the FASB issued ASU 2014-09, “Revenue from Contracts with Customers” (Topic 606), which replaces most existing U.S. GAAP revenue recognition guidance and permits the use of either the retrospective or cumulative effect transition method. In August 2015, “Deferral of the Effective Date” (Topic 606), deferred the effective date of this guidance to interim and annual reporting periods beginning after December 15, 2017. Subsequently, in 2016, the FASB issued implementation guidance related to ASU 2014-09, including: • ASU 2016-08, “Principal versus Agent considerations (Reporting Revenue Gross versus Net)” (Topic 606), which is intended to provide further clarification on the application of the principal versus agent implementations; • ASU 2016-10, “Identifying Performance Obligations and Licensing” (Topic 606), which is intended to clarify the guidance for identifying promised goods or service in a contract with a customer; • ASU 2016-11, “Rescission of SEC Guidance Because of Accounting Standards Updates 2014-09 and 2014-16 Pursuant to Staff Announcements at the March 3, 2016 EITF Meeting” (Topics 605 & 815); • ASU 2016-12, “Narrow-Scope Improvements and Practical Expedients” (Topic 606), provides additional guidance for quantitative and qualitative disclosures in certain cases, and makes 12 additional technical corrections and improvements to the new revenue standard. We adopted this ASU effective January 1, 2018. The adoption of this standard did not have a material impact on our consolidated financial results.

Acquisition of Ariscom

Acquisition of Ariscom	3 Months Ended
	Mar. 31, 2018
Business Combinations [Abstract]
Acquisition of Ariscom	3. 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 provides Argo Group with an existing Italian insurance platform and access to Ariscom’s broker and client network throughout Italy, with longer-term opportunities to expand our presence in continental Europe, particularly Spain and Portugal. 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. We are in the process of finalizing our determination of fair values, including an independent appraisal of certain assets and liabilities, including intangible assets. We anticipate closing the fair value measurement period during the third quarter of 2018.

Investments

Investments	3 Months Ended
	Mar. 31, 2018
Investments Debt And Equity Securities [Abstract]
Investments	4. Investments Composition of Invested Assets The amortized cost, gross unrealized gains, gross unrealized losses and fair value of investments were as follows: March 31, 2018 (in millions) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Fixed maturities U.S. Governments $ 374.2 $ 0.2 $ 7.4 $ 367.0 Foreign Governments 233.0 3.9 3.5 233.4 Obligations of states and political subdivisions 315.9 6.6 2.0 320.5 Corporate bonds 1,580.7 15.0 24.1 1,571.6 Commercial mortgage-backed securities 103.5 — 2.3 101.2 Residential mortgage-backed securities 341.5 2.3 8.1 335.7 Asset-backed securities 165.6 1.0 1.3 165.3 Collateralized loan obligations 192.3 4.8 0.3 196.8 Total fixed maturities 3,306.7 33.8 49.0 3,291.5 Equity securities 358.3 131.9 13.7 476.5 Other investments 513.7 9.6 — 523.3 Short-term investments 542.4 — 0.2 542.2 Total investments $ 4,721.1 $ 175.3 $ 62.9 $ 4,833.5 December 31, 2017 (in millions) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Fixed maturities U.S. Governments $ 419.9 $ 0.2 $ 5.0 $ 415.1 Foreign Governments 229.0 6.7 2.5 233.2 Obligations of states and political subdivisions 327.7 9.3 1.1 335.9 Corporate bonds 1,514.5 24.4 13.2 1,525.7 Commercial mortgage-backed securities 136.3 0.1 1.5 134.9 Residential mortgage-backed securities 309.3 2.8 2.7 309.4 Asset-backed securities 161.3 0.7 0.8 161.2 Collateralized loan obligations 222.6 5.9 0.5 228.0 Total fixed maturities 3,320.6 50.1 27.3 3,343.4 Equity securities 338.2 154.0 4.8 487.4 Other investments 534.1 9.6 0.1 543.6 Short-term investments 368.5 — — 368.5 Total investments $ 4,561.4 $ 213.7 $ 32.2 $ 4,742.9 Included in “Total investments” in our Consolidated Balance Sheets at March 31, 2018 and December 31, 2017 is $120.6 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. Contractual Maturity The amortized cost and fair values of fixed maturity investments as of March 31, 2018, by contractual maturity, were as follows: (in millions) Amortized Cost Fair Value Due in one year or less $ 268.5 $ 267.7 Due after one year through five years 1,467.0 1,460.2 Due after five years through ten years 600.4 595.6 Thereafter 167.9 169.0 Structured securities 802.9 799.0 Total $ 3,306.7 $ 3,291.5 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 March 31, 2018 and December 31, 2017 were as follows: March 31, 2018 (in millions) Carrying Value Unfunded Commitments Investment Type Hedge funds $ 138.8 $ — Private equity 186.5 123.8 Long only funds 193.7 — Other investments 4.3 — Total other invested assets $ 523.3 $ 123.8 December 31, 2017 (in millions) Carrying Value Unfunded Commitments Investment Type Hedge funds $ 163.6 $ — Private equity 179.2 129.9 Long only funds 196.5 — Other investments 4.3 — Total other invested assets $ 543.6 $ 129.9 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 investments: Other investments include participation in investment pools, foreign exchange currency forward contracts to manage our foreign currency exposure and a portfolio of foreign exchange currency forward contracts that are actively traded by an external currency manager for a total return strategy. Unrealized Losses and Other-Than-Temporary Impairments An aging of unrealized losses on our investments in fixed maturities, equity securities (as of December 31, 2017), other investments and short-term investments is presented below: March 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 $ 316.8 $ 6.5 $ 17.3 $ 0.9 $ 334.1 $ 7.4 Foreign Governments 207.2 3.4 5.0 0.1 212.2 3.5 Obligations of states and political subdivisions 92.1 1.4 14.7 0.6 106.8 2.0 Corporate bonds 1,078.7 22.2 32.0 1.9 1,110.7 24.1 Commercial mortgage-backed securities 69.6 1.4 16.0 0.9 85.6 2.3 Residential mortgage-backed securities 263.9 7.5 14.4 0.6 278.3 8.1 Asset-backed securities 111.3 1.0 7.9 0.3 119.2 1.3 Collateralized loan obligations (1) 41.0 0.3 0.9 — 41.9 0.3 Total fixed maturities 2,180.6 43.7 108.2 5.3 2,288.8 49.0 Other investments (1) — — 0.3 — 0.3 — Short-term investments 58.4 0.2 — — 58.4 0.2 Total $ 2,239.0 $ 43.9 $ 108.5 $ 5.3 $ 2,347.5 $ 49.2 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 Equity securities 46.9 4.8 — — 46.9 4.8 Other investments — — 0.3 0.1 0.3 0.1 Total $ 1,585.8 $ 22.0 $ 343.0 $ 10.2 $ 1,928.8 $ 32.2 (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 2016-01. 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 8,233 securities, of which 2,901 were in an unrealized loss position for less than one year and 121 were in an unrealized loss position for a period one year or greater as of March 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 March 31, 2018. We recognized other-than-temporary losses on our fixed maturities and equity portfolio as follows: For the Three Months Ended March 31, (in millions) 2018 2017 Other-than-temporary impairment: Corporate bonds $ (1.0 ) $ — Equity securities — (0.4 ) Other-than-temporary impairment losses $ (1.0 ) $ (0.4 ) Net Realized Investment Gains and Losses The following table presents our gross realized investment gains (losses): For the Three Months Ended March 31, (in millions) 2018 2017 Realized gains on fixed maturities and other Fixed maturities $ 7.5 $ 7.1 Equity securities (1) — 15.7 Other investments 11.7 6.2 19.2 29.0 Realized losses on fixed maturities and other Fixed maturities (4.9 ) (6.2 ) Equity securities (1) — (0.7 ) Other investments (9.3 ) (7.1 ) Other-than-temporary impairment losses on fixed maturities (1.0 ) — Other-than-temporary impairment losses on equity securities — (0.4 ) (15.2 ) (14.4 ) Equity Securities (1) Net realized gains on equity securities sold during the period 11.2 — Change in unrealized (losses) on equity securities held at the end of the period (30.9 ) — Net realized (losses) on equity securities (19.7 ) — Net realized investment (losses) gains before income taxes (15.7 ) 14.6 Income tax benefit (expense) 2.9 (4.3 ) Net realized investment (losses) gains, net of income taxes $ (12.8 ) $ 10.3 (1) Changes in unrealized appreciation (depreciation) related to investments are summarized as follows: For the Three Months Ended March 31, (in millions) 2018 2017 Change in unrealized gains (losses) Fixed maturities $ (37.1 ) $ 12.6 Equity securities — 5.3 Short-term investments (0.1 ) — Net unrealized investment (losses) gains before income taxes (37.2 ) 17.9 Income tax benefit (expense) 6.3 (4.6 ) Net unrealized investment (losses) gains, net of income taxes $ (30.9 ) $ 13.3 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 assets” and “Other liabilities” at March 31, 2018 and December 31, 2017, respectively. The gains and losses are included in “Net realized investment (losses) gains” in our Consolidated Statements of Income. The fair value of our foreign currency exchange forward contracts as of March 31, 2018 and December 31, 2017 was as follows: (in millions) March 31, 2018 December 31, 2017 Operational currency exposure $ 3.4 $ (0.2 ) Asset manager investment exposure (1.8 ) (0.9 ) Total return strategy 0.5 0.7 Total $ 2.1 $ (0.4 ) The following table represents our gross investment realized gains and losses on our foreign currency exchange forward contracts: For the Three Months Ended March 31, (in millions) 2018 2017 Realized gains Operational currency exposure $ 4.3 $ 2.0 Asset manager investment exposure 4.3 0.5 Total return strategy 2.6 3.1 Gross realized investment gains 11.2 5.6 Realized losses Operational currency exposure (0.6 ) (2.2 ) Asset manager investment exposure (5.9 ) (1.4 ) Total return strategy (2.1 ) (1.9 ) Gross realized investment losses (8.6 ) (5.5 ) Net realized investment gains on foreign currency exchange forward contracts $ 2.6 $ 0.1 Regulatory Deposits, Pledged Securities and Letters of Credit We are required to maintain assets on deposit with various regulatory authorities to support our insurance and reinsurance operations. We maintain assets pledged as collateral in support of irrevocable letters of credit issued under the terms of certain reinsurance agreements for reported loss and loss expense reserves. The following table presents our components of restricted assets at March 31: (in millions) March 31, 2018 March 31, 2017 Securities on deposit for regulatory and other purposes $ 171.7 $ 183.9 Securities pledged as collateral for letters of credit 62.8 37.6 Securities and cash on deposit supporting Lloyd’s business 373.9 396.8 Total restricted investments $ 608.4 $ 618.3 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 approach, income approach and/or cost 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 judgments 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 March 31, 2018. 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 (Available-for-Sale) Levels 3: • We own a term loan that is valued using unobservable inputs. Equity Securities Level 1: Equity securities are principally reported at fair value using Level 1 inputs. For these securities, we obtain fair value measurements from a third-party pricing service using quoted prices (unadjusted) in active markets at the reporting date. Equity Securities Level 2: We own interests in a mutual fund that is reported at fair value using Level 2 inputs. The valuation is based on the fund’s net asset value per share, at the end of each month. The underlying assets in the fund are valued primarily on the basis of closing market quotations or official closing prices on each valuation day. Equity Securities Level 3: We own certain equity securities that are reported at fair value using Level 3 inputs. The valuation techniques for these securities include the following: • Fair value measurements obtained from the National Association of Insurance Commissioners’ Security Valuation Office at the reporting date. • Fair value measurements for an investment in an equity fund obtained by applying final prices provided by the administrator of the fund, which is based upon certain estimates and assumptions. • Fair value measurements from a broker and an independent valuation service, both based upon estimates and assumptions. Other Investments Level 2: Foreign regulatory deposits are assets held in trust in jurisdictions where there is a legal and regulatory requirement to maintain funds locally in order to protect policyholders. Lloyd’s is the appointed investment manager for the funds. These assets are invested in short-term government securities, agency securities and corporate bonds and are valued using Level 2 inputs based upon values obtained from Lloyd’s. Short-term Investments: Short-term investments are principally reported at fair value using Level 1 inputs, with the exception of short-term corporate and governmental bonds reported at fair value using Level 2 inputs as described in the fixed maturities section above. Values for the investments categorized as Level 1 are obtained from various financial institutions as of the reporting date. Transfers Between Level 1 and Level 2 Securities: There were no transfers between Level 1 and Level 2 securities during the three months ended March 31, 2018. Based on an analysis of the inputs, our financial assets measured at fair value on a recurring basis have been categorized as follows: Fair (in millions) March 31, 2018 Level 1 (a) Level 2 (b) Level 3 (c) Fixed maturities U.S. Governments $ 367.0 $ 359.3 $ 7.7 $ — Foreign Governments 233.4 — 233.4 — Obligations of states and political subdivisions 320.5 — 320.5 — Corporate bonds 1,571.6 — 1,569.7 1.9 Commercial mortgage-backed securities 101.2 — 101.2 — Residential mortgage-backed securities 335.7 — 335.7 — Asset-backed securities 165.3 — 165.3 — Collateralized loan obligations 196.8 — 196.8 — Total fixed maturities 3,291.5 359.3 2,930.3 1.9 Equity securities 476.5 472.1 2.1 2.3 Other investments 103.8 — 103.8 — Short-term investments 542.2 464.5 77.7 — $ 4,414.0 $ 1,295.9 $ 3,113.9 $ 4.2 (a) Quoted prices in active markets for identical assets (b) Significant other observable inputs (c) Significant unobservable inputs Fair Value Measurements at Reporting Date Using (in millions) December 31, 2017 Level 1 (a) Level 2 (b) Level 3 (c) Fixed maturities U.S. Governments $ 415.1 $ 410.6 $ 4.5 $ — Foreign Governments 233.2 — 233.2 — Obligations of states and political subdivisions 335.9 — 335.9 — Corporate bonds 1,525.7 — 1,523.8 1.9 Commercial mortgage-backed securities 134.9 — 134.9 — Residential mortgage-backed securities 309.4 — 309.4 — Asset-backed securities 161.2 — 161.2 — Collateralized loan obligations 228.0 — 228.0 — Total fixed maturities 3,343.4 410.6 2,930.9 1.9 Equity securities 487.4 483.0 2.1 2.3 Other investments 108.8 — 108.8 — Short-term investments 368.5 333.7 34.8 — $ 4,308.1 $ 1,227.3 $ 3,076.6 $ 4.2 (a) Quoted prices in active markets for identical assets (b) Significant other observable inputs (c) Significant unobservable inputs 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) — — — Included in other comprehensive income (loss) — — — Purchases, issuances, sales, and settlements: Purchases — — — Issuances — — — Sales — — — Settlements — — — Ending balance, March 31, 2018 $ 1.9 $ 2.3 $ 4.2 Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at December 31, 2017 $ — $ — $ — (in millions) Credit Financial Equity Securities Total Beginning balance, January 1, 2017 $ 2.0 $ 0.4 $ 2.4 Transfers into Level 3 — — — Transfers out of Level 3 — — — Total gains or losses (realized/unrealized): — Included in net income (loss) — — — Included in other comprehensive income (loss) (0.1 ) 0.2 0.1 Purchases, issuances, sales, and settlements: — Purchases — 1.7 1.7 Issuances — — — Sales — — — Settlements — — — Ending balance, December 31, 2017 $ 1.9 $ 2.3 $ 4.2 Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at December 31, 2017 $ — $ — $ — At March 31, 2018 and December 31, 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.

Reserves for Losses and Loss Ad

Reserves for Losses and Loss Adjustment Expenses	3 Months Ended
	Mar. 31, 2018
Insurance [Abstract]
Reserves for Losses and Loss Adjustment Expenses	5. Reserves for Losses and Loss Adjustment Expenses The following table provides a reconciliation of reserves for losses and loss adjustment expenses (“LAE”): For the Three Months Ended March 31, (in millions) 2018 2017 Net reserves beginning of the year $ 2,488.0 $ 2,180.2 Net Maybrooke reserves acquired — $ 132.6 Net Ariscom reserves acquired 47.2 — Add: Losses and LAE incurred during current calendar year, net of reinsurance: Current accident year 239.2 215.7 Prior accident years (2.0 ) 6.8 Losses and LAE incurred during calendar year, net of reinsurance 237.2 222.5 Deduct: Losses and LAE payments made during current calendar year, net of reinsurance: Current accident year 33.3 27.4 Prior accident years 186.6 155.3 Losses and LAE payments made during current calendar year, net of reinsurance: 219.9 182.7 Change in participation interest (1) (16.9 ) (23.2 ) Foreign exchange adjustments (0.8 ) 3.6 Net reserves - end of period 2,534.8 2,333.0 Add: Reinsurance recoverables on unpaid losses and LAE, end of period 1,748.8 1,247.3 Gross reserves - end of period $ 4,283.6 $ 3,580.3 (1) Amount represents the change in reserves due to changing our participation in Syndicates 1200 and 1910. 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’ loss and LAE reserves on each reporting segment is presented below: For the Three Months Ended March 31, (in millions) 2018 2017 U.S. Operations $ (1.0 ) $ (5.2 ) International Operations (2.8 ) 9.6 Run-off Lines 1.8 2.4 Total unfavorable (favorable) prior-year development $ (2.0 ) $ 6.8 The following describes the primary factors behind each segment’s prior accident year reserve development for three months ended March 31, 2018, and 2017: Three months ended March 31, 2018: • U.S. Operations: Favorable development in liability and specialty lines. • International Operations: Favorable development in property lines driven by 2017 catastrophe events, partially offset by unfavorable development within specialty lines. • Run-off Lines: Unfavorable development in risk management lines and other run-off lines . Three months ended March 31, 2017: • U.S. Operations: Favorable development for workers compensation, commercial auto and commercial multi-peril lines, partially offset by unfavorable development in professional liability lines. • International Operations: Unfavorable development in the liabilities lines primarily attributable to the impact of the change in discount rate used to calculate awards for personal injury claims (“U.K. Ogden rate”), and to the late reporting of Hurricane Matthew claims within property lines. • Run-off Lines: Unfavorable development in risk management lines and other run-off lines . In the opinion of management, our reserves represent the best estimate of our ultimate liabilities, based on currently known facts, current law, current technology and assumptions considered reasonable where facts are not known. Due to the significant uncertainties and related management judgments, there can be no assurance that future favorable or unfavorable loss development, which may be material, will not occur.

Junior Subordinated Debentures

Junior Subordinated Debentures	3 Months Ended
	Mar. 31, 2018
Junior Subordinated Debentures [Member]
Junior Subordinated Debentures	6. Junior Subordinated Debentures Trust Preferred 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 March 31, 2018 and December 31, 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: March 31, 2018 (in millions) Issue Date Trust Preferred Pools Maturity Rate Structure Interest Rate at March 31, 2018 Amount Argo Group 05/15/2003 PXRE Capital Statutory Trust II 05/15/2033 3M LIBOR + 4.10% 5.94% $ 18.1 11/06/2003 PXRE Capital Trust VI 09/30/2033 3M LIBOR + 3.90% 6.21% 10.3 Argo Group US 05/15/2003 Argonaut Group Statutory Trust I 05/15/2033 3M LIBOR + 4.10% 5.94% 15.5 12/16/2003 Argonaut Group Statutory Trust III 01/08/2034 3M LIBOR + 4.10% 5.82% 12.3 04/29/2004 Argonaut Group Statutory Trust IV 04/29/2034 3M LIBOR + 3.85% 5.69% 13.4 05/26/2004 Argonaut Group Statutory Trust V 05/24/2034 3M LIBOR + 3.85% 5.79% 12.3 05/12/2004 Argonaut Group Statutory Trust VI 05/12/2034 3M LIBOR + 3.80% 5.98% 13.4 09/17/2004 Argonaut Group Statutory Trust VII 12/15/2034 3M LIBOR + 3.60% 5.72% 15.5 09/22/2004 Argonaut Group Statutory Trust VIII 09/22/2034 3M LIBOR + 3.55% 5.80% 15.5 10/22/2004 Argonaut Group Statutory Trust IX 12/15/2034 3M LIBOR + 3.60% 5.72% 15.5 09/15/2005 Argonaut Group Statutory Trust X 09/15/2035 3M LIBOR + 3.40% 5.52% 30.9 Total Outstanding $ 172.7 December 31, 2017 (in millions) Issue Date Trust Preferred Pools Maturity Rate Structure Interest Rate at December 31, 2017 Amount Argo Group 05/15/2003 PXRE Capital Statutory Trust II 05/15/2033 3M LIBOR + 4.10% 5.52% $ 18.1 11/06/2003 PXRE Capital Trust VI 09/30/2033 3M LIBOR + 3.90% 5.59% 10.3 Argo Group US 05/15/2003 Argonaut Group Statutory Trust I 05/15/2033 3M LIBOR + 4.10% 5.52% 15.5 12/16/2003 Argonaut Group Statutory Trust III 01/08/2034 3M LIBOR + 4.10% 5.46% 12.3 04/29/2004 Argonaut Group Statutory Trust IV 04/29/2034 3M LIBOR + 3.85% 5.27% 13.4 05/26/2004 Argonaut Group Statutory Trust V 05/24/2034 3M LIBOR + 3.85% 5.30% 12.3 05/12/2004 Argonaut Group Statutory Trust VI 05/12/2034 3M LIBOR + 3.80% 5.40% 13.4 09/17/2004 Argonaut Group Statutory Trust VII 12/15/2034 3M LIBOR + 3.60% 5.19% 15.5 09/22/2004 Argonaut Group Statutory Trust VIII 09/22/2034 3M LIBOR + 3.55% 5.21% 15.5 10/22/2004 Argonaut Group Statutory Trust IX 12/15/2034 3M LIBOR + 3.60% 5.19% 15.5 09/15/2005 Argonaut Group Statutory Trust X 09/15/2035 3M LIBOR + 3.40% 4.99% 30.9 Total Outstanding $ 172.7 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 Maybrooke debt”). As part of the ongoing liquidation of the Maybrooke holding company, which began subsequent to our acquisition in 2017, the acquired debt was ultimately assigned to Argo Re and is carried on our consolidated balance sheet at $83.9 million, which represents the debt’s fair value at the date of acquisition plus accumulated accretion of discount to par value, as required by accounting for business combinations under ASC 805. At March 31, 2018 and December 31, 2017, the acquired debt was eligible for redemption at par. Interest accrues on the acquired debt based on a variable rate, which is reset quarterly. Interest payments are payable quarterly. A summary of the terms of the acquired debt outstanding is presented below: (in millions) Issue Date Maturity Rate Structure Interest Rate at March 31, 2018 Principal at March 31, 2018 Carrying Value at March 31, 2018 9/15/2007 9/15/2037 3 month LIBOR + 3.15% 5.27 % $ 91.8 $ 83.9 (in millions) Issue Date Maturity Rate Structure Interest Rate at December 31, 2017 Principal at December 31, 2017 Carrying Value at December 31, 2017 9/15/2007 9/15/2037 3 month LIBOR + 3.15% 4.74 % $ 91.8 $ 83.9

Other Indebtedness

Other Indebtedness	3 Months Ended
	Mar. 31, 2018
Debt Disclosure [Abstract]
Other Indebtedness	7. 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 March 31, 2018 December 31, 2017 Floating rate loan stock $ 60.8 $ 58.9 Term loan 125.0 125.0 Other debt 0.6 0.6 Total other indebtedness $ 186.4 $ 184.5 Floating Rate Loan Stock This debt was assumed through the acquisition of Argo Underwriting Agency, Ltd. These notes are unsecured. At March 31, 2018 and December 31, 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 March 31, 2018 and December 31, 2017 is presented below: March 31, 2018 (in millions) Currency Issue Date Maturity Rate Structure Interest Rate at March 31, 2018 Amount U.S. Dollar 12/08/2004 11/15/2034 6 month LIBOR + 4.2% 6.09% $ 6.5 U.S. Dollar 10/31/2006 01/15/2036 6 month LIBOR + 4.0% 5.89% 10.0 Total U.S. Dollar 16.5 Euro 09/06/2005 08/22/2035 3 month LIBOR + 4.0% 3.67% 14.8 Euro 10/31/2006 11/22/2036 3 month LIBOR + 4.0% 3.67% 12.9 Euro 06/08/2007 09/15/2037 3 month LIBOR + 3.9% 3.57% 16.6 Total Euro 44.3 Total notes outstanding $ 60.8 December 31, 2017 (in millions) Currency Issue Date Maturity Rate Structure Interest Rate at December 31, 2017 Amount U.S. Dollar 12/08/2004 11/15/2034 6 month LIBOR + 4.2% 5.66% $ 6.5 U.S. Dollar 10/31/2006 01/15/2036 6 month LIBOR + 4.0% 5.46% 10.0 Total U.S. Dollar 16.5 Euro 09/06/2005 08/22/2035 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 06/08/2007 09/15/2037 3 month LIBOR + 3.9% 3.57% 15.9 Total Euro 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 $ million and Borrowing Under Credit Facility On March 3, 2017, each of Argo Group, Argo Group US, Inc., Argo International Holdings Limited and Argo Underwriting Agency Limited (the “Borrowers”) entered into a $325.0 million Credit Agreement (“New Credit Agreement”) with JPMorgan Chase Bank, N.A., as administrative agent. The New Credit Agreement replaced and terminated the previous $175.0 million Credit Agreement (“Prior Agreement”). The New Credit Agreement provides for a $200.0 million revolving credit facility with a maturity date of March 3, 2022 unless extended in accordance with the terms of the New Credit Agreement. In addition, the New Credit Agreement includes a $125.0 million term loan borrowing, which Argo Group used to pay off in its entirety the $125.0 million borrowing drawn on January 31, 2017 under the Prior Agreement to help fund the acquisition of Maybrooke. Interest accrues based on a variable rate, which resets and is payable based on reset options selected by Argo Group pursuant to the terms of the New Credit Agreement. A summary of the terms of the outstanding balance at March 31, 2018 and December 31, 2017 is presented below: (in millions) Issue Date Maturity Rate Structure Interest Rate at March 31, 2018 Amount 3/3/2017 3/3/2019 2 month LIBOR + 1.5% 3.08% $ 125.0 (in millions) Issue Date Maturity Rate Structure Interest Rate at December 31, 2017 Amount 3/3/2017 3/3/2019 2 month LIBOR + 1.5% 2.94% $ 125.0 Borrowings under the New Credit Agreement may be used for general corporate purposes, including working capital, permitted acquisitions and letters of credit, and each of the Borrowers has agreed to be jointly and severally liable for the obligations of the other Borrowers under the New Credit Agreement. The New Credit Agreement contains customary events of default. If an event of default occurs and is continuing, the Borrowers could be required immediately to repay all amounts outstanding under the New Credit Agreement. Lenders holding at least a majority of the loans and commitments under the New Credit Agreement could elect to accelerate the maturity of the loans and/or terminate the commitments under the New Credit Agreement upon the occurrence and during the continuation of an event of default. Included in the New Credit Agreement is a provision that allows up to $200.0 million of the revolving credit facility to be used for LOCs, subject to availability. On March 3, 2017, the $0.2 million LOC outstanding under the Prior Credit Agreement was transferred to the New Credit Agreement. At March 31, 2018 and December 31, 2017, there were no borrowings outstanding under the revolving portions of the credit facilities. At March 31, 2018 and December 31, 2017, there was $0.5 million in LOCs against the New and Prior Credit Agreement. Other Debt As part of the ARIS Title Insurance Corporation (“ARIS”) acquisition, at March 31, 2018 and December 31, 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	3 Months Ended
	Mar. 31, 2018
Fair Value Disclosures [Abstract]
Disclosures about Fair Value of Financial Instruments	8. Disclosures about Fair Value of Financial Instruments Cash. The carrying amount approximates fair value. Investment securities and short-term investments . See Note 4, “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 March 31, 2018 and December 31, 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: March 31, 2018 December 31, 2017 (in millions) Carrying Amount Fair Value Carrying Amount Fair Value Junior subordinated debentures Trust preferred debentures $ 172.7 $ 172.9 $ 172.7 $ 172.9 Subordinated debentures acquired with Maybrooke 83.9 85.5 83.9 85.0 Senior unsecured fixed rate notes 139.7 141.1 139.6 141.2 Floating rate loan stock 60.8 60.9 58.9 59.0

Shareholders' Equity

Shareholders' Equity	3 Months Ended
	Mar. 31, 2018
Equity [Abstract]
Shareholders' Equity	9 . Shareholders’ Equity On February 20, 2018, the Board of Directors declared a 15% stock dividend, payable March 21, 2018 to all 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 February 20, 2018, our Board of Directors declared a quarterly cash dividend in the amount of $0.27 on each share of common stock outstanding. On March 23, 2018, we paid $9.3 million to our shareholders of record on March 7, 2018. On February 21, 2017 our Board of Directors declared a quarterly cash dividend in the amount of $0.23 on each share of common stock outstanding, on a post-stock dividend basis. On March 15, 2017 we paid $8.3 million to our shareholders of record on March 3, 2017. 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 previous Repurchase Authorizations. As of March 31, 2018, availability under the 2016 Repurchase Authorization for future repurchases of our common shares was $66.4 million. For the three months ended March 31, 2018, we repurchased a total of 314,586 common shares for $18.6 million. A summary of activity from January 1, 2018 through March 31, 2018 follows. A summary of common shares repurchased for the three months ended March 31, 2018 is shown below: Repurchase Type Date Trading Plan Initiated 2018 Purchase Period Number of Shares Repurchased Average Price of Shares Repurchased Total Cost (in millions) Repurchase Authorization Year 10b5-1 Trading Plan 12/15/2017 01/02/2018-02/16/2018 225,281 $ 59.31 $ 13.3 2016 Open Market N/A 02/20/2018-3/02/2018 89,305 $ 59.03 5.3 2016 Total 314,586 $ 59.23 $ 18.6

Accumulated Other Comprehensive

Accumulated Other Comprehensive (Loss) Income	3 Months Ended
	Mar. 31, 2018
Equity [Abstract]
Accumulated Other Comprehensive (Loss) Income	10. Accumulated Other Comprehensive (Loss) Income A summary of changes in accumulated other comprehensive (loss) income, net of taxes (where applicable) by component for the three months ended March 31, 2018, and 2017 is presented below: (in millions) Foreign Currency Translation Adjustments Unrealized Holding Gains on Securities Defined Benefit Pension Plans Total Balance, January 1, 2018 $ (19.0 ) $ 121.9 $ (6.3 ) $ 96.6 Cumulative effect of adoption of ASU 2016-01 — (117.5 ) — (117.5 ) Cumulative effect of adoption of ASU 2018-02 — 20.9 — 20.9 Other comprehensive income (loss) before reclassifications (1.1 ) (20.7 ) — (21.8 ) Amounts reclassified from accumulated other comprehensive income (loss) — (10.2 ) — (10.2 ) Net current-period other comprehensive income (loss) (1.1 ) (127.5 ) — (128.6 ) Balance at March 31, 2018 $ (20.1 ) $ (5.6 ) $ (6.3 ) $ (32.0 ) (in millions) Foreign Currency Translation Adjustments Unrealized Holding Gains on Securities Defined Benefit Pension Plans Total Balance, January 1, 2017 $ (17.6 ) $ 72.4 $ (7.1 ) $ 47.7 Other comprehensive income (loss) before reclassifications 0.6 24.3 — 24.9 Amounts reclassified from accumulated other comprehensive income (loss) — (11.0 ) — (11.0 ) Net current-period other comprehensive income (loss) 0.6 13.3 — 13.9 Balance at March 31, 2017 $ (17.0 ) $ 85.7 $ (7.1 ) $ 61.6 The amounts reclassified from accumulated other comprehensive (loss) income shown in the above table have been included in the following captions in our Consolidated Statements of Income: For the Three Months Ended March 31, (in millions) 2018 2017 Unrealized gains and losses on securities: Net realized investment gains $ (12.5 ) $ (16.2 ) Provision for income taxes 2.3 5.2 Net of taxes $ (10.2 ) $ (11.0 )

Net Income Per Common Share

Net Income Per Common Share	3 Months Ended
	Mar. 31, 2018
Earnings Per Share [Abstract]
Net Income Per Common Share	11 . 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 Three Months Ended March 31, (in millions, except number of shares and per share amounts) 2018 2017 Net income $ 24.8 $ 36.7 Weighted average common shares outstanding - basic 33,868,749 34,554,341 Effect of dilutive securities: Equity compensation awards 871,594 1,028,475 Weighted average common shares outstanding - diluted 34,740,343 35,582,816 Net income per common share: Basic $ 0.73 $ 1.06 Diluted $ 0.71 $ 1.03 Excluded from the weighted average common shares outstanding calculation at March 31, 2018 and 2017 are 11,099,593 shares and 10,028,755 shares, respectively, which are held as treasury shares. The shares are excluded as of their repurchase date. For the three months ended March 31, 2018 and 2017, there were no equity compensation awards with an anti-dilutive effect.

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	3 Months Ended
	Mar. 31, 2018
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information	12. Supplemental Cash Flow Information Interest paid and income taxes paid (recovered) were as follows: For the Three Months Ended March 31, (in millions) 2018 2017 Senior unsecured fixed rate notes $ 2.3 $ 2.3 Junior subordinated debentures 3.4 2.9 Other indebtedness 1.5 0.8 Revolving credit facility — 0.3 Total interest paid $ 7.2 $ 6.3 Income taxes paid 1.5 — Income taxes recovered — (2.2 ) Income taxes paid (recovered), net $ 1.5 $ (2.2 ) Total $ 8.7 $ 4.1

Share-based Compensation

Share-based Compensation	3 Months Ended
	Mar. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Share-based Compensation	13. Share-based Compensation The fair value method of accounting is used for share-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 risk-free rate of return assumption is based on the five-year U.S. Treasury constant maturity rate on the measurement date. The expected dividend yield is based on our history and expected dividend payouts. The expected award life is based upon the average holding period over the history of the incentive plan. The expected volatility assumption is based on the historical change in our stock price over the previous five years preceding the measurement date. The following table summarizes the assumptions we used for the three months ended March 31, 2018 and 2017: For the Three Months Ended March 31, 2018 2017 Risk-free rate of return 2.61% 1.93% Expected dividend yields 1.71% 1.63% Expected award life (years) 4.48 4.49 Expected volatility 18.44% 18.70% All outstanding awards were adjusted to reflect the 15% stock dividend, resulting in a 15% increase to the number of awards outstanding and a 13.04% reduction in the exercise price. Argo Group’s Long-Term Incentive Plans In November 2007, our shareholders approved the 2007 Long-Term Incentive Plan (the “2007 Plan”), which provides 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, 1.46 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 unit 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. Restricted Shares A summary of restricted share activity as of March 31, 2018 and changes during the three months then ended is as follows: Shares Weighted-Average Grant Date Fair Value Outstanding at January 1, 2018 767,501 $ 42.70 Granted 214,048 $ 55.67 Vested and issued (137,834 ) $ 44.68 Expired or forfeited (81,171 ) $ 43.77 Outstanding at March 31, 2018 762,544 $ 45.87 The restricted shares vest over one to eight years. Expense recognized under this plan for the restricted shares was $3.4 million and $2.8 million for the three months ended March 31, 2018, and 2017, respectively. Compensation expense for all share-based compensation awards is included in “Underwriting, acquisition and insurance expenses” in the accompanying Consolidated Statements of Income. As of March 31, 2018, there was $26.4 million of total unrecognized compensation cost related to restricted share compensation arrangements granted by Argo Group. Stock-Settled SARs A summary of stock-settled SARs activity as of March 31, 2018 and changes during the three months then ended is as follows: Shares Weighted-Average Exercise Price Outstanding at January 1, 2018 1,494,458 $ 30.85 Exercised (136,849 ) $ 27.45 Expired or forfeited (14,947 ) $ 36.25 Outstanding at March 31, 2018 1,342,662 $ 31.13 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. Expense recognized for the stock-settled SARs was $1.1 million and $0.7 million for the three months ended March 31, 2018, and 2017, respectively. As of March 31, 2018, there was $1.5 million of total unrecognized compensation cost related to stock-settled SARs outstanding. Cash-Settled SARs A summary of cash-settled SARs activity as of March 31, 2018 and changes during the three months then ended is as follows: Shares Weighted-Average Exercise Price Outstanding at January 1, 2018 189,235 $ 28.97 Exercised (42,432 ) $ 29.43 Expired or forfeited (2,235 ) $ 21.23 Outstanding at March 31, 2018 144,568 $ 28.95 As of March 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. Expense recognized for the cash-settle SARs was $0.5 million and $1.4 for the three months ended March 31, 2018, and 2017, respectively. As of March 31, 2018, there was no unrecognized compensation cost related to cash-settled SARs outstanding.

Underwriting, Acquisition and I

Underwriting, Acquisition and Insurance Expenses	3 Months Ended
	Mar. 31, 2018
Underwriting Acquisition And Insurance Expenses [Abstract]
Underwriting, Acquisition and Insurance Expenses	14. Underwriting, Acquisition and Insurance Expenses Underwriting, acquisition and insurance expenses were as follows: For the Three Months Ended March 31, (in millions) 2018 2017 Commissions $ 68.0 $ 61.2 General expenses 87.8 88.7 Premium taxes, boards and bureaus 9.3 7.4 165.1 157.3 Net deferral of policy acquisition costs (4.9 ) (3.7 ) Total underwriting, acquisition and insurance expenses $ 160.2 $ 153.6

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	15. Income Taxes We are incorporated under the laws of Bermuda and, under current Bermuda law, are not obligated to pay any taxes in Bermuda based upon income or capital gains. We have received an undertaking from the Supervisor of Insurance in Bermuda pursuant to the provisions of the Exempted Undertakings Tax Protection Act, 2011, which exempts us from any Bermuda taxes computed on profits, income or any capital asset, gain or appreciation or any tax in the nature of estate duty or inheritance tax, at least until the year 2035. We do not consider ourselves to be engaged in a trade or business in the United States or the United Kingdom and, accordingly, do not expect to be subject to direct United States or United Kingdom income taxation. We have subsidiaries based in the United Kingdom that are subject to the tax laws of that country. Under current law, these subsidiaries are taxed at the applicable corporate tax rates. Eight of the United Kingdom subsidiaries are deemed to be engaged in business in the United States, and therefore, are subject to United States corporate tax in respect of a proportion of their United States underwriting business only. Relief is available against the United Kingdom tax liabilities in respect of overseas taxes paid that arise from the underwriting business. Our United Kingdom subsidiaries file separate United Kingdom income tax returns. We have subsidiaries based in the United States that are subject to United States tax laws. Under current law, these subsidiaries are taxed at the applicable corporate tax rates. Our United States subsidiaries generally file a consolidated United States federal income tax return. We also have operations in Belgium, Brazil, France, Ireland, Italy, Luxembourg, Malta, Spain, and Switzerland, which also are subject to income taxes imposed by the jurisdiction in which they operate. We have operations in Barbados and the United Arab Emirates, which are not subject to income tax under the laws of those countries. On December 22, 2017, U.S. tax legislation referred to as the TCJA was enacted. The effects of changes in tax laws and tax rates are recognized in the period of enactment. Accordingly, we recorded the impacts of the TCJA in our 2017 consolidated financial statements which, among other changes, primarily includes the remeasurement of our deferred tax assets and liabilities for the reduced US federal tax rate from 35% to 21% beginning on January 1, 2018, and the computation of a provisional amount for the loss reserve discounting modifications. We are still analyzing certain aspects of the TCJA and refining our calculations, which could potentially affect the measurement of these balances or potentially give rise to new deferred tax amounts. We are still analyzing certain aspects of the TCJA and refining our calculations, which could potentially affect the measurement of these balances or potentially give rise to new deferred tax amounts. We have not made any adjustments to the provisional amounts we recorded during the year ended December 31, 2017. Beginning in 2018, the TCJA introduces an additional Base Erosion and Anti-abuse Tax (“BEAT”) provision computed to be an additional liability in excess of the regular US tax liability based upon an alternative BEAT computation. At this time, we have not recorded any provisional amounts for BEAT as we are awaiting information from Lloyd’s to determine the effect of BEAT on our US branches within our UK operations. Thus, our 2018 consolidated financial statements reflect a reasonably estimated provisional amount based on information available and in accordance with SAB 118. SAB 118 provides guidance on accounting for the effects of the U.S. tax reform where our determinations are incomplete but we are able to determine a reasonable estimate. A final determination is required to be made within a measurement period not to extend beyond one year from the enactment date of the U.S. tax reform. As additional guidance is released, the estimate will be updated as necessary. 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 three months ended March 31, 2018 and 2017, pre-tax income (loss) attributable to our operations and the operations’ effective tax rates were as follows: For the Three Months Ended March 31, (in millions) 2018 2017 Pre-Tax Income (Loss) Effective Tax Rate Pre-Tax Income (Loss) Effective Tax Rate Bermuda $ 25.3 0.0 % $ 26.5 0.0 % United States (6.3 ) 23.0 % 24.0 26.9 % United Kingdom 6.2 24.5 % (6.9 ) 6.7 % Belgium 0.1 35.8 % — (1) 0.0 % Brazil (1.4 ) 0.0 % (0.8 ) 0.0 % United Arab Emirates 0.2 0.0 % — (1) 0.0 % Ireland — (1) 0.0 % — (1) 0.0 % Italy — (1) 0.0 % N/A N/A Malta 0.9 0.0 % 0.6 0.0 % Luxembourg — (1) 0.0 % (0.7 ) 0.0 % Switzerland — 21.2 % — (1) 21.0 % Pre-tax income $ 25.0 0.7 % $ 42.7 14.1 % (1) Pre-tax income for the respective year was less than $0.1 million. Our effective tax rate may vary significantly from period to period depending on the jurisdiction generating the pre-tax income (loss) and its corresponding statutory tax rate. The geographic distribution of pre-tax income (loss) can fluctuate significantly between periods given the inherit nature of our business. A reconciliation of the difference between the provision for income taxes and the expected tax provision at the weighted average tax rate is as follows: For the Three Months Ended March 31, (in millions) 2018 2017 Income tax (benefit) provision at expected rate $ (0.4 ) $ 7.2 Tax effect of: Nontaxable investment income (0.5 ) (1.3 ) Foreign exchange adjustments (0.2 ) 0.6 Withholding taxes 0.1 0.1 Change in valuation allowance (2.7 ) (0.1 ) Other 3.9 (0.5 ) Income tax provision $ 0.2 $ 6.0 Our gross deferred tax assets (liabilities) are supported by taxes paid in previous periods, reversal of taxable temporary differences and recognition of future taxable income. Management regularly evaluates the recoverability of the deferred tax assets and makes any necessary adjustments to them based upon any changes in management’s expectations of future taxable income. Realization of deferred tax assets is dependent upon our generation of future taxable income sufficient to recover tax benefits that cannot be recovered from taxes paid in the carryback period, generally for our US property and casualty insurers two years for net operating losses and for all our US subsidiaries three years for capital losses. If a company determines that any of its deferred tax assets will not result in future tax benefits, a valuation allowance must be established for the portion of these assets that are not expected to be realized. The net change in valuation allowance for deferred tax assets was a decrease of $2.7 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 $7.0 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 as Internal Revenue Code Section 382 limits regarding the application of net operating loss carryforwards following an ownership change. The loss carryforwards available per year for both of these items are $2.8 million, as required by Internal Revenue Code Section 382. Furthermore, due to cumulative losses incurred since inception, management has concluded that a valuation allowance is required for the full amount of the tax-effected net operating losses generated by our Brazil and Malta entities as well as our Luxembourg and US affiliates acquired in the Maybrooke transaction. Accordingly, a valuation allowance is required as of March 31, 2018 of which $7.7 million relates to Brazil operations, and $0.9 million relates to Malta operations. During the three months ended March 31, 2018, the affiliates of Maybrooke Holdings SA have a valuation allowance established in the amount of $1.2 million. The change from 2017 was primarily driven from the liquidation of our Luxembourg affiliate as of March 23, 2018. 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 March 31, 2018 and 2017. Our United States subsidiaries are no longer subject to U.S. federal and state income tax examinations by tax authorities for years before 2014. Our United Kingdom subsidiaries are no longer subject to United Kingdom income tax examinations by Her Majesty’s Revenue and Customs for years before 2014.

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	16. Commitments and Contingencies Argo Group’s subsidiaries are parties to legal actions incidental to their business. Based on the opinion of counsel, management believes that the resolution of these matters will not materially affect our financial condition or results of operations. We have contractual commitments to invest up to $123.8 million related to our limited partnership investments at March 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.

Segment Information

Segment Information	3 Months Ended
	Mar. 31, 2018
Segment Reporting [Abstract]
Segment Information	17. 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 (loss) before income taxes for each segment were as follows: For the Three Months Ended March 31, (in millions) 2018 2017 Revenue: Earned premiums U.S. Operations $ 262.3 $ 221.2 International Operations 152.4 158.2 Run-off Lines — — Total earned premiums 414.7 379.4 Net investment income U.S. Operations 22.6 19.9 International Operations 8.8 6.6 Run-off Lines 2.2 2.1 Corporate and Other 2.4 1.9 Total net investment income 36.0 30.5 Fee and other income 2.0 3.6 Net realized investment (losses) gains (15.7 ) 14.6 Total revenue $ 437.0 $ 428.1 For the Three Months Ended March 31, (in millions) 2018 2017 Income (loss) before income taxes U.S. Operations $ 34.0 $ 37.2 International Operations 22.9 8.3 Run-off Lines (0.7 ) (2.5 ) Total segment income before taxes 56.2 43.0 Corporate and Other (15.5 ) (14.9 ) Net realized investment (losses) gains (15.7 ) 14.6 Total income before income taxes $ 25.0 $ 42.7 The table below presents earned premiums by geographic location for the three months ended March 31, 2018 and 2017. 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 Three Months Ended March 31, (in millions) 2018 2017 Bermuda $ 20.5 $ 25.6 Brazil 13.0 12.8 Malta 5.7 1.0 United Kingdom 114.1 118.8 United States 261.4 221.2 Total earned premiums $ 414.7 $ 379.4 The following table represents identifiable assets: (in millions) March 31, 2018 December 31, 2017 U.S. Operations $ 4,527.1 $ 4,298.4 International Operations 3,736.8 3,553.8 Run-off Lines 456.3 449.6 Corporate and Other 421.1 462.2 Total $ 9,141.3 $ 8,764.0 Included in total assets at March 31, 2018 and December 31, 2017 are $734.2 million and $731.8 million, respectively, in assets associated with trade capital providers.

Senior Unsecured Fixed Rate Not

Senior Unsecured Fixed Rate Notes	3 Months Ended
	Mar. 31, 2018
Debt Disclosure [Abstract]
Senior Unsecured Fixed Rate Notes	18. Senior Unsecured Fixed Rate Notes In September 2012, Argo Group (the “Parent Guarantor”), through its subsidiary Argo Group US (the “Subsidiary Issuer”), issued $143,750,000 aggregate principal amount of the Subsidiary Issuer’s 6.5% Senior Notes due September 15, 2042 (the “Notes”). The Notes are unsecured and unsubordinated obligations of the Subsidiary Issuer and rank equally in right of payment with all of the Subsidiary Issuer’s other unsecured and unsubordinated debt. The Notes are guaranteed on a full and unconditional senior unsecured basis by the Parent Guarantor. The Notes may be redeemed, for cash, in whole or in part, on or after September 15, 2017, at the Subsidiary Issuer’s option, at any time and from time to time, prior to maturity at a redemption price equal to 100% of the principal amount of the Notes to be redeemed, plus accrued but unpaid interest on the principal amount being redeemed to, but not including, the redemption date. In accordance with ASU 2015-03, “Simplifying the Presentation of Debt Issuance Costs” (Topic 835), we present the unamortized debt issuance costs in the balance sheet as a direct deduction from the carrying value of the debt liability. At March 31, 2018 and December 31, 2017, the Notes consisted of the following: (in millions) March 31, 2018 December 31, 2017 Senior unsecured fixed rate notes Principal $ 143.8 $ 143.8 Less: unamortized debt issuance costs (4.1 ) (4.2 ) Senior unsecured fixed rate notes, less unamortized debt issuance costs $ 139.7 $ 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 March 31, 2018 and December 31, 2017 and for the three months ended March 31, 2018 and 2017, 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 MARCH 31, 2018 (in millions) (Unaudited) Argo Group International Holdings, Ltd (Parent Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Assets Investments $ 0.4 $ 3,085.9 $ 1,747.2 $ — $ 4,833.5 Cash 0.3 59.4 114.0 — 173.7 Accrued investment income — 18.0 6.3 — 24.3 Premiums receivable — 210.6 489.0 — 699.6 Reinsurance recoverables — 1,537.4 550.1 — 2,087.5 Goodwill and other intangible assets, net 42.9 124.7 101.1 — 268.7 Deferred acquisition costs, net — 81.1 82.8 — 163.9 Ceded unearned premiums — 211.0 290.9 — 501.9 Other assets 14.9 179.6 193.7 — 388.2 Intercompany note receivable — 51.5 (51.5 ) — — Investments in subsidiaries 1,939.4 — — (1,939.4 ) — Total assets $ 1,997.9 $ 5,559.2 $ 3,523.6 $ (1,939.4 ) $ 9,141.3 Liabilities and Shareholders' Equity Reserves for losses and loss adjustment expenses $ — $ 2,538.1 $ 1,745.5 $ — $ 4,283.6 Unearned premiums — 702.6 567.5 — 1,270.1 Funds held and ceded reinsurance payable, net — 863.1 49.8 — 912.9 Long-term debt 153.4 284.6 144.7 — 582.7 Current income taxes payable, net — 0.8 1.4 — 2.2 Deferred tax liabilities, net — 21.7 (0.4 ) — 21.3 Accrued underwriting expenses and other liabilities 6.3 90.6 184.2 — 281.1 Due to (from) affiliates 50.8 5.0 (5.0 ) (50.8 ) — Total liabilities 210.5 4,506.5 2,687.7 (50.8 ) 7,353.9 Total shareholders' equity 1,787.4 1,052.7 835.9 (1,888.6 ) 1,787.4 Total liabilities and shareholders' equity $ 1,997.9 $ 5,559.2 $ 3,523.6 $ (1,939.4 ) $ 9,141.3 (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 BALANCE SHEET DECEMBER 31, 2017 (in millions) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Assets Investments $ 0.4 $ 3,079.0 $ 1,663.5 $ — $ 4,742.9 Cash 0.9 47.8 127.9 — 176.6 Accrued investment income — 18.0 5.5 — 23.5 Premiums receivable — 216.5 382.1 — 598.6 Reinsurance recoverables — 1,487.3 606.0 — 2,093.3 Goodwill and other intangible assets, net 43.2 124.9 90.1 — 258.2 Current income taxes receivable, net — 2.4 (1.0 ) — 1.4 Deferred acquisition costs, net — 80.7 79.7 — 160.4 Ceded unearned premiums — 198.5 201.0 — 399.5 Other assets 9.2 171.5 128.9 — 309.6 Intercompany note receivable — 50.9 (50.9 ) — — Investments in subsidiaries 1,940.0 — — (1,940.0 ) — Total assets $ 1,993.7 $ 5,477.5 $ 3,232.8 $ (1,940.0 ) $ 8,764.0 Liabilities and Shareholders' Equity Reserves for losses and loss adjustment expenses $ — $ 2,483.9 $ 1,717.1 $ — $ 4,201.0 Unearned premiums — 704.0 503.7 — 1,207.7 Funds held and ceded reinsurance payable, net — 799.4 (22.7 ) — 776.7 Long-term debt 153.4 284.5 142.8 — 580.7 Deferred tax liabilities, net — 32.5 (1.2 ) — 31.3 Accrued underwriting expenses and other liabilities 8.9 95.0 43.0 — 146.9 Due to affiliates 11.7 (0.4 ) 0.4 (11.7 ) — Total liabilities 174.0 4,398.9 2,383.1 (11.7 ) 6,944.3 Total shareholders' equity 1,819.7 1,078.6 849.7 (1,928.3 ) 1,819.7 Total liabilities and shareholders' equity $ 1,993.7 $ 5,477.5 $ 3,232.8 $ (1,940.0 ) $ 8,764.0 (1) Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations. (2) Includes all Argo Group parent company eliminations. CONDENSED CONSOLIDATING STATEMENT OF INCOME FOR THE THREE MONTHS ENDED MARCH 31, 2018 (in millions) (Unaudited) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Premiums and other revenue: Earned premiums $ — $ 156.9 $ 257.8 $ — $ 414.7 Net investment (expense) income (0.7 ) 22.1 14.6 — 36.0 Fee and other income — 0.8 1.2 — 2.0 Net realized investment (losses) gains (0.4 ) (15.4 ) 0.1 — (15.7 ) Total revenue (1.1 ) 164.4 273.7 — 437.0 Expenses: Losses and loss adjustment expenses — 95.4 141.8 — 237.2 Underwriting, acquisition and insurance expenses 2.4 69.8 88.0 — 160.2 Interest expense 1.3 4.3 2.1 — 7.7 Fee and other expense — 1.4 0.6 — 2.0 Foreign currency exchange loss — 0.1 4.8 — 4.9 Total expenses 3.7 171.0 237.3 — 412.0 (Loss) Income before income taxes (4.8 ) (6.6 ) 36.4 — 25.0 (Benefit) Provision for income taxes — (1.4 ) 1.6 — 0.2 Net (loss) income before equity in earnings of subsidiaries (4.8 ) (5.2 ) 34.8 — 24.8 Equity in undistributed earnings of subsidiaries 29.6 — — (29.6 ) — Net income $ 24.8 $ (5.2 ) $ 34.8 $ (29.6 ) $ 24.8 (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 THREE MONTHS ENDED MARCH 31, 2017 (in millions) (Unaudited) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Premiums and other revenue: Earned premiums $ — $ 130.9 $ 248.5 $ — $ 379.4 Net investment (expense) income (1.7 ) 18.9 13.3 — 30.5 Fee and other income — 2.8 0.8 — 3.6 Net realized investment gains 0.5 13.8 0.3 — 14.6 Total revenue (1.2 ) 166.4 262.9 — 428.1 Expenses: Losses and loss adjustment expenses — 76.1 146.4 — 222.5 Underwriting, acquisition and insurance expenses 8.4 58.2 87.0 — 153.6 Interest expense 0.3 4.3 1.3 — 5.9 Fee and other expense — 3.4 0.7 — 4.1 Foreign currency exchange loss (gains) — 0.1 (0.8 ) — (0.7 ) Total expenses 8.7 142.1 234.6 — 385.4 (Loss) Income before income taxes (9.9 ) 24.3 28.3 — 42.7 Provision (benefit) for income taxes — 6.4 (0.4 ) — 6.0 Net (loss) income before equity in earnings of subsidiaries (9.9 ) 17.9 28.7 — 36.7 Equity in undistributed earnings of subsidiaries 46.6 — — (46.6 ) — Net income $ 36.7 $ 17.9 $ 28.7 $ (46.6 ) $ 36.7 (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 THE THREE MONTHS ENDED MARCH 31, 2018 (in millions) (Unaudited) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Net cash flows from operating activities $ 27.8 $ 53.2 $ 72.6 $ — $ 153.6 Cash flows from investing activities: Proceeds from sales of investments — 325.0 230.4 — 555.4 Maturities and mandatory calls of fixed maturity investments — 113.1 31.7 — 144.8 Purchases of investments — (475.2 ) (212.3 ) — (687.5 ) Change in short-term investments and foreign regulatory deposits — (4.5 ) (156.5 ) — (161.0 ) Settlements of foreign currency exchange forward contracts (0.6 ) (0.7 ) 3.0 — 1.7 Cash included in acquisition of Ariscom — — 15.6 — 15.6 Purchases of fixed assets and other, net (0.1 ) 0.7 1.9 — 2.5 Cash used in investing activities (0.7 ) (41.6 ) (86.2 ) — (128.5 ) Cash flows from financing activities: Activity under stock incentive plans 0.2 — — — 0.2 Repurchase of Company's common shares (18.6 ) — — — (18.6 ) Payment of cash dividend to common shareholders (9.3 ) — — — (9.3 ) Cash used in financing activities (27.7 ) — — — (27.7 ) Effect of exchange rate changes on cash — — (0.3 ) — (0.3 ) Change in cash (0.6 ) 11.6 (13.9 ) — (2.9 ) Cash, beginning of year 0.9 47.8 127.9 — 176.6 Cash, end of period $ 0.3 $ 59.4 $ 114.0 $ — $ 173.7 (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 THE THREE MONTHS ENDED MARCH 31, 2017 (in millions) (Unaudited) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Net cash flows from operating activities $ (2.0 ) $ (14.7 ) $ (20.2 ) $ — $ (36.9 ) Cash flows from investing activities: Proceeds from sales of investments — 228.9 225.1 — 454.0 Maturities and mandatory calls of fixed maturity investments — 135.5 46.6 — 182.1 Purchases of investments — (446.5 ) (197.9 ) — (644.4 ) Change in short-term investments and foreign regulatory deposits 0.5 65.5 54.3 — 120.3 Settlements of foreign currency exchange forward contracts — — (2.8 ) — (2.8 ) Acquisition of Maybrooke, net of cash and cash equivalents acquired (235.3 ) 152.2 (83.1 ) Issuance of intercompany note, net — — (120.0 ) 120.0 — Purchases of fixed assets and other, net (0.1 ) 18.7 (19.9 ) — (1.3 ) Cash (used in) provided by investing activities (234.9 ) 2.1 137.6 $ 120.0 24.8 Cash flows from financing activities: Additional long-term borrowings 125.0 — — — 125.0 Borrowing under intercompany note, net 120.0 — — (120.0 ) — Activity under stock incentive plans 0.2 — — — 0.2 Payment of cash dividend to common shareholders (8.3 ) — — — (8.3 ) Cash provided by (used in) financing activities 236.9 — — (120.0 ) 116.9 Effect of exchange rate changes on cash — — (0.1 ) — (0.1 ) Change in cash — (12.6 ) 117.3 — 104.7 Cash, beginning of year — 53.7 32.3 — 86.0 Cash, end of period $ — $ 41.1 $ 149.6 $ — $ 190.7 (1) Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations. (2) Includes all Argo Group parent company eliminations.

Basis of Presentation (Policies

Basis of Presentation (Policies)	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements of Argo Group International Holdings, Ltd. (“Argo Group,” “we” or the “Company”) and its subsidiaries have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial information and with the instructions for Form 10-Q and Article 10 of Regulation S-X. The preparation of interim 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 intangibles and our deferred tax asset valuation allowance. Actual results could differ from those estimates. Certain financial information that normally is included in annual financial statements, including certain financial statement footnotes, prepared in accordance with GAAP, is not required for interim reporting purposes and has been condensed or omitted. These statements should be read in conjunction with the consolidated financial statements and notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2017, filed with the Securities and Exchange Commission on February 27, 2018. 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 3, “Acquisition of Ariscom,” for additional discussion regarding the acquisition. The Consolidated Financial Statements as of and for the three months ended March 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. The interim financial information as of, and for the three months ended March 31, 2018 and 2017 is unaudited. However, in the opinion of management, the interim information includes all adjustments, consisting of normal recurring accruals, necessary for a fair presentation of the results presented for the interim periods. The operating results for the interim periods are not necessarily indicative of the results to be expected for the full year. All significant intercompany amounts have been eliminated in consolidation.

Investments (Tables)

Investments (Tables)	3 Months Ended
	Mar. 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 of investments were as follows: March 31, 2018 (in millions) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Fixed maturities U.S. Governments $ 374.2 $ 0.2 $ 7.4 $ 367.0 Foreign Governments 233.0 3.9 3.5 233.4 Obligations of states and political subdivisions 315.9 6.6 2.0 320.5 Corporate bonds 1,580.7 15.0 24.1 1,571.6 Commercial mortgage-backed securities 103.5 — 2.3 101.2 Residential mortgage-backed securities 341.5 2.3 8.1 335.7 Asset-backed securities 165.6 1.0 1.3 165.3 Collateralized loan obligations 192.3 4.8 0.3 196.8 Total fixed maturities 3,306.7 33.8 49.0 3,291.5 Equity securities 358.3 131.9 13.7 476.5 Other investments 513.7 9.6 — 523.3 Short-term investments 542.4 — 0.2 542.2 Total investments $ 4,721.1 $ 175.3 $ 62.9 $ 4,833.5 December 31, 2017 (in millions) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Fixed maturities U.S. Governments $ 419.9 $ 0.2 $ 5.0 $ 415.1 Foreign Governments 229.0 6.7 2.5 233.2 Obligations of states and political subdivisions 327.7 9.3 1.1 335.9 Corporate bonds 1,514.5 24.4 13.2 1,525.7 Commercial mortgage-backed securities 136.3 0.1 1.5 134.9 Residential mortgage-backed securities 309.3 2.8 2.7 309.4 Asset-backed securities 161.3 0.7 0.8 161.2 Collateralized loan obligations 222.6 5.9 0.5 228.0 Total fixed maturities 3,320.6 50.1 27.3 3,343.4 Equity securities 338.2 154.0 4.8 487.4 Other investments 534.1 9.6 0.1 543.6 Short-term investments 368.5 — — 368.5 Total investments $ 4,561.4 $ 213.7 $ 32.2 $ 4,742.9
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 March 31, 2018, by contractual maturity, were as follows: (in millions) Amortized Cost Fair Value Due in one year or less $ 268.5 $ 267.7 Due after one year through five years 1,467.0 1,460.2 Due after five years through ten years 600.4 595.6 Thereafter 167.9 169.0 Structured securities 802.9 799.0 Total $ 3,306.7 $ 3,291.5
Schedule Of Carrying Value 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 March 31, 2018 and December 31, 2017 were as follows: March 31, 2018 (in millions) Carrying Value Unfunded Commitments Investment Type Hedge funds $ 138.8 $ — Private equity 186.5 123.8 Long only funds 193.7 — Other investments 4.3 — Total other invested assets $ 523.3 $ 123.8 December 31, 2017 (in millions) Carrying Value Unfunded Commitments Investment Type Hedge funds $ 163.6 $ — Private equity 179.2 129.9 Long only funds 196.5 — Other investments 4.3 — Total other invested assets $ 543.6 $ 129.9
Schedule of Aging of Unrealized Losses on Company's Investments in Fixed Maturities, Equity Securities and Other Investments	An aging of unrealized losses on our investments in fixed maturities, equity securities (as of December 31, 2017), other investments and short-term investments is presented below: March 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 $ 316.8 $ 6.5 $ 17.3 $ 0.9 $ 334.1 $ 7.4 Foreign Governments 207.2 3.4 5.0 0.1 212.2 3.5 Obligations of states and political subdivisions 92.1 1.4 14.7 0.6 106.8 2.0 Corporate bonds 1,078.7 22.2 32.0 1.9 1,110.7 24.1 Commercial mortgage-backed securities 69.6 1.4 16.0 0.9 85.6 2.3 Residential mortgage-backed securities 263.9 7.5 14.4 0.6 278.3 8.1 Asset-backed securities 111.3 1.0 7.9 0.3 119.2 1.3 Collateralized loan obligations (1) 41.0 0.3 0.9 — 41.9 0.3 Total fixed maturities 2,180.6 43.7 108.2 5.3 2,288.8 49.0 Other investments (1) — — 0.3 — 0.3 — Short-term investments 58.4 0.2 — — 58.4 0.2 Total $ 2,239.0 $ 43.9 $ 108.5 $ 5.3 $ 2,347.5 $ 49.2 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 Equity securities 46.9 4.8 — — 46.9 4.8 Other investments — — 0.3 0.1 0.3 0.1 Total $ 1,585.8 $ 22.0 $ 343.0 $ 10.2 $ 1,928.8 $ 32.2 (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 portfolio as follows: For the Three Months Ended March 31, (in millions) 2018 2017 Other-than-temporary impairment: Corporate bonds $ (1.0 ) $ — Equity securities — (0.4 ) Other-than-temporary impairment losses $ (1.0 ) $ (0.4 )
Schedule of Company's Gross Realized Investment Gains (Losses)	The following table presents our gross realized investment gains (losses): For the Three Months Ended March 31, (in millions) 2018 2017 Realized gains on fixed maturities and other Fixed maturities $ 7.5 $ 7.1 Equity securities (1) — 15.7 Other investments 11.7 6.2 19.2 29.0 Realized losses on fixed maturities and other Fixed maturities (4.9 ) (6.2 ) Equity securities (1) — (0.7 ) Other investments (9.3 ) (7.1 ) Other-than-temporary impairment losses on fixed maturities (1.0 ) — Other-than-temporary impairment losses on equity securities — (0.4 ) (15.2 ) (14.4 ) Equity Securities (1) Net realized gains on equity securities sold during the period 11.2 — Change in unrealized (losses) on equity securities held at the end of the period (30.9 ) — Net realized (losses) on equity securities (19.7 ) — Net realized investment (losses) gains before income taxes (15.7 ) 14.6 Income tax benefit (expense) 2.9 (4.3 ) Net realized investment (losses) gains, net of income taxes $ (12.8 ) $ 10.3 (1)
Schedule of Changes in Unrealized Appreciation (Depreciation)	Changes in unrealized appreciation (depreciation) related to investments are summarized as follows: For the Three Months Ended March 31, (in millions) 2018 2017 Change in unrealized gains (losses) Fixed maturities $ (37.1 ) $ 12.6 Equity securities — 5.3 Short-term investments (0.1 ) — Net unrealized investment (losses) gains before income taxes (37.2 ) 17.9 Income tax benefit (expense) 6.3 (4.6 ) Net unrealized investment (losses) gains, net of income taxes $ (30.9 ) $ 13.3
Schedule of Fair Value of Foreign Currency Exchange Forward Contracts	The fair value of our foreign currency exchange forward contracts as of March 31, 2018 and December 31, 2017 was as follows: (in millions) March 31, 2018 December 31, 2017 Operational currency exposure $ 3.4 $ (0.2 ) Asset manager investment exposure (1.8 ) (0.9 ) Total return strategy 0.5 0.7 Total $ 2.1 $ (0.4 )
Schedule of Realized Gains and Losses of Investment on Foreign Currency Exchange Forward Contracts	The following table represents our gross investment realized gains and losses on our foreign currency exchange forward contracts: For the Three Months Ended March 31, (in millions) 2018 2017 Realized gains Operational currency exposure $ 4.3 $ 2.0 Asset manager investment exposure 4.3 0.5 Total return strategy 2.6 3.1 Gross realized investment gains 11.2 5.6 Realized losses Operational currency exposure (0.6 ) (2.2 ) Asset manager investment exposure (5.9 ) (1.4 ) Total return strategy (2.1 ) (1.9 ) Gross realized investment losses (8.6 ) (5.5 ) Net realized investment gains on foreign currency exchange forward contracts $ 2.6 $ 0.1
Schedule of Restricted Assets	The following table presents our components of restricted assets at March 31: (in millions) March 31, 2018 March 31, 2017 Securities on deposit for regulatory and other purposes $ 171.7 $ 183.9 Securities pledged as collateral for letters of credit 62.8 37.6 Securities and cash on deposit supporting Lloyd’s business 373.9 396.8 Total restricted investments $ 608.4 $ 618.3
Financial Assets Measured at Fair Value on Recurring Basis	Based on an analysis of the inputs, our financial assets measured at fair value on a recurring basis have been categorized as follows: Fair (in millions) March 31, 2018 Level 1 (a) Level 2 (b) Level 3 (c) Fixed maturities U.S. Governments $ 367.0 $ 359.3 $ 7.7 $ — Foreign Governments 233.4 — 233.4 — Obligations of states and political subdivisions 320.5 — 320.5 — Corporate bonds 1,571.6 — 1,569.7 1.9 Commercial mortgage-backed securities 101.2 — 101.2 — Residential mortgage-backed securities 335.7 — 335.7 — Asset-backed securities 165.3 — 165.3 — Collateralized loan obligations 196.8 — 196.8 — Total fixed maturities 3,291.5 359.3 2,930.3 1.9 Equity securities 476.5 472.1 2.1 2.3 Other investments 103.8 — 103.8 — Short-term investments 542.2 464.5 77.7 — $ 4,414.0 $ 1,295.9 $ 3,113.9 $ 4.2 (a) Quoted prices in active markets for identical assets (b) Significant other observable inputs (c) Significant unobservable inputs Fair Value Measurements at Reporting Date Using (in millions) December 31, 2017 Level 1 (a) Level 2 (b) Level 3 (c) Fixed maturities U.S. Governments $ 415.1 $ 410.6 $ 4.5 $ — Foreign Governments 233.2 — 233.2 — Obligations of states and political subdivisions 335.9 — 335.9 — Corporate bonds 1,525.7 — 1,523.8 1.9 Commercial mortgage-backed securities 134.9 — 134.9 — Residential mortgage-backed securities 309.4 — 309.4 — Asset-backed securities 161.2 — 161.2 — Collateralized loan obligations 228.0 — 228.0 — Total fixed maturities 3,343.4 410.6 2,930.9 1.9 Equity securities 487.4 483.0 2.1 2.3 Other investments 108.8 — 108.8 — Short-term investments 368.5 333.7 34.8 — $ 4,308.1 $ 1,227.3 $ 3,076.6 $ 4.2 (a) Quoted prices in active markets for identical assets (b) Significant other observable inputs (c) Significant unobservable inputs
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) — — — Included in other comprehensive income (loss) — — — Purchases, issuances, sales, and settlements: Purchases — — — Issuances — — — Sales — — — Settlements — — — Ending balance, March 31, 2018 $ 1.9 $ 2.3 $ 4.2 Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at December 31, 2017 $ — $ — $ — (in millions) Credit Financial Equity Securities Total Beginning balance, January 1, 2017 $ 2.0 $ 0.4 $ 2.4 Transfers into Level 3 — — — Transfers out of Level 3 — — — Total gains or losses (realized/unrealized): — Included in net income (loss) — — — Included in other comprehensive income (loss) (0.1 ) 0.2 0.1 Purchases, issuances, sales, and settlements: — Purchases — 1.7 1.7 Issuances — — — Sales — — — Settlements — — — Ending balance, December 31, 2017 $ 1.9 $ 2.3 $ 4.2 Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at December 31, 2017 $ — $ — $ —

Reserves for Losses and Loss 29

Reserves for Losses and Loss Adjustment Expenses (Tables)	3 Months Ended
	Mar. 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 Three Months Ended March 31, (in millions) 2018 2017 Net reserves beginning of the year $ 2,488.0 $ 2,180.2 Net Maybrooke reserves acquired — $ 132.6 Net Ariscom reserves acquired 47.2 — Add: Losses and LAE incurred during current calendar year, net of reinsurance: Current accident year 239.2 215.7 Prior accident years (2.0 ) 6.8 Losses and LAE incurred during calendar year, net of reinsurance 237.2 222.5 Deduct: Losses and LAE payments made during current calendar year, net of reinsurance: Current accident year 33.3 27.4 Prior accident years 186.6 155.3 Losses and LAE payments made during current calendar year, net of reinsurance: 219.9 182.7 Change in participation interest (1) (16.9 ) (23.2 ) Foreign exchange adjustments (0.8 ) 3.6 Net reserves - end of period 2,534.8 2,333.0 Add: Reinsurance recoverables on unpaid losses and LAE, end of period 1,748.8 1,247.3 Gross reserves - end of period $ 4,283.6 $ 3,580.3 (1) Amount represents the change in reserves due to changing our participation in Syndicates 1200 and 1910.
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’ loss and LAE reserves on each reporting segment is presented below: For the Three Months Ended March 31, (in millions) 2018 2017 U.S. Operations $ (1.0 ) $ (5.2 ) International Operations (2.8 ) 9.6 Run-off Lines 1.8 2.4 Total unfavorable (favorable) prior-year development $ (2.0 ) $ 6.8

Junior Subordinated Debentures

Junior Subordinated Debentures (Tables)	3 Months Ended
	Mar. 31, 2018
Junior Subordinated Debentures [Member]
Schedule of Junior Subordinated Debentures	A summary of our outstanding junior subordinated debentures is presented below: March 31, 2018 (in millions) Issue Date Trust Preferred Pools Maturity Rate Structure Interest Rate at March 31, 2018 Amount Argo Group 05/15/2003 PXRE Capital Statutory Trust II 05/15/2033 3M LIBOR + 4.10% 5.94% $ 18.1 11/06/2003 PXRE Capital Trust VI 09/30/2033 3M LIBOR + 3.90% 6.21% 10.3 Argo Group US 05/15/2003 Argonaut Group Statutory Trust I 05/15/2033 3M LIBOR + 4.10% 5.94% 15.5 12/16/2003 Argonaut Group Statutory Trust III 01/08/2034 3M LIBOR + 4.10% 5.82% 12.3 04/29/2004 Argonaut Group Statutory Trust IV 04/29/2034 3M LIBOR + 3.85% 5.69% 13.4 05/26/2004 Argonaut Group Statutory Trust V 05/24/2034 3M LIBOR + 3.85% 5.79% 12.3 05/12/2004 Argonaut Group Statutory Trust VI 05/12/2034 3M LIBOR + 3.80% 5.98% 13.4 09/17/2004 Argonaut Group Statutory Trust VII 12/15/2034 3M LIBOR + 3.60% 5.72% 15.5 09/22/2004 Argonaut Group Statutory Trust VIII 09/22/2034 3M LIBOR + 3.55% 5.80% 15.5 10/22/2004 Argonaut Group Statutory Trust IX 12/15/2034 3M LIBOR + 3.60% 5.72% 15.5 09/15/2005 Argonaut Group Statutory Trust X 09/15/2035 3M LIBOR + 3.40% 5.52% 30.9 Total Outstanding $ 172.7 December 31, 2017 (in millions) Issue Date Trust Preferred Pools Maturity Rate Structure Interest Rate at December 31, 2017 Amount Argo Group 05/15/2003 PXRE Capital Statutory Trust II 05/15/2033 3M LIBOR + 4.10% 5.52% $ 18.1 11/06/2003 PXRE Capital Trust VI 09/30/2033 3M LIBOR + 3.90% 5.59% 10.3 Argo Group US 05/15/2003 Argonaut Group Statutory Trust I 05/15/2033 3M LIBOR + 4.10% 5.52% 15.5 12/16/2003 Argonaut Group Statutory Trust III 01/08/2034 3M LIBOR + 4.10% 5.46% 12.3 04/29/2004 Argonaut Group Statutory Trust IV 04/29/2034 3M LIBOR + 3.85% 5.27% 13.4 05/26/2004 Argonaut Group Statutory Trust V 05/24/2034 3M LIBOR + 3.85% 5.30% 12.3 05/12/2004 Argonaut Group Statutory Trust VI 05/12/2034 3M LIBOR + 3.80% 5.40% 13.4 09/17/2004 Argonaut Group Statutory Trust VII 12/15/2034 3M LIBOR + 3.60% 5.19% 15.5 09/22/2004 Argonaut Group Statutory Trust VIII 09/22/2034 3M LIBOR + 3.55% 5.21% 15.5 10/22/2004 Argonaut Group Statutory Trust IX 12/15/2034 3M LIBOR + 3.60% 5.19% 15.5 09/15/2005 Argonaut Group Statutory Trust X 09/15/2035 3M LIBOR + 3.40% 4.99% 30.9 Total Outstanding $ 172.7
Maybrooke Junior Subordinated Debentures [Member]
Schedule of Junior Subordinated Debentures	A summary of the terms of the acquired debt outstanding is presented below: (in millions) Issue Date Maturity Rate Structure Interest Rate at March 31, 2018 Principal at March 31, 2018 Carrying Value at March 31, 2018 9/15/2007 9/15/2037 3 month LIBOR + 3.15% 5.27 % $ 91.8 $ 83.9 (in millions) Issue Date Maturity Rate Structure Interest Rate at December 31, 2017 Principal at December 31, 2017 Carrying Value at December 31, 2017 9/15/2007 9/15/2037 3 month LIBOR + 3.15% 4.74 % $ 91.8 $ 83.9

Other Indebtedness (Tables)

Other Indebtedness (Tables)	3 Months Ended
	Mar. 31, 2018
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 March 31, 2018 December 31, 2017 Floating rate loan stock $ 60.8 $ 58.9 Term loan 125.0 125.0 Other debt 0.6 0.6 Total other indebtedness $ 186.4 $ 184.5
Floating Rate Loan Stock [Member]
Schedule of Junior Subordinated Debentures	A summary of the notes outstanding at March 31, 2018 and December 31, 2017 is presented below: March 31, 2018 (in millions) Currency Issue Date Maturity Rate Structure Interest Rate at March 31, 2018 Amount U.S. Dollar 12/08/2004 11/15/2034 6 month LIBOR + 4.2% 6.09% $ 6.5 U.S. Dollar 10/31/2006 01/15/2036 6 month LIBOR + 4.0% 5.89% 10.0 Total U.S. Dollar 16.5 Euro 09/06/2005 08/22/2035 3 month LIBOR + 4.0% 3.67% 14.8 Euro 10/31/2006 11/22/2036 3 month LIBOR + 4.0% 3.67% 12.9 Euro 06/08/2007 09/15/2037 3 month LIBOR + 3.9% 3.57% 16.6 Total Euro 44.3 Total notes outstanding $ 60.8 December 31, 2017 (in millions) Currency Issue Date Maturity Rate Structure Interest Rate at December 31, 2017 Amount U.S. Dollar 12/08/2004 11/15/2034 6 month LIBOR + 4.2% 5.66% $ 6.5 U.S. Dollar 10/31/2006 01/15/2036 6 month LIBOR + 4.0% 5.46% 10.0 Total U.S. Dollar 16.5 Euro 09/06/2005 08/22/2035 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 06/08/2007 09/15/2037 3 month LIBOR + 3.9% 3.57% 15.9 Total Euro 42.4 Total notes outstanding $ 58.9
New Credit Agreement [Member]
Schedule of Floating Rate Loan Stock and Borrowing Under Credit Facility, Notes Outstanding	A summary of the terms of the outstanding balance at March 31, 2018 and December 31, 2017 is presented below: (in millions) Issue Date Maturity Rate Structure Interest Rate at March 31, 2018 Amount 3/3/2017 3/3/2019 2 month LIBOR + 1.5% 3.08% $ 125.0 (in millions) Issue Date Maturity Rate Structure Interest Rate at December 31, 2017 Amount 3/3/2017 3/3/2019 2 month LIBOR + 1.5% 2.94% $ 125.0

Disclosures about Fair Value 32

Disclosures about Fair Value of Financial Instruments (Tables)	3 Months Ended
	Mar. 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: March 31, 2018 December 31, 2017 (in millions) Carrying Amount Fair Value Carrying Amount Fair Value Junior subordinated debentures Trust preferred debentures $ 172.7 $ 172.9 $ 172.7 $ 172.9 Subordinated debentures acquired with Maybrooke 83.9 85.5 83.9 85.0 Senior unsecured fixed rate notes 139.7 141.1 139.6 141.2 Floating rate loan stock 60.8 60.9 58.9 59.0

Shareholders' Equity (Tables)

Shareholders' Equity (Tables)	3 Months Ended
	Mar. 31, 2018
Equity [Abstract]
Schedule of Repurchase of Shares	A summary of common shares repurchased for the three months ended March 31, 2018 is shown below: Repurchase Type Date Trading Plan Initiated 2018 Purchase Period Number of Shares Repurchased Average Price of Shares Repurchased Total Cost (in millions) Repurchase Authorization Year 10b5-1 Trading Plan 12/15/2017 01/02/2018-02/16/2018 225,281 $ 59.31 $ 13.3 2016 Open Market N/A 02/20/2018-3/02/2018 89,305 $ 59.03 5.3 2016 Total 314,586 $ 59.23 $ 18.6

Accumulated Other Comprehensi34

Accumulated Other Comprehensive (Loss) Income (Tables)	3 Months Ended
	Mar. 31, 2018
Equity [Abstract]
Changes in Accumulated Other Comprehensive (Loss) Income	A summary of changes in accumulated other comprehensive (loss) income, net of taxes (where applicable) by component for the three months ended March 31, 2018, and 2017 is presented below: (in millions) Foreign Currency Translation Adjustments Unrealized Holding Gains on Securities Defined Benefit Pension Plans Total Balance, January 1, 2018 $ (19.0 ) $ 121.9 $ (6.3 ) $ 96.6 Cumulative effect of adoption of ASU 2016-01 — (117.5 ) — (117.5 ) Cumulative effect of adoption of ASU 2018-02 — 20.9 — 20.9 Other comprehensive income (loss) before reclassifications (1.1 ) (20.7 ) — (21.8 ) Amounts reclassified from accumulated other comprehensive income (loss) — (10.2 ) — (10.2 ) Net current-period other comprehensive income (loss) (1.1 ) (127.5 ) — (128.6 ) Balance at March 31, 2018 $ (20.1 ) $ (5.6 ) $ (6.3 ) $ (32.0 ) (in millions) Foreign Currency Translation Adjustments Unrealized Holding Gains on Securities Defined Benefit Pension Plans Total Balance, January 1, 2017 $ (17.6 ) $ 72.4 $ (7.1 ) $ 47.7 Other comprehensive income (loss) before reclassifications 0.6 24.3 — 24.9 Amounts reclassified from accumulated other comprehensive income (loss) — (11.0 ) — (11.0 ) Net current-period other comprehensive income (loss) 0.6 13.3 — 13.9 Balance at March 31, 2017 $ (17.0 ) $ 85.7 $ (7.1 ) $ 61.6
Amounts Reclassified from Accumulated Other Comprehensive (Loss) Income	The amounts reclassified from accumulated other comprehensive (loss) income shown in the above table have been included in the following captions in our Consolidated Statements of Income: For the Three Months Ended March 31, (in millions) 2018 2017 Unrealized gains and losses on securities: Net realized investment gains $ (12.5 ) $ (16.2 ) Provision for income taxes 2.3 5.2 Net of taxes $ (10.2 ) $ (11.0 )

Net Income Per Common Share (Ta

Net Income Per Common Share (Tables)	3 Months Ended
	Mar. 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 Three Months Ended March 31, (in millions, except number of shares and per share amounts) 2018 2017 Net income $ 24.8 $ 36.7 Weighted average common shares outstanding - basic 33,868,749 34,554,341 Effect of dilutive securities: Equity compensation awards 871,594 1,028,475 Weighted average common shares outstanding - diluted 34,740,343 35,582,816 Net income per common share: Basic $ 0.73 $ 1.06 Diluted $ 0.71 $ 1.03

Supplemental Cash Flow Inform36

Supplemental Cash Flow Information (Tables)	3 Months Ended
	Mar. 31, 2018
Supplemental Cash Flow Elements [Abstract]
Schedule of Interest Paid and Income Taxes Paid (Recovered)	Interest paid and income taxes paid (recovered) were as follows: For the Three Months Ended March 31, (in millions) 2018 2017 Senior unsecured fixed rate notes $ 2.3 $ 2.3 Junior subordinated debentures 3.4 2.9 Other indebtedness 1.5 0.8 Revolving credit facility — 0.3 Total interest paid $ 7.2 $ 6.3 Income taxes paid 1.5 — Income taxes recovered — (2.2 ) Income taxes paid (recovered), net $ 1.5 $ (2.2 ) Total $ 8.7 $ 4.1

Share-based Compensation (Table

Share-based Compensation (Tables)	3 Months Ended
	Mar. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Fair Value Assumptions	The following table summarizes the assumptions we used for the three months ended March 31, 2018 and 2017: For the Three Months Ended March 31, 2018 2017 Risk-free rate of return 2.61% 1.93% Expected dividend yields 1.71% 1.63% Expected award life (years) 4.48 4.49 Expected volatility 18.44% 18.70%
Summary of Restricted Share Activity	A summary of restricted share activity as of March 31, 2018 and changes during the three months then ended is as follows: Shares Weighted-Average Grant Date Fair Value Outstanding at January 1, 2018 767,501 $ 42.70 Granted 214,048 $ 55.67 Vested and issued (137,834 ) $ 44.68 Expired or forfeited (81,171 ) $ 43.77 Outstanding at March 31, 2018 762,544 $ 45.87
Summary of Stock-Settled SARs Activity	A summary of stock-settled SARs activity as of March 31, 2018 and changes during the three months then ended is as follows: Shares Weighted-Average Exercise Price Outstanding at January 1, 2018 1,494,458 $ 30.85 Exercised (136,849 ) $ 27.45 Expired or forfeited (14,947 ) $ 36.25 Outstanding at March 31, 2018 1,342,662 $ 31.13
Summary of Cash-Settled SARs Activity	A summary of cash-settled SARs activity as of March 31, 2018 and changes during the three months then ended is as follows: Shares Weighted-Average Exercise Price Outstanding at January 1, 2018 189,235 $ 28.97 Exercised (42,432 ) $ 29.43 Expired or forfeited (2,235 ) $ 21.23 Outstanding at March 31, 2018 144,568 $ 28.95

Underwriting, Acquisition and38

Underwriting, Acquisition and Insurance Expenses (Tables)	3 Months Ended
	Mar. 31, 2018
Underwriting Acquisition And Insurance Expenses [Abstract]
Underwriting, Acquisition and Insurance Expenses	Underwriting, acquisition and insurance expenses were as follows: For the Three Months Ended March 31, (in millions) 2018 2017 Commissions $ 68.0 $ 61.2 General expenses 87.8 88.7 Premium taxes, boards and bureaus 9.3 7.4 165.1 157.3 Net deferral of policy acquisition costs (4.9 ) (3.7 ) Total underwriting, acquisition and insurance expenses $ 160.2 $ 153.6

Income Taxes (Tables)

Income Taxes (Tables)	3 Months Ended
	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of Pre-Tax Income (Loss) and Effective Income Tax Rates	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 three months ended March 31, 2018 and 2017, pre-tax income (loss) attributable to our operations and the operations’ effective tax rates were as follows: For the Three Months Ended March 31, (in millions) 2018 2017 Pre-Tax Income (Loss) Effective Tax Rate Pre-Tax Income (Loss) Effective Tax Rate Bermuda $ 25.3 0.0 % $ 26.5 0.0 % United States (6.3 ) 23.0 % 24.0 26.9 % United Kingdom 6.2 24.5 % (6.9 ) 6.7 % Belgium 0.1 35.8 % — (1) 0.0 % Brazil (1.4 ) 0.0 % (0.8 ) 0.0 % United Arab Emirates 0.2 0.0 % — (1) 0.0 % Ireland — (1) 0.0 % — (1) 0.0 % Italy — (1) 0.0 % N/A N/A Malta 0.9 0.0 % 0.6 0.0 % Luxembourg — (1) 0.0 % (0.7 ) 0.0 % Switzerland — 21.2 % — (1) 21.0 % Pre-tax income $ 25.0 0.7 % $ 42.7 14.1 % (1) Pre-tax income for the respective year was less than $0.1 million.
Reconciliation of Difference Between Provision for Income Taxes and Expected Tax Provision at Weighted Average Tax Rate	Our effective tax rate may vary significantly from period to period depending on the jurisdiction generating the pre-tax income (loss) and its corresponding statutory tax rate. The geographic distribution of pre-tax income (loss) can fluctuate significantly between periods given the inherit nature of our business. A reconciliation of the difference between the provision for income taxes and the expected tax provision at the weighted average tax rate is as follows: For the Three Months Ended March 31, (in millions) 2018 2017 Income tax (benefit) provision at expected rate $ (0.4 ) $ 7.2 Tax effect of: Nontaxable investment income (0.5 ) (1.3 ) Foreign exchange adjustments (0.2 ) 0.6 Withholding taxes 0.1 0.1 Change in valuation allowance (2.7 ) (0.1 ) Other 3.9 (0.5 ) Income tax provision $ 0.2 $ 6.0

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2018
Segment Reporting [Abstract]
Revenue and Income (Loss) Before Income Taxes for Each Segment	Revenue and income (loss) before income taxes for each segment were as follows: For the Three Months Ended March 31, (in millions) 2018 2017 Revenue: Earned premiums U.S. Operations $ 262.3 $ 221.2 International Operations 152.4 158.2 Run-off Lines — — Total earned premiums 414.7 379.4 Net investment income U.S. Operations 22.6 19.9 International Operations 8.8 6.6 Run-off Lines 2.2 2.1 Corporate and Other 2.4 1.9 Total net investment income 36.0 30.5 Fee and other income 2.0 3.6 Net realized investment (losses) gains (15.7 ) 14.6 Total revenue $ 437.0 $ 428.1
Schedule of Earned Premiums by Geographic Location	The table below presents earned premiums by geographic location for the three months ended March 31, 2018 and 2017. 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.
Identifiable Assets	The following table represents identifiable assets: (in millions) March 31, 2018 December 31, 2017 U.S. Operations $ 4,527.1 $ 4,298.4 International Operations 3,736.8 3,553.8 Run-off Lines 456.3 449.6 Corporate and Other 421.1 462.2 Total $ 9,141.3 $ 8,764.0

Senior Unsecured Fixed Rate N41

Senior Unsecured Fixed Rate Notes (Tables)	3 Months Ended
	Mar. 31, 2018
Debt Disclosure [Abstract]
Schedule Of Unamortized Debt Issuance Costs Deducted From Carrying Value Of Debt Liability	At March 31, 2018 and December 31, 2017, the Notes consisted of the following: (in millions) March 31, 2018 December 31, 2017 Senior unsecured fixed rate notes Principal $ 143.8 $ 143.8 Less: unamortized debt issuance costs (4.1 ) (4.2 ) Senior unsecured fixed rate notes, less unamortized debt issuance costs $ 139.7 $ 139.6
Condensed Consolidating Balance Sheet	CONDENSED CONSOLIDATING BALANCE SHEET MARCH 31, 2018 (in millions) (Unaudited) Argo Group International Holdings, Ltd (Parent Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Assets Investments $ 0.4 $ 3,085.9 $ 1,747.2 $ — $ 4,833.5 Cash 0.3 59.4 114.0 — 173.7 Accrued investment income — 18.0 6.3 — 24.3 Premiums receivable — 210.6 489.0 — 699.6 Reinsurance recoverables — 1,537.4 550.1 — 2,087.5 Goodwill and other intangible assets, net 42.9 124.7 101.1 — 268.7 Deferred acquisition costs, net — 81.1 82.8 — 163.9 Ceded unearned premiums — 211.0 290.9 — 501.9 Other assets 14.9 179.6 193.7 — 388.2 Intercompany note receivable — 51.5 (51.5 ) — — Investments in subsidiaries 1,939.4 — — (1,939.4 ) — Total assets $ 1,997.9 $ 5,559.2 $ 3,523.6 $ (1,939.4 ) $ 9,141.3 Liabilities and Shareholders' Equity Reserves for losses and loss adjustment expenses $ — $ 2,538.1 $ 1,745.5 $ — $ 4,283.6 Unearned premiums — 702.6 567.5 — 1,270.1 Funds held and ceded reinsurance payable, net — 863.1 49.8 — 912.9 Long-term debt 153.4 284.6 144.7 — 582.7 Current income taxes payable, net — 0.8 1.4 — 2.2 Deferred tax liabilities, net — 21.7 (0.4 ) — 21.3 Accrued underwriting expenses and other liabilities 6.3 90.6 184.2 — 281.1 Due to (from) affiliates 50.8 5.0 (5.0 ) (50.8 ) — Total liabilities 210.5 4,506.5 2,687.7 (50.8 ) 7,353.9 Total shareholders' equity 1,787.4 1,052.7 835.9 (1,888.6 ) 1,787.4 Total liabilities and shareholders' equity $ 1,997.9 $ 5,559.2 $ 3,523.6 $ (1,939.4 ) $ 9,141.3 (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 BALANCE SHEET DECEMBER 31, 2017 (in millions) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Assets Investments $ 0.4 $ 3,079.0 $ 1,663.5 $ — $ 4,742.9 Cash 0.9 47.8 127.9 — 176.6 Accrued investment income — 18.0 5.5 — 23.5 Premiums receivable — 216.5 382.1 — 598.6 Reinsurance recoverables — 1,487.3 606.0 — 2,093.3 Goodwill and other intangible assets, net 43.2 124.9 90.1 — 258.2 Current income taxes receivable, net — 2.4 (1.0 ) — 1.4 Deferred acquisition costs, net — 80.7 79.7 — 160.4 Ceded unearned premiums — 198.5 201.0 — 399.5 Other assets 9.2 171.5 128.9 — 309.6 Intercompany note receivable — 50.9 (50.9 ) — — Investments in subsidiaries 1,940.0 — — (1,940.0 ) — Total assets $ 1,993.7 $ 5,477.5 $ 3,232.8 $ (1,940.0 ) $ 8,764.0 Liabilities and Shareholders' Equity Reserves for losses and loss adjustment expenses $ — $ 2,483.9 $ 1,717.1 $ — $ 4,201.0 Unearned premiums — 704.0 503.7 — 1,207.7 Funds held and ceded reinsurance payable, net — 799.4 (22.7 ) — 776.7 Long-term debt 153.4 284.5 142.8 — 580.7 Deferred tax liabilities, net — 32.5 (1.2 ) — 31.3 Accrued underwriting expenses and other liabilities 8.9 95.0 43.0 — 146.9 Due to affiliates 11.7 (0.4 ) 0.4 (11.7 ) — Total liabilities 174.0 4,398.9 2,383.1 (11.7 ) 6,944.3 Total shareholders' equity 1,819.7 1,078.6 849.7 (1,928.3 ) 1,819.7 Total liabilities and shareholders' equity $ 1,993.7 $ 5,477.5 $ 3,232.8 $ (1,940.0 ) $ 8,764.0 (1) Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations. (2) Includes all Argo Group parent company eliminations.
Condensed Consolidating Statement of Income	CONDENSED CONSOLIDATING STATEMENT OF INCOME FOR THE THREE MONTHS ENDED MARCH 31, 2018 (in millions) (Unaudited) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Premiums and other revenue: Earned premiums $ — $ 156.9 $ 257.8 $ — $ 414.7 Net investment (expense) income (0.7 ) 22.1 14.6 — 36.0 Fee and other income — 0.8 1.2 — 2.0 Net realized investment (losses) gains (0.4 ) (15.4 ) 0.1 — (15.7 ) Total revenue (1.1 ) 164.4 273.7 — 437.0 Expenses: Losses and loss adjustment expenses — 95.4 141.8 — 237.2 Underwriting, acquisition and insurance expenses 2.4 69.8 88.0 — 160.2 Interest expense 1.3 4.3 2.1 — 7.7 Fee and other expense — 1.4 0.6 — 2.0 Foreign currency exchange loss — 0.1 4.8 — 4.9 Total expenses 3.7 171.0 237.3 — 412.0 (Loss) Income before income taxes (4.8 ) (6.6 ) 36.4 — 25.0 (Benefit) Provision for income taxes — (1.4 ) 1.6 — 0.2 Net (loss) income before equity in earnings of subsidiaries (4.8 ) (5.2 ) 34.8 — 24.8 Equity in undistributed earnings of subsidiaries 29.6 — — (29.6 ) — Net income $ 24.8 $ (5.2 ) $ 34.8 $ (29.6 ) $ 24.8 (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 THREE MONTHS ENDED MARCH 31, 2017 (in millions) (Unaudited) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Premiums and other revenue: Earned premiums $ — $ 130.9 $ 248.5 $ — $ 379.4 Net investment (expense) income (1.7 ) 18.9 13.3 — 30.5 Fee and other income — 2.8 0.8 — 3.6 Net realized investment gains 0.5 13.8 0.3 — 14.6 Total revenue (1.2 ) 166.4 262.9 — 428.1 Expenses: Losses and loss adjustment expenses — 76.1 146.4 — 222.5 Underwriting, acquisition and insurance expenses 8.4 58.2 87.0 — 153.6 Interest expense 0.3 4.3 1.3 — 5.9 Fee and other expense — 3.4 0.7 — 4.1 Foreign currency exchange loss (gains) — 0.1 (0.8 ) — (0.7 ) Total expenses 8.7 142.1 234.6 — 385.4 (Loss) Income before income taxes (9.9 ) 24.3 28.3 — 42.7 Provision (benefit) for income taxes — 6.4 (0.4 ) — 6.0 Net (loss) income before equity in earnings of subsidiaries (9.9 ) 17.9 28.7 — 36.7 Equity in undistributed earnings of subsidiaries 46.6 — — (46.6 ) — Net income $ 36.7 $ 17.9 $ 28.7 $ (46.6 ) $ 36.7 (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 THE THREE MONTHS ENDED MARCH 31, 2018 (in millions) (Unaudited) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Net cash flows from operating activities $ 27.8 $ 53.2 $ 72.6 $ — $ 153.6 Cash flows from investing activities: Proceeds from sales of investments — 325.0 230.4 — 555.4 Maturities and mandatory calls of fixed maturity investments — 113.1 31.7 — 144.8 Purchases of investments — (475.2 ) (212.3 ) — (687.5 ) Change in short-term investments and foreign regulatory deposits — (4.5 ) (156.5 ) — (161.0 ) Settlements of foreign currency exchange forward contracts (0.6 ) (0.7 ) 3.0 — 1.7 Cash included in acquisition of Ariscom — — 15.6 — 15.6 Purchases of fixed assets and other, net (0.1 ) 0.7 1.9 — 2.5 Cash used in investing activities (0.7 ) (41.6 ) (86.2 ) — (128.5 ) Cash flows from financing activities: Activity under stock incentive plans 0.2 — — — 0.2 Repurchase of Company's common shares (18.6 ) — — — (18.6 ) Payment of cash dividend to common shareholders (9.3 ) — — — (9.3 ) Cash used in financing activities (27.7 ) — — — (27.7 ) Effect of exchange rate changes on cash — — (0.3 ) — (0.3 ) Change in cash (0.6 ) 11.6 (13.9 ) — (2.9 ) Cash, beginning of year 0.9 47.8 127.9 — 176.6 Cash, end of period $ 0.3 $ 59.4 $ 114.0 $ — $ 173.7 (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 THE THREE MONTHS ENDED MARCH 31, 2017 (in millions) (Unaudited) Argo Group International Holdings, Ltd (Parent Guarantor) Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) Other Subsidiaries and Eliminations (1) Consolidating Adjustments (2) Total Net cash flows from operating activities $ (2.0 ) $ (14.7 ) $ (20.2 ) $ — $ (36.9 ) Cash flows from investing activities: Proceeds from sales of investments — 228.9 225.1 — 454.0 Maturities and mandatory calls of fixed maturity investments — 135.5 46.6 — 182.1 Purchases of investments — (446.5 ) (197.9 ) — (644.4 ) Change in short-term investments and foreign regulatory deposits 0.5 65.5 54.3 — 120.3 Settlements of foreign currency exchange forward contracts — — (2.8 ) — (2.8 ) Acquisition of Maybrooke, net of cash and cash equivalents acquired (235.3 ) 152.2 (83.1 ) Issuance of intercompany note, net — — (120.0 ) 120.0 — Purchases of fixed assets and other, net (0.1 ) 18.7 (19.9 ) — (1.3 ) Cash (used in) provided by investing activities (234.9 ) 2.1 137.6 $ 120.0 24.8 Cash flows from financing activities: Additional long-term borrowings 125.0 — — — 125.0 Borrowing under intercompany note, net 120.0 — — (120.0 ) — Activity under stock incentive plans 0.2 — — — 0.2 Payment of cash dividend to common shareholders (8.3 ) — — — (8.3 ) Cash provided by (used in) financing activities 236.9 — — (120.0 ) 116.9 Effect of exchange rate changes on cash — — (0.1 ) — (0.1 ) Change in cash — (12.6 ) 117.3 — 104.7 Cash, beginning of year — 53.7 32.3 — 86.0 Cash, end of period $ — $ 41.1 $ 149.6 $ — $ 190.7 (1) Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations. (2) Includes all Argo Group parent company eliminations.

Basis of Presentation - Additio

Basis of Presentation - Additional Information (Detail) - shares	Mar. 05, 2018	Feb. 20, 2018	Feb. 21, 2017	Mar. 31, 2018
Basis Of Presentation [Line Items]
Stock dividend declared				15.00%
Dividends declaration date		Feb. 20, 2018	Feb. 21, 2017
Dividends payable date		Mar. 23, 2018	Mar. 15, 2017
Dividend payable, date of record		Mar. 7, 2018	Mar. 3, 2017
Stock Dividend [Member]
Basis Of Presentation [Line Items]
Stock dividend declared		15.00%
Dividends declaration date		Feb. 20, 2018
Dividends payable date		Mar. 21, 2018
Dividend payable, date of record		Mar. 7, 2018
Additional stock issued as dividend		4,397,520
Ariscom [Member]
Basis Of Presentation [Line Items]
Effective date of acquisition	Mar. 5, 2018
Percentage of capital stock acquired	100.00%

Recently Issued Accounting Pr43

Recently Issued Accounting Pronouncements - Additional Information (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2018USD ($)
Accounting Policies [Abstract]
Cumulative effect of adoption, net of taxes	$ 20.9
Cumulative effect of adoption, net of taxes	$ 117.5

Acquisition of Ariscom - Additi

Acquisition of Ariscom - Additional Information (Detail) - Ariscom [Member]	Mar. 05, 2018
Business Acquisition [Line Items]
Business acquisition effective date	Mar. 5, 2018
Percentage of capital stock acquired	100.00%

Investments - Schedule of Amort

Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost	$ 4,721.1	$ 4,561.4
Investments Gross Unrealized Gains	175.3	213.7
Investments Gross Unrealized losses	62.9	32.2
Total investments	4,833.5	4,742.9	[1]
Amortized Cost	3,306.7	3,320.6	[1]
Fair Value	3,291.5	3,343.4	[1]
Amortized Cost	358.3	338.2	[1]
Fair Value	476.5	487.4	[1]
Fixed Maturities [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	3,306.7	3,320.6
Gross Unrealized Gains	33.8	50.1
Gross Unrealized Losses	49	27.3
Fair Value	3,291.5	3,343.4
Fixed Maturities [Member] | Collateralized Loan Obligations [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	192.3	222.6
Gross Unrealized Gains	4.8	5.9
Gross Unrealized Losses	0.3	0.5
Fair Value	196.8	228
Fixed Maturities [Member] | Residential Mortgage-backed Securities [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	341.5	309.3
Gross Unrealized Gains	2.3	2.8
Gross Unrealized Losses	8.1	2.7
Fair Value	335.7	309.4
Fixed Maturities [Member] | U.S. Governments [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	374.2	419.9
Gross Unrealized Gains	0.2	0.2
Gross Unrealized Losses	7.4	5
Fair Value	367	415.1
Fixed Maturities [Member] | Foreign Governments [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	233	229
Gross Unrealized Gains	3.9	6.7
Gross Unrealized Losses	3.5	2.5
Fair Value	233.4	233.2
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	315.9	327.7
Gross Unrealized Gains	6.6	9.3
Gross Unrealized Losses	2	1.1
Fair Value	320.5	335.9
Fixed Maturities [Member] | Corporate Bonds [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	1,580.7	1,514.5
Gross Unrealized Gains	15	24.4
Gross Unrealized Losses	24.1	13.2
Fair Value	1,571.6	1,525.7
Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	103.5	136.3
Gross Unrealized Gains	0	0.1
Gross Unrealized Losses	2.3	1.5
Fair Value	101.2	134.9
Fixed Maturities [Member] | Asset-backed Securities [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	165.6	161.3
Gross Unrealized Gains	1	0.7
Gross Unrealized Losses	1.3	0.8
Fair Value	165.3	161.2
Equity Securities [Member]
Schedule of Invested Assets [Line Items]
Amortized Cost	358.3	338.2
Gross Unrealized Gains	131.9	154
Gross Unrealized Losses	13.7	4.8
Fair Value	476.5	487.4
Other Investments [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost	513.7	534.1
Investments Gross Unrealized Gains	9.6	9.6
Investments Gross Unrealized losses	0	0.1
Total investments	523.3	543.6
Short-Term Investments [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost	542.4	368.5
Investments Gross Unrealized Gains	0	0
Investments Gross Unrealized losses	0.2	0
Total investments	$ 542.2	$ 368.5
[1]	Derived from audited consolidated financial statements.

Investments - Additional Inform

Investments - Additional Information (Detail)	Mar. 31, 2018USD ($)Security	Dec. 31, 2017USD ($)
Amortized Cost And Fair Value Debt Securities [Abstract]
Fair value of investments, assets managed on behalf of the trade capital providers | $	$ 120,600,000	$ 130,800,000
Number of securities in an unrealized loss position, total | Security	8,233
Number of securities in an unrealized loss position for less than one year | Security	2,901
Number of securities in an unrealized loss position for a period of one year or greater | Security	121
Transfers between Level 1 and Level 2 securities | $	$ 0
Transfers between Level 2 and Level 1 securities | $	$ 0

Investments - Schedule of Amo47

Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) $ in Millions	Mar. 31, 2018USD ($)
Investments Debt And Equity Securities [Abstract]
Fair Value, Due in one year or less	$ 267.7
Fair Value, Due after one year through five years	1,460.2
Fair Value, Due after five years through ten years	595.6
Fair Value, Thereafter	169
Fair Value, Structured securities	799
Fair Value, Total	3,291.5
Amortized Cost, Due in one year or less	268.5
Amortized Cost, Due after one year through five years	1,467
Amortized Cost, Due after five years through ten years	600.4
Amortized Cost, Thereafter	167.9
Amortized Cost, Structured securities	802.9
Amortized Cost, Total	$ 3,306.7

Investments - Schedule of Carry

Investments - Schedule of Carrying Value and Unfunded Investment Commitments of Other Invested Assets Portfolio (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Carrying Value	$ 4,833.5	$ 4,742.9	[1]
Hedge funds [Member]
Schedule of Investments [Line Items]
Carrying Value	138.8	163.6
Unfunded Commitments	0	0
Private equity [Member]
Schedule of Investments [Line Items]
Carrying Value	186.5	179.2
Unfunded Commitments	123.8	129.9
Long only funds [Member]
Schedule of Investments [Line Items]
Carrying Value	193.7	196.5
Unfunded Commitments	0	0
Other Investments [Member]
Schedule of Investments [Line Items]
Carrying Value	4.3	4.3
Unfunded Commitments	0	0
Other Invested Assets [Member]
Schedule of Investments [Line Items]
Carrying Value	523.3	543.6
Unfunded Commitments	$ 123.8	$ 129.9
[1]	Derived from audited consolidated financial statements.

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		Mar. 31, 2018		Dec. 31, 2017
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		$ 2,239		$ 1,585.8
Unrealized Losses, Less Than One Year		43.9		22
Fair Value, One Year or Greater		108.5		343
Unrealized Losses, One Year or Greater		5.3		10.2
Fair Value, Total		2,347.5		1,928.8
Unrealized Losses, Total		49.2		32.2
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		2,180.6		1,538.9
Unrealized Losses, Less Than One Year		43.7		17.2
Fair Value, One Year or Greater		108.2		342.7
Unrealized Losses, One Year or Greater		5.3		10.1
Fair Value, Total		2,288.8		1,881.6
Unrealized Losses, Total		49		27.3
Fixed Maturities [Member] | Collateralized Loan Obligations [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	[1]	41		34.6
Unrealized Losses, Less Than One Year	[1]	0.3		0.5
Fair Value, One Year or Greater	[1]	0.9		0.9
Unrealized Losses, One Year or Greater	[1]	0		0
Fair Value, Total	[1]	41.9		35.5
Unrealized Losses, Total	[1]	0.3		0.5
Fixed Maturities [Member] | Residential Mortgage-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		263.9		164.4
Unrealized Losses, Less Than One Year		7.5		1.6
Fair Value, One Year or Greater		14.4		52.4
Unrealized Losses, One Year or Greater		0.6		1.1
Fair Value, Total		278.3		216.8
Unrealized Losses, Total		8.1		2.7
Fixed Maturities [Member] | U.S. Governments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		316.8		313.7
Unrealized Losses, Less Than One Year		6.5		1.9
Fair Value, One Year or Greater		17.3		83.7
Unrealized Losses, One Year or Greater		0.9		3.1
Fair Value, Total		334.1		397.4
Unrealized Losses, Total		7.4		5
Fixed Maturities [Member] | Foreign Governments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		207.2		175.2
Unrealized Losses, Less Than One Year		3.4		2
Fair Value, One Year or Greater		5		35.9
Unrealized Losses, One Year or Greater		0.1		0.5
Fair Value, Total		212.2		211.1
Unrealized Losses, Total		3.5		2.5
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		92.1		33.3
Unrealized Losses, Less Than One Year		1.4		0.5
Fair Value, One Year or Greater		14.7		22.4
Unrealized Losses, One Year or Greater		0.6		0.6
Fair Value, Total		106.8		55.7
Unrealized Losses, Total		2		1.1
Fixed Maturities [Member] | Corporate Bonds [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		1,078.7		674.1
Unrealized Losses, Less Than One Year		22.2		9.9
Fair Value, One Year or Greater		32		77.7
Unrealized Losses, One Year or Greater		1.9		3.3
Fair Value, Total		1,110.7		751.8
Unrealized Losses, Total		24.1		13.2
Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		69.6		58.2
Unrealized Losses, Less Than One Year		1.4		0.4
Fair Value, One Year or Greater		16		37.8
Unrealized Losses, One Year or Greater		0.9		1.1
Fair Value, Total		85.6		96
Unrealized Losses, Total		2.3		1.5
Fixed Maturities [Member] | Asset-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		111.3		85.4
Unrealized Losses, Less Than One Year		1		0.4
Fair Value, One Year or Greater		7.9		31.9
Unrealized Losses, One Year or Greater		0.3		0.4
Fair Value, Total		119.2		117.3
Unrealized Losses, Total		1.3		0.8
Other Investments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		0	[1]	0
Unrealized Losses, Less Than One Year		0	[1]	0
Fair Value, One Year or Greater		0.3	[1]	0.3
Unrealized Losses, One Year or Greater		0	[1]	0.1
Fair Value, Total		0.3	[1]	0.3
Unrealized Losses, Total		0	[1]	0.1
Short-Term Investments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year		58.4
Unrealized Losses, Less Than One Year		0.2
Fair Value, One Year or Greater		0
Unrealized Losses, One Year or Greater		0
Fair Value, Total		58.4
Unrealized Losses, Total		$ 0.2
Equity Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year				46.9
Unrealized Losses, Less Than One Year				4.8
Fair Value, One Year or Greater				0
Unrealized Losses, One Year or Greater				0
Fair Value, Total				46.9
Unrealized Losses, Total				$ 4.8
[1]	Unrealized losses one year or greater are less than $0.1 million.

Investments - Schedule of Agi50

Investments - Schedule of Aging of Unrealized Losses on Company's Investments in Fixed Maturities, Equity Securities and Other Investments (Parenthetical) (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Unrealized losses one year or greater	$ 0.1	$ 0.1

Investments - Schedule of Recog

Investments - Schedule of Recognized Other-than-temporary Losses on Fixed Maturities and Equity Portfolios (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Other-than-temporary impairment:
Other-than-temporary impairment losses	$ (1)	$ (0.4)
Corporate Bonds [Member]
Other-than-temporary impairment:
Other-than-temporary impairment losses	(1)	0
Equity Securities [Member]
Other-than-temporary impairment:
Other-than-temporary impairment losses	$ 0	$ (0.4)

Investments - Schedule of Compa

Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Schedule of Investments [Line Items]
Gross realized investment on fixed maturities and other gains		$ 19.2	$ 29
Gross realized investment on fixed maturities and other losses		(15.2)	(14.4)
Net realized gains on equity securities sold during the period	[1]	11.2	0
Change in unrealized (losses) on equity securities held at the end of the period	[1]	(30.9)	0
Net realized (losses) on equity securities	[1]	(19.7)	0
Net realized investment (losses) gains before income taxes		(15.7)	14.6
Income tax benefit (expense)		2.9	(4.3)
Net realized investment (losses) gains, net of income taxes		(12.8)	10.3
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Gross realized investment on fixed maturities and other gains		7.5	7.1
Gross realized investment on fixed maturities and other losses		(4.9)	(6.2)
Equity Securities [Member]
Schedule of Investments [Line Items]
Gross realized investment on fixed maturities and other gains	[1]	0	15.7
Gross realized investment on fixed maturities and other losses	[1]	0	(0.7)
Other Investments [Member]
Schedule of Investments [Line Items]
Gross realized investment on fixed maturities and other gains		11.7	6.2
Gross realized investment on fixed maturities and other losses		(9.3)	(7.1)
Other-Than-Temporary Impairment Losses on Fixed Maturities [Member]
Schedule of Investments [Line Items]
Gross realized investment on fixed maturities and other losses		(1)	0
Other-Than-Temporary Impairment Losses on Equity Securities [Member]
Schedule of Investments [Line Items]
Gross realized investment on fixed maturities and other losses		$ 0	$ (0.4)
[1]	Effective January 1, 2018, we adopted ASU 2016-01. 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.

Investments - Schedule of Chang

Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	$ (37.2)	$ 17.9
Income tax benefit (expense)	6.3	(4.6)
Net unrealized investment (losses) gains, net of income taxes	(30.9)	13.3
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	(37.1)	12.6
Equity Securities [Member]
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	0	5.3
Short-Term Investments [Member]
Schedule of Investments [Line Items]
Net unrealized investment (losses) gains before income taxes	$ (0.1)	$ 0

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	Mar. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Fair value of foreign currency exchange forward contracts, Asset	$ 2.1
Fair value of foreign currency exchange forward contracts, Liability	0	$ (0.4)
Operational Currency Exposure [Member]
Schedule of Investments [Line Items]
Fair value of foreign currency exchange forward contracts, Asset	3.4
Fair value of foreign currency exchange forward contracts, Liability	0	(0.2)
Asset Manager Investment Exposure [Member]
Schedule of Investments [Line Items]
Fair value of foreign currency exchange forward contracts, Liability	(1.8)	(0.9)
Total return strategy [Member]
Schedule of Investments [Line Items]
Fair value of foreign currency exchange forward contracts, Asset	$ 0.5	$ 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	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Schedule of Investments [Line Items]
Realized gains from foreign currency exchange forward contracts	$ 11.2	$ 5.6
Realized losses from foreign currency exchange forward contracts	(8.6)	(5.5)
Net realized investment gains on foreign currency exchange forward contracts	2.6	0.1
Operational Currency Exposure [Member]
Schedule of Investments [Line Items]
Realized gains from foreign currency exchange forward contracts	4.3	2
Realized losses from foreign currency exchange forward contracts	(0.6)	(2.2)
Asset Manager Investment Exposure [Member]
Schedule of Investments [Line Items]
Realized gains from foreign currency exchange forward contracts	4.3	0.5
Realized losses from foreign currency exchange forward contracts	(5.9)	(1.4)
Total return strategy [Member]
Schedule of Investments [Line Items]
Realized gains from foreign currency exchange forward contracts	2.6	3.1
Realized losses from foreign currency exchange forward contracts	$ (2.1)	$ (1.9)

Investments - Components of Res

Investments - Components of Restricted Assets (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2016
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Securities and cash on deposit supporting Lloyd’s business	$ 373.9	$ 396.8
Total restricted investments	608.4	618.3
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	171.7	183.9
Securities Pledged as Collateral [Member]
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Securities pledged as collateral for letters of credit	$ 62.8	$ 37.6

Investments - Financial Assets

Investments - Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		$ 4,833.5	$ 4,742.9	[1]
Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		523.3	543.6
Short-Term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		542.2	368.5
Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		4,414	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	[2]	1,295.9	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]	3,113.9	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	[4]	4.2	4.2
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		3,291.5	3,343.4
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		196.8	228
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		335.7	309.4
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	[2]	359.3	410.6
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[2]	0	0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Residential Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[2]	0	0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[3]	2,930.3	2,930.9
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[3]	196.8	228
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[3]	335.7	309.4
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	[4]	1.9	1.9
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[4]	0	0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[4]	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		367	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	[2]	359.3	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	[3]	7.7	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	[4]	0	0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Foreign Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		233.4	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	[2]	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	[3]	233.4	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	[4]	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		320.5	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	[2]	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	[3]	320.5	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	[4]	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		1,571.6	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	[2]	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	[3]	1,569.7	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	[4]	1.9	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		101.2	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	[2]	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	[3]	101.2	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	[4]	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		165.3	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	[2]	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	[3]	165.3	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	[4]	0	0
Fair Value, Measurements, Recurring [Member] | Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		103.8	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	[2]	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	[3]	103.8	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	[4]	0	0
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		476.5	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	[2]	472.1	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	[3]	2.1	2.1
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	[4]	2.3	2.3
Fair Value, Measurements, Recurring [Member] | Short-Term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		542.2	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	[2]	464.5	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	[3]	77.7	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	[4]	$ 0	$ 0
[1]	Derived from audited consolidated financial statements.
[2]	Quoted prices in active markets for identical assets
[3]	Significant other observable inputs
[4]	Significant unobservable inputs

Investments - Schedule of Recon

Investments - Schedule of Reconciliation of Beginning and Ending Balances for Investments Categorized as Level 3 (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 4.2	$ 2.4
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Included in net income (loss)	0	0
Included in other comprehensive income (loss)	0	0.1
Purchases	0	1.7
Issuances	0	0
Sales	0	0
Settlements	0	0
Ending balance	4.2	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 [Line Items]
Beginning balance	1.9	2
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Included in net income (loss)	0	0
Included in other comprehensive income (loss)	0	(0.1)
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Ending balance	1.9	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 [Line Items]
Beginning balance	2.3	0.4
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Included in net income (loss)	0	0
Included in other comprehensive income (loss)	0	0.2
Purchases	0	1.7
Issuances	0	0
Sales	0	0
Settlements	0	0
Ending balance	2.3	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

Reserves for Losses and Loss 59

Reserves for Losses and Loss Adjustment Expenses - Reserves for Losses and Loss Adjustment Expenses (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	[2]
Liability for Claims and Claims Adjustment Expense [Abstract]
Net reserves beginning of the year		$ 2,488	$ 2,180.2
Losses and LAE incurred during current calendar year, net of reinsurance:
Current accident year		239.2	215.7
Prior accident years		(2)	6.8
Losses and LAE incurred during calendar year, net of reinsurance		237.2	222.5
Current accident year		33.3	27.4
Prior accident years		186.6	155.3
Losses and LAE payments made during current calendar year, net of reinsurance:		219.9	182.7
Change in participation interest	[1]	(16.9)	(23.2)
Foreign exchange adjustments		(0.8)	3.6
Net reserves - end of period		2,534.8	2,333
Reinsurance recoverables on unpaid losses and LAE, end of period		1,748.8	1,247.3
Reserves for losses and loss adjustment expenses		4,283.6	3,580.3	$ 4,201
Maybrooke [Member]
Liability for Claims and Claims Adjustment Expense [Abstract]
Net reserves acquired		0	132.6
Ariscom [Member]
Liability for Claims and Claims Adjustment Expense [Abstract]
Net reserves acquired		$ 47.2	$ 0
[1]	Amount represents the change in reserves due to changing our participation in Syndicates 1200 and 1910.
[2]	Derived from audited consolidated financial statements.

Reserves for Losses and Loss 60

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	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	$ (2)	$ 6.8
U.S. Operations [Member]
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	(1)	(5.2)
International Operations [Member]
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	(2.8)	9.6
Run Off Lines [Member]
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	$ 1.8	$ 2.4

Junior Subordinated Debenture61

Junior Subordinated Debentures - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Junior subordinated debentures	$ 256.6	$ 256.6	[1]
Maybrooke Junior Subordinated Debentures [Member]
Debt Instrument [Line Items]
Junior subordinated debentures	91.8
Junior subordinated debentures, Amount	83.9	83.9
Trust Preferred Debentures [Member]
Debt Instrument [Line Items]
Junior subordinated debentures	$ 172.7	$ 172.7
Trust Preferred Debentures [Member] | Junior Subordinated Debentures [Member]
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage	100.00%	100.00%
[1]	Derived from audited consolidated financial statements.

Junior Subordinated Debenture62

Junior Subordinated Debentures - Schedule of Junior Subordinated Debentures (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Junior subordinated debentures, Amount	$ 256.6	$ 256.6	[1]
Trust Preferred Debentures [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Amount	$ 172.7	$ 172.7
Trust Preferred Debentures [Member] | PXRE Capital Statutory Trust II [Member] | Argo Group [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	May 15, 2003	May 15, 2003
Junior subordinated debentures, Maturity date	May 15, 2033	May 15, 2033
Junior subordinated debentures, Rate Structure	2018 and 2017: 3M LIBOR + 4.10%
Junior subordinated debentures, interest rate stated percentage	5.94%	5.52%
Junior subordinated debentures, Amount	$ 18.1	$ 18.1
Trust Preferred Debentures [Member] | PXRE Capital Trust VI [Member] | Argo Group [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	Nov. 6, 2003	Nov. 6, 2003
Junior subordinated debentures, Maturity date	Sep. 30, 2033	Sep. 30, 2033
Junior subordinated debentures, Rate Structure	2018 and 2017: 3M LIBOR + 3.90%
Junior subordinated debentures, interest rate stated percentage	6.21%	5.59%
Junior subordinated debentures, Amount	$ 10.3	$ 10.3
Trust Preferred Debentures [Member] | Argonaut Group Statutory Trust I [Member] | Argo Group US [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	May 15, 2003	May 15, 2003
Junior subordinated debentures, Maturity date	May 15, 2033	May 15, 2033
Junior subordinated debentures, Rate Structure	2018 and 2017: 3M LIBOR + 4.10%
Junior subordinated debentures, interest rate stated percentage	5.94%	5.52%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust Preferred Debentures [Member] | Argonaut Group Statutory Trust III [Member] | Argo Group US [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	Dec. 16, 2003	Dec. 16, 2003
Junior subordinated debentures, Maturity date	Jan. 8, 2034	Jan. 8, 2034
Junior subordinated debentures, Rate Structure	2018 and 2017: 3M LIBOR + 4.10%
Junior subordinated debentures, interest rate stated percentage	5.82%	5.46%
Junior subordinated debentures, Amount	$ 12.3	$ 12.3
Trust Preferred Debentures [Member] | Argonaut Group Statutory Trust IV [Member] | Argo Group US [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	Apr. 29, 2004	Apr. 29, 2004
Junior subordinated debentures, Maturity date	Apr. 29, 2034	Apr. 29, 2034
Junior subordinated debentures, Rate Structure	2018 and 2017: 3M LIBOR + 3.85%
Junior subordinated debentures, interest rate stated percentage	5.69%	5.27%
Junior subordinated debentures, Amount	$ 13.4	$ 13.4
Trust Preferred Debentures [Member] | Argonaut Group Statutory Trust V [Member] | Argo Group US [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	May 26, 2004	May 26, 2004
Junior subordinated debentures, Maturity date	May 24, 2034	May 24, 2034
Junior subordinated debentures, Rate Structure	2018 and 2017: 3M LIBOR + 3.85%
Junior subordinated debentures, interest rate stated percentage	5.79%	5.30%
Junior subordinated debentures, Amount	$ 12.3	$ 12.3
Trust Preferred Debentures [Member] | Argonaut Group Statutory Trust VI [Member] | Argo Group US [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	May 12, 2004	May 12, 2004
Junior subordinated debentures, Maturity date	May 12, 2034	May 12, 2034
Junior subordinated debentures, Rate Structure	2018 and 2017: 3M LIBOR + 3.80%
Junior subordinated debentures, interest rate stated percentage	5.98%	5.40%
Junior subordinated debentures, Amount	$ 13.4	$ 13.4
Trust Preferred Debentures [Member] | Argonaut Group Statutory Trust VII [Member] | Argo Group US [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	Sep. 17, 2004	Sep. 17, 2004
Junior subordinated debentures, Maturity date	Dec. 15, 2034	Dec. 15, 2034
Junior subordinated debentures, Rate Structure	2018 and 2017: 3M LIBOR + 3.60%
Junior subordinated debentures, interest rate stated percentage	5.72%	5.19%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust Preferred Debentures [Member] | Argonaut Group Statutory Trust VIII [Member] | Argo Group US [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	Sep. 22, 2004	Sep. 22, 2004
Junior subordinated debentures, Maturity date	Sep. 22, 2034	Sep. 22, 2034
Junior subordinated debentures, Rate Structure	2018 and 2017: 3M LIBOR + 3.55%
Junior subordinated debentures, interest rate stated percentage	5.80%	5.21%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust Preferred Debentures [Member] | Argonaut Group Statutory Trust IX [Member] | Argo Group US [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	Oct. 22, 2004	Oct. 22, 2004
Junior subordinated debentures, Maturity date	Dec. 15, 2034	Dec. 15, 2034
Junior subordinated debentures, Rate Structure	2018 and 2017: 3M LIBOR + 3.60%
Junior subordinated debentures, interest rate stated percentage	5.72%	5.19%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust Preferred Debentures [Member] | Argonaut Group Statutory Trust X [Member] | Argo Group US [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Issue date	Sep. 15, 2005	Sep. 15, 2005
Junior subordinated debentures, Maturity date	Sep. 15, 2035	Sep. 15, 2035
Junior subordinated debentures, Rate Structure	2018 and 2017: 3M LIBOR + 3.40%
Junior subordinated debentures, interest rate stated percentage	5.52%	4.99%
Junior subordinated debentures, Amount	$ 30.9	$ 30.9
[1]	Derived from audited consolidated financial statements.

Junior Subordinated Debenture63

Junior Subordinated Debentures - Schedule of Maybrooke Junior Subordinated Debentures (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Principal	$ 143.8	$ 143.8
Maybrooke Junior Subordinated Debentures [Member]
Debt Instrument [Line Items]
Issue Date	Sep. 15, 2007	Sep. 15, 2007
Maturity	Sep. 15, 2037	Sep. 15, 2037
Rate Structure	3M LIBOR + 3.15%
Interest Rate	5.27%	4.74%
Principal	$ 91.8	$ 91.8
Carrying Value	$ 83.9	$ 83.9

Other Indebtedness - Schedule o

Other Indebtedness - Schedule of Terms and Principal Amounts of Each Debt Instrument (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Other indebtedness	$ 186.4	$ 184.5	[1]
Term Loan [Member]
Debt Instrument [Line Items]
Other indebtedness	125	125
Floating Rate Loan Stock [Member]
Debt Instrument [Line Items]
Other indebtedness	60.8	58.9
Other Debt [Member]
Debt Instrument [Line Items]
Other indebtedness	$ 0.6	$ 0.6
[1]	Derived from audited consolidated financial statements.

Other Indebtedness - Additional

Other Indebtedness - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended	12 Months Ended
	Jan. 31, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 03, 2017	Mar. 07, 2014
Debt Instrument [Line Items]
Floating rate loan stock denominated in Euros		$ 44,300,000	$ 42,400,000
New Credit Agreement [Member]
Debt Instrument [Line Items]
Junior subordinated debentures, Maturity date		Mar. 3, 2019	Mar. 3, 2019
New Credit Agreement [Member] | J P Morgan Chase Bank [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing amount				$ 325,000,000
New Credit Agreement [Member] | Revolving Credit Facility [Member] | J P Morgan Chase Bank [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing amount				200,000,000
Revolving credit facility maturity date		Mar. 3, 2022
Letter of credit facility amount outstanding		$ 0	$ 0
New Credit Agreement [Member] | Term Loan [Member] | J P Morgan Chase Bank [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing amount				125,000,000
New Credit Agreement [Member] | Letter Of Credit [Member] | J P Morgan Chase Bank [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing amount		200,000,000
Letter of credit facility amount		$ 500,000		$ 200,000
Prior Agreement [Member] | Revolving Credit Facility [Member] | Maybrooke [Member]
Debt Instrument [Line Items]
Repayments of borrowings	$ 125,000,000
Prior Agreement [Member] | Revolving Credit Facility [Member] | J P Morgan Chase Bank [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing amount					$ 175,000,000
Prior Agreement [Member] | Letter Of Credit [Member] | J P Morgan Chase Bank [Member]
Debt Instrument [Line Items]
Letter of credit facility amount			$ 500,000
Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage		100.00%	100.00%
Other Debt - Note Payable [Member]
Debt Instrument [Line Items]
Note payable as part of ARIS acquisition		$ 600,000	$ 600,000
Junior subordinated debentures, Maturity date		Apr. 1, 2019
Debt instrument, interest rate terms		The note had a variable interest rate of 2.00% above 30-day LIBOR, with the variable interest rate being reset quarterly and subject to certain interest rate ceilings. Interest payments are payable quarterly.
Other Debt - Note Payable [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Variable interest rate of note payable, above 30-day LIBOR		2.00%

Other Indebtedness - Schedule66

Other Indebtedness - Schedule of Floating Rate Loan Stock, Notes Outstanding (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Amount	$ 60.8	$ 58.9
Other indebtedness, Amount	$ 186.4	$ 184.5	[1]
Floating Rate Loan Stock, U.S. Dollar, Start Year 2004 [Member]
Debt Instrument [Line Items]
Issue Date	Dec. 8, 2004	Dec. 8, 2004
Maturity	Nov. 15, 2034	Nov. 15, 2034
Rate Structure	First Quarter of 2018 and 2017: 6 month LIBOR + 4.2%
Interest Rate	6.09%	5.66%
Amount	$ 6.5	$ 6.5
Floating Rate Loan Stock, U.S. Dollar, Start Year 2006 [Member]
Debt Instrument [Line Items]
Issue Date	Oct. 31, 2006	Oct. 31, 2006
Maturity	Jan. 15, 2036	Jan. 15, 2036
Rate Structure	First Quarter of 2018 and 2017: 6 month LIBOR + 4.0%
Interest Rate	5.89%	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]
Issue Date	Sep. 6, 2005	Sep. 6, 2005
Maturity	Aug. 22, 2035	Aug. 22, 2035
Rate Structure	First Quarter of 2018 and 2017: 3 month LIBOR + 4.0%
Interest Rate	3.67%	3.67%
Amount	$ 14.8	$ 14.1
Floating Rate Loan Stock, Euro, Start Year 2006 [Member]
Debt Instrument [Line Items]
Issue Date	Oct. 31, 2006	Oct. 31, 2006
Maturity	Nov. 22, 2036	Nov. 22, 2036
Rate Structure	First Quarter of 2018 and 2017: 3 month LIBOR + 4.0%
Interest Rate	3.67%	3.67%
Amount	$ 12.9	$ 12.4
Floating Rate Loan Stock, Euro, Start Year 2007 [Member]
Debt Instrument [Line Items]
Issue Date	Jun. 8, 2007	Jun. 8, 2007
Maturity	Sep. 15, 2037	Sep. 15, 2037
Rate Structure	First Quarter of 2018 and 2017: 3 month LIBOR + 3.9%
Interest Rate	3.57%	3.57%
Amount	$ 16.6	$ 15.9
Floating Rate Loan Stock, Euro [Member]
Debt Instrument [Line Items]
Amount	$ 44.3	$ 42.4
New Credit Agreement [Member]
Debt Instrument [Line Items]
Issue Date	Mar. 3, 2017	Mar. 3, 2017
Maturity	Mar. 3, 2019	Mar. 3, 2019
Rate Structure	First Quarter of 2018 and 2017: 2 month LIBOR + 1.5%
Interest Rate	3.08%	2.94%
Other indebtedness, Amount	$ 125	$ 125
[1]	Derived from audited consolidated financial statements.

Disclosures about Fair Value 67

Disclosures about Fair Value of Financial Instruments - Summary of Company's Financial Instruments Whose Carrying Amount Did Not Equal Fair Value (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Junior Subordinated Debentures [Member] | Trust Preferred Debentures [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	$ 172.7	$ 172.7
Junior Subordinated Debentures [Member] | Trust Preferred Debentures [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	172.9	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	83.9	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	85.5	85
Senior Unsecured Fixed Rate Notes [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	139.7	139.6
Senior Unsecured Fixed Rate Notes [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	141.1	141.2
Floating Rate Loan Stock [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	60.8	58.9
Floating Rate Loan Stock [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	$ 60.9	$ 59

Shareholders Equity - Additiona

Shareholders Equity - Additional Information (Detail) - USD ($)	Feb. 20, 2018	Mar. 15, 2017	Feb. 21, 2017	Mar. 31, 2018	Mar. 31, 2017	May 03, 2016
Class Of Stock [Line Items]
Dividends declaration date	Feb. 20, 2018		Feb. 21, 2017
Stock dividend declared				15.00%
Dividends payable date	Mar. 23, 2018		Mar. 15, 2017
Dividend payable, date of record	Mar. 7, 2018		Mar. 3, 2017
Dividend declared per common share	$ 0.27		$ 0.23	$ 0.27	$ 0.23
Cash dividends paid - common shares, total	$ 9,300,000	$ 8,300,000		$ 9,300,000	$ 8,300,000
Common shares repurchased				314,586
Common shares repurchased on open market, total cost				$ 18,600,000
2016 Repurchase Authorization [Member]
Class Of Stock [Line Items]
Remaining number of shares available under repurchase authorization, Value				$ 66,400,000
Common shares repurchased				314,586
Common shares repurchased on open market, total cost				$ 18,600,000
2016 Repurchase Authorization [Member] | Maximum [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]
Dividends declaration date	Feb. 20, 2018
Stock dividend declared	15.00%
Dividends payable date	Mar. 21, 2018
Dividend payable, date of record	Mar. 7, 2018
Additional stock issued as dividend	4,397,520

Shareholders Equity - Schedule

Shareholders Equity - Schedule of Repurchase of Shares (Detail) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018USD ($)$ / sharesshares
Equity Class Of Treasury Stock [Line Items]
Common shares repurchased on open market | shares	314,586
Average Price of Shares Repurchased | $ / shares	$ 59.23
Total Cost (in millions) | $	$ 18.6
10b5-1 Trading Plan Initiated In 12/15/2017 Member]
Equity Class Of Treasury Stock [Line Items]
Date Trading Plan Initiated	Dec. 15, 2017
Trading Plan Purchase Period Start Date	Jan. 2, 2018
Common shares repurchased on open market | shares	225,281
Average Price of Shares Repurchased | $ / shares	$ 59.31
Total Cost (in millions) | $	$ 13.3
Repurchase Authorization Year	2,016
Trading Plan Purchase Period End Date	Feb. 16, 2018
Open Market Repurchase Authorization Two Thousand And Sixteen [Member]
Equity Class Of Treasury Stock [Line Items]
Trading Plan Purchase Period Start Date	Feb. 20, 2018
Common shares repurchased on open market | shares	89,305
Average Price of Shares Repurchased | $ / shares	$ 59.03
Total Cost (in millions) | $	$ 5.3
Repurchase Authorization Year	2,016
Trading Plan Purchase Period End Date	Mar. 2, 2018

Accumulated Other Comprehensi70

Accumulated Other Comprehensive (Loss) Income - Changes in Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018		Mar. 31, 2017
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Beginning Balance	$ 96.6	[1]	$ 47.7
Other comprehensive income (loss) before reclassifications	(21.8)		24.9
Amounts reclassified from accumulated other comprehensive income (loss)	(10.2)		(11)
Net current-period other comprehensive income (loss)	(128.6)		13.9
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Ending Balance	(32)		61.6
ASU 2016-01 [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Cumulative effect of adoption	(117.5)
ASU 2018-02 [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Cumulative effect of adoption	20.9
Foreign Currency Translation Adjustments [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Beginning Balance	(19)		(17.6)
Other comprehensive income (loss) before reclassifications	(1.1)		0.6
Amounts reclassified from accumulated other comprehensive income (loss)	0		0
Net current-period other comprehensive income (loss)	(1.1)		0.6
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Ending Balance	(20.1)		(17)
Foreign Currency Translation Adjustments [Member] | ASU 2016-01 [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Cumulative effect of adoption	0
Foreign Currency Translation Adjustments [Member] | ASU 2018-02 [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Cumulative effect of adoption	0
Unrealized Holding Gains on Securities [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Beginning Balance	121.9		72.4
Other comprehensive income (loss) before reclassifications	(20.7)		24.3
Amounts reclassified from accumulated other comprehensive income (loss)	(10.2)		(11)
Net current-period other comprehensive income (loss)	(127.5)		13.3
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Ending Balance	(5.6)		85.7
Unrealized Holding Gains on Securities [Member] | ASU 2016-01 [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Cumulative effect of adoption	(117.5)
Unrealized Holding Gains on Securities [Member] | ASU 2018-02 [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Cumulative effect of adoption	20.9
Defined Benefit Pension Plans [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Beginning Balance	(6.3)		(7.1)
Other comprehensive income (loss) before reclassifications	0		0
Amounts reclassified from accumulated other comprehensive income (loss)	0		0
Net current-period other comprehensive income (loss)	0		0
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Ending Balance	(6.3)		$ (7.1)
Defined Benefit Pension Plans [Member] | ASU 2016-01 [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Cumulative effect of adoption	0
Defined Benefit Pension Plans [Member] | ASU 2018-02 [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Cumulative effect of adoption	$ 0
[1]	Derived from audited consolidated financial statements.

Accumulated Other Comprehensi71

Accumulated Other Comprehensive (Loss) Income - Amounts Reclassified from Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Unrealized gains and losses on securities:
Reclassification adjustment for gains included in net income	$ (12.5)	$ (16.2)
Provision for income taxes	2.3	5.2
Net of taxes	(10.2)	(11)
Unrealized Gains and Losses on Securities [Member]
Unrealized gains and losses on securities:
Net of taxes	(10.2)	(11)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains and Losses on Securities [Member]
Unrealized gains and losses on securities:
Reclassification adjustment for gains included in net income	(12.5)	(16.2)
Provision for income taxes	2.3	5.2
Net of taxes	$ (10.2)	$ (11)

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
	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Net income	$ 24.8	$ 36.7
Weighted average common shares outstanding - basic	33,868,749	34,554,341
Effect of dilutive securities:
Equity compensation awards	871,594	1,028,475
Weighted average common shares outstanding - diluted	34,740,343	35,582,816
Net income per common share - basic	$ 0.73	$ 1.06
Net income per common share - diluted	$ 0.71	$ 1.03

Net Income Per Common Share - A

Net Income Per Common Share - Additional Information (Detail) - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	[1]
Earnings Per Share [Abstract]
Treasury shares, shares	11,099,593	10,028,755	10,785,007
Equity compensation awards with anti-dilutive effect	0	0
[1]	Derived from audited consolidated financial statements.

Supplemental Cash Flow Inform74

Supplemental Cash Flow Information - Schedule of Interest Paid and Income Taxes Paid (Recovered) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Supplemental Cash Flow [Line Items]
Total interest paid	$ 7.2	$ 6.3
Income taxes paid	1.5	0
Income taxes recovered	0	(2.2)
Income taxes paid (recovered), net	1.5	(2.2)
Total	8.7	4.1
Revolving Credit Facility [Member]
Supplemental Cash Flow [Line Items]
Total interest paid	0	0.3
Senior Unsecured Fixed Rate Notes [Member]
Supplemental Cash Flow [Line Items]
Total interest paid	2.3	2.3
Junior Subordinated Debentures [Member]
Supplemental Cash Flow [Line Items]
Total interest paid	3.4	2.9
Other Indebtedness [Member]
Supplemental Cash Flow [Line Items]
Total interest paid	$ 1.5	$ 0.8

Share-based Compensation - Addi

Share-based Compensation - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Share-based compensation arrangement by share-based payment award, award vesting period, years	5 years	5 years
Stock dividend declared	15.00%
Percentage increase in number of awards outstanding due to stock dividend	15.00%
Reduction in exercise price of awards outstanding due to stock dividend	13.04%

Share-based Compensation - Fair

Share-based Compensation - Fair Value Assumptions (Detail)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Risk-free rate of return	2.61%	1.93%
Expected dividend yields	1.71%	1.63%
Expected award life (years)	4 years 5 months 23 days	4 years 5 months 26 days
Expected volatility	18.44%	18.70%

Share-Based Compensation - 2014

Share-Based Compensation - 2014 Long-Term Incentive Plan - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	May 31, 2014	Mar. 31, 2018	Mar. 31, 2017	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	5 years
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, compensation expense		$ 3.4	$ 2.8
Employee service share-based compensation, unrecognized compensation costs on non-vested awards		$ 26.4
Restricted Stock Units (RSUs) [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
Restricted Stock Units (RSUs) [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		8 years
Stock-Settled SARs Activity [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, compensation expense		$ 1.1	0.7
Employee service share-based compensation, unrecognized compensation costs on non-vested awards		$ 1.5
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]
Share-based compensation arrangement by share-based payment award, compensation expense		$ 0.5	$ 1.4
Employee service share-based compensation, unrecognized compensation costs on non-vested awards		$ 0
2007 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, number of shares authorized				4,500,000
Share-based compensation arrangement by share-based payment award, number of shares available for grant	1,460,000
2014 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, additional number of shares authorized	2,800,000
Count of restricted shares, settle in common shares		2.75

Share-based Compensation - Summ

Share-based Compensation - Summary of Restricted Share Activity (Detail) - Restricted Stock Units (RSUs) [Member]	3 Months Ended
	Mar. 31, 2018$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares, Outstanding at January 1, 2018 | shares	767,501
Granted, Shares | shares	214,048
Vested and issued, Shares | shares	(137,834)
Expired or forfeited, Shares | shares	(81,171)
Shares, Outstanding at March 31, 2018 | shares	762,544
Weighted-Average Grant Date Fair Value, Outstanding at January 1, 2018 | $ / shares	$ 42.70
Granted, Weighted-Average Grant Date Fair Value | $ / shares	55.67
Vested and issued, Weighted-Average Grant Date Fair Value | $ / shares	44.68
Expired or forfeited, Weighted-Average Grant Date Fair Value | $ / shares	43.77
Weighted-Average Grant Date Fair Value, Outstanding at March 31, 2018 | $ / shares	$ 45.87

Share-based Compensation - Su79

Share-based Compensation - Summary of Stock-Settled SARs Activity (Detail) - Stock-Settled SARs Activity [Member]	3 Months Ended
	Mar. 31, 2018$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares, Outstanding at January 1, 2018 | shares	1,494,458
Exercised, Shares | shares	(136,849)
Expired or forfeited, Shares | shares	(14,947)
Shares, Outstanding at March 31, 2018 | shares	1,342,662
Weighted-Average Exercise Price, Outstanding at January 1, 2018 | $ / shares	$ 30.85
Exercised, Weighted-Average Exercise Price | $ / shares	27.45
Expired or forfeited, Weighted-Average Exercise Price | $ / shares	36.25
Weighted-Average Exercise Price, Outstanding at March 31, 2018 | $ / shares	$ 31.13

Share-based Compensation - Su80

Share-based Compensation - Summary of Cash-Settled SARs Activity (Detail) - Cash-Settled SARs Activity [Member]	3 Months Ended
	Mar. 31, 2018$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares, Outstanding at January 1, 2018 | shares	189,235
Exercised, Shares | shares	(42,432)
Expired or forfeited, Shares | shares	(2,235)
Shares, Outstanding at March 31, 2018 | shares	144,568
Weighted-Average Exercise Price, Outstanding at January 1, 2018 | $ / shares	$ 28.97
Exercised, Weighted-Average Exercise Price | $ / shares	29.43
Expired or forfeited, Weighted-Average Exercise Price | $ / shares	21.23
Weighted-Average Exercise Price, Outstanding at March 31, 2018 | $ / shares	$ 28.95

Underwriting, Acquisition and81

Underwriting, Acquisition and Insurance Expenses - Underwriting, Acquisition and Insurance Expenses (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Underwriting Acquisition And Insurance Expenses [Abstract]
Commissions	$ 68	$ 61.2
General expenses	87.8	88.7
Premium taxes, boards and bureaus	9.3	7.4
Underwriting, acquisition and insurance expenses, total	165.1	157.3
Net deferral of policy acquisition costs	(4.9)	(3.7)
Total underwriting, acquisition and insurance expenses	$ 160.2	$ 153.6

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Income Taxes Disclosure [Line Items]
US federal tax rate	21.00%	35.00%
Net operating losses carryback period	2 years
Capital losses carryback period	3 years
Net change in valuation allowance, for deferred tax asset	$ 2,700,000
Net operating loss carryforward	7,000,000
Loss carryforwards per year	2,800,000
Unrecognized tax benefits	0		$ 0
Brazil [Member]
Income Taxes Disclosure [Line Items]
Valuation allowance	7,700,000
Maybrooke [Member]
Income Taxes Disclosure [Line Items]
Valuation allowance	1,200,000
Malta [Member]
Income Taxes Disclosure [Line Items]
Valuation allowance	900,000
ARIS [Member]
Income Taxes Disclosure [Line Items]
Net operating loss carryforward	$ 600,000

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
		Mar. 31, 2018		Mar. 31, 2017
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ 25		$ 42.7
Effective Tax Rate		0.70%		14.10%
Bermuda [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ 25.3		$ 26.5
Effective Tax Rate		0.00%		0.00%
United States [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ (6.3)		$ 24
Effective Tax Rate		23.00%		26.90%
United Kingdom [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ 6.2		$ (6.9)
Effective Tax Rate		24.50%		6.70%
Belgium [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ 0.1		$ 0	[1]
Effective Tax Rate		35.80%		0.00%
Brazil [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ (1.4)		$ (0.8)
Effective Tax Rate		0.00%		0.00%
United Arab Emirates [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ 0.2		$ 0	[1]
Effective Tax Rate		0.00%		0.00%
Ireland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	[1]	$ 0		$ 0
Effective Tax Rate		0.00%		0.00%
Italy [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ 0	[1]	$ 0
Effective Tax Rate		0.00%		0.00%
Malta [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ 0.9		$ 0.6
Effective Tax Rate		0.00%		0.00%
Luxembourg [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ 0	[1]	$ (0.7)
Effective Tax Rate		0.00%		0.00%
Switzerland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ 0		$ 0	[1]
Effective Tax Rate		21.20%		21.00%
[1]	Pre-tax income for the respective year was less than $0.1 million.

Income Taxes - Schedule of Pr84

Income Taxes - Schedule of Pre-Tax Income (Loss) and Effective Income Tax Rates (Parenthetical) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018		Mar. 31, 2017
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ 25		$ 42.7
United Arab Emirates [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		0.2		0	[1]
Ireland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	[1]	0		0
Luxembourg [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		0	[1]	(0.7)
Switzerland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		0		0	[1]
Belgium [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)		$ 0.1		0	[1]
Maximum [Member] | United Arab Emirates [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)				0.1
Maximum [Member] | Ireland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)				0.1
Maximum [Member] | Luxembourg [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)				0.1
Maximum [Member] | Switzerland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)				0.1
Maximum [Member] | Belgium [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)				$ 0.1
[1]	Pre-tax income for the respective year was less than $0.1 million.

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	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Income tax (benefit) provision at expected rate	$ (0.4)	$ 7.2
Nontaxable investment income	(0.5)	(1.3)
Foreign exchange adjustments	(0.2)	0.6
Withholding taxes	0.1	0.1
Change in valuation allowance	(2.7)	(0.1)
Other	3.9	(0.5)
Income tax provision	$ 0.2	$ 6

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2018USD ($)
Commitments And Contingencies [Line Items]
Contractual commitments related to its limited partnership investments	$ 123.8
Maximum [Member]
Commitments And Contingencies [Line Items]
Contractual commitments period	13 years

Segment Information - Additiona

Segment Information - Additional Information (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2018USD ($)Segment	Dec. 31, 2017USD ($)
Segment Reporting [Abstract]
Number of reportable segments | Segment	2
Assets associated with trade capital providers | $	$ 734.2	$ 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
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Earned premiums	$ 414.7	$ 379.4
Net investment income	36	30.5
Fee and other income	2	3.6
Net realized investment (losses) gains before income taxes	(15.7)	14.6
Total revenue	437	428.1
Income (loss) before income taxes	25	42.7
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Net investment income	2.4	1.9
Income (loss) before income taxes	(15.5)	(14.9)
Operating Segments [Members]
Segment Reporting Information [Line Items]
Income (loss) before income taxes	56.2	43
U.S. Operations [Member]
Segment Reporting Information [Line Items]
Earned premiums	262.3	221.2
Net investment income	22.6	19.9
U.S. Operations [Member] | Operating Segments [Members]
Segment Reporting Information [Line Items]
Income (loss) before income taxes	34	37.2
International Operations [Member]
Segment Reporting Information [Line Items]
Earned premiums	152.4	158.2
Net investment income	8.8	6.6
International Operations [Member] | Operating Segments [Members]
Segment Reporting Information [Line Items]
Income (loss) before income taxes	22.9	8.3
Run Off Lines [Member]
Segment Reporting Information [Line Items]
Earned premiums	0	0
Net investment income	2.2	2.1
Run Off Lines [Member] | Operating Segments [Members]
Segment Reporting Information [Line Items]
Income (loss) before income taxes	$ (0.7)	$ (2.5)

Segment Information - Schedule

Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 414.7	$ 379.4
Bermuda [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	20.5	25.6
Brazil [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	13	12.8
Malta [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	5.7	1
United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	114.1	118.8
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 261.4	$ 221.2

Segment Information - Identifia

Segment Information - Identifiable Assets (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 9,141.3	$ 8,764	[1]
U.S. Operations [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	4,527.1	4,298.4
International Operations [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	3,736.8	3,553.8
Run Off Lines [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	456.3	449.6
Corporate and Other [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 421.1	$ 462.2
[1]	Derived from audited consolidated financial statements.

Senior Unsecured Fixed Rate N91

Senior Unsecured Fixed Rate Notes - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended
	Sep. 30, 2012	Mar. 31, 2018	Dec. 31, 2017
Schedule Of Equity Method Investments [Line Items]
Principal amount of senior note		$ 143,800,000	$ 143,800,000
Senior Unsecured Fixed Rate Notes [Member]
Schedule Of Equity Method Investments [Line Items]
Principal amount of senior note	$ 143,750,000
Debt instrument interest rate stated percentage	6.50%
Junior subordinated debentures, Maturity date		Sep. 15, 2042
Initial maturity date		Sep. 15, 2017
Debt Instrument, Redemption Price, Percentage	100.00%

Senior Unsecured Fixed Rate N92

Senior Unsecured Fixed Rate Notes - Schedule Of Unamortized Debt Issuance Costs Deducted From Carrying Value Of Debt Liability (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Debt Disclosure [Abstract]
Principal	$ 143.8	$ 143.8
Less: unamortized debt issuance costs	(4.1)	(4.2)
Senior unsecured fixed rate notes, less unamortized debt issuance costs	$ 139.7	$ 139.6	[1]
[1]	Derived from audited consolidated financial statements.

Senior Unsecured Fixed Rate N93

Senior Unsecured Fixed Rate Notes - Condensed Consolidating Balance Sheet (Detail) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017		Mar. 31, 2017	Dec. 31, 2016
Assets
Investments		$ 4,833.5	$ 4,742.9	[1]
Cash		173.7	176.6	[1]	$ 190.7	$ 86
Accrued investment income		24.3	23.5	[1]
Premiums receivable		699.6	598.6	[1]
Reinsurance recoverables		2,087.5	2,093.3	[1]
Goodwill and other intangible assets, net		268.7	258.2
Current income taxes receivable, net		0	1.4	[1]
Deferred acquisition costs, net		163.9	160.4	[1]
Ceded unearned premiums		501.9	399.5	[1]
Other assets		388.2	309.6	[1]
Intercompany note receivable		0	0
Investments in subsidiaries		0	0
Total assets		9,141.3	8,764	[1]
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses		4,283.6	4,201	[1]	3,580.3
Unearned premiums		1,270.1	1,207.7	[1]
Funds held and ceded reinsurance payable, net		912.9	776.7
Long-term debt		582.7	580.7
Current income taxes payable, net		2.2	0	[1]
Deferred tax liabilities, net		21.3	31.3	[1]
Accrued underwriting expenses and other liabilities		281.1	146.9
Due to (from) affiliates		0
Due to affiliates			0
Total liabilities		7,353.9	6,944.3	[1]
Total shareholders' equity		1,787.4	1,819.7	[1]	1,834.6	1,792.7
Total liabilities and shareholders' equity		9,141.3	8,764	[1]
Argo Group [Member]
Assets
Investments		0.4	0.4
Cash		0.3	0.9		0	0
Accrued investment income		0	0
Premiums receivable		0	0
Reinsurance recoverables		0	0
Goodwill and other intangible assets, net		42.9	43.2
Current income taxes receivable, net			0
Deferred acquisition costs, net		0	0
Ceded unearned premiums		0	0
Other assets		14.9	9.2
Intercompany note receivable		0	0
Investments in subsidiaries		1,939.4	1,940
Total assets		1,997.9	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
Long-term debt		153.4	153.4
Current income taxes payable, net		0
Deferred tax liabilities, net		0	0
Accrued underwriting expenses and other liabilities		6.3	8.9
Due to (from) affiliates		50.8
Due to affiliates			11.7
Total liabilities		210.5	174
Total shareholders' equity		1,787.4	1,819.7
Total liabilities and shareholders' equity		1,997.9	1,993.7
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Assets
Investments		3,085.9	3,079
Cash		59.4	47.8		41.1	53.7
Accrued investment income		18	18
Premiums receivable		210.6	216.5
Reinsurance recoverables		1,537.4	1,487.3
Goodwill and other intangible assets, net		124.7	124.9
Current income taxes receivable, net			2.4
Deferred acquisition costs, net		81.1	80.7
Ceded unearned premiums		211	198.5
Other assets		179.6	171.5
Intercompany note receivable		51.5	50.9
Investments in subsidiaries		0	0
Total assets		5,559.2	5,477.5
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses		2,538.1	2,483.9
Unearned premiums		702.6	704
Funds held and ceded reinsurance payable, net		863.1	799.4
Long-term debt		284.6	284.5
Current income taxes payable, net		0.8
Deferred tax liabilities, net		21.7	32.5
Accrued underwriting expenses and other liabilities		90.6	95
Due to (from) affiliates		5
Due to affiliates			(0.4)
Total liabilities		4,506.5	4,398.9
Total shareholders' equity		1,052.7	1,078.6
Total liabilities and shareholders' equity		5,559.2	5,477.5
Other Subsidiaries and Eliminations [Member]
Assets
Investments	[2]	1,747.2	1,663.5
Cash	[2]	114	127.9		149.6	32.3
Accrued investment income	[2]	6.3	5.5
Premiums receivable	[2]	489	382.1
Reinsurance recoverables	[2]	550.1	606
Goodwill and other intangible assets, net	[2]	101.1	90.1
Current income taxes receivable, net	[2]		(1)
Deferred acquisition costs, net	[2]	82.8	79.7
Ceded unearned premiums	[2]	290.9	201
Other assets	[2]	193.7	128.9
Intercompany note receivable	[2]	(51.5)	(50.9)
Investments in subsidiaries	[2]	0	0
Total assets	[2]	3,523.6	3,232.8
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	[2]	1,745.5	1,717.1
Unearned premiums	[2]	567.5	503.7
Funds held and ceded reinsurance payable, net	[2]	49.8	(22.7)
Long-term debt	[2]	144.7	142.8
Current income taxes payable, net	[2]	1.4
Deferred tax liabilities, net	[2]	(0.4)	(1.2)
Accrued underwriting expenses and other liabilities	[2]	184.2	43
Due to (from) affiliates	[2]	(5)
Due to affiliates	[2]		0.4
Total liabilities	[2]	2,687.7	2,383.1
Total shareholders' equity	[2]	835.9	849.7
Total liabilities and shareholders' equity	[2]	3,523.6	3,232.8
Consolidating Adjustments [Member]
Assets
Investments	[3]	0	0
Cash	[3]	0	0		$ 0	$ 0
Accrued investment income	[3]	0	0
Premiums receivable	[3]	0	0
Reinsurance recoverables	[3]	0	0
Goodwill and other intangible assets, net	[3]	0	0
Current income taxes receivable, net	[3]		0
Deferred acquisition costs, net	[3]	0	0
Ceded unearned premiums	[3]	0	0
Other assets	[3]	0	0
Intercompany note receivable	[3]	0	0
Investments in subsidiaries	[3]	(1,939.4)	(1,940)
Total assets	[3]	(1,939.4)	(1,940)
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	[3]	0	0
Unearned premiums	[3]	0	0
Funds held and ceded reinsurance payable, net	[3]	0	0
Long-term debt	[3]	0	0
Current income taxes payable, net	[3]	0
Deferred tax liabilities, net	[3]	0	0
Accrued underwriting expenses and other liabilities	[3]	0	0
Due to (from) affiliates	[3]	(50.8)
Due to affiliates	[3]		(11.7)
Total liabilities	[3]	(50.8)	(11.7)
Total shareholders' equity	[3]	(1,888.6)	(1,928.3)
Total liabilities and shareholders' equity	[3]	$ (1,939.4)	$ (1,940)
[1]	Derived from audited consolidated financial statements.
[2]	Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations.
[3]	Includes all Argo Group parent company eliminations.

Senior Unsecured Fixed Rate N94

Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Income (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Premiums and other revenue:
Earned premiums		$ 414.7	$ 379.4
Net investment (expense) income		36	30.5
Fee and other income		2	3.6
Net realized investment (losses) gains before income taxes		(15.7)	14.6
Total revenue		437	428.1
Expenses:
Losses and loss adjustment expenses		237.2	222.5
Underwriting, acquisition and insurance expenses		160.2	153.6
Interest expense		7.7	5.9
Fee and other expense		2	4.1
Foreign currency exchange loss (gains)		4.9	(0.7)
Total expenses		412	385.4
Income before income taxes		25	42.7
(Benefit) Provision for income taxes		0.2	6
Net (loss) income before equity in earnings of subsidiaries		24.8	36.7
Equity in undistributed earnings of subsidiaries		0	0
Net income		24.8	36.7
Argo Group [Member]
Premiums and other revenue:
Earned premiums		0	0
Net investment (expense) income		(0.7)	(1.7)
Fee and other income		0	0
Net realized investment (losses) gains before income taxes		(0.4)	0.5
Total revenue		(1.1)	(1.2)
Expenses:
Losses and loss adjustment expenses		0	0
Underwriting, acquisition and insurance expenses		2.4	8.4
Interest expense		1.3	0.3
Fee and other expense		0	0
Foreign currency exchange loss (gains)		0	0
Total expenses		3.7	8.7
Income before income taxes		(4.8)	(9.9)
(Benefit) Provision for income taxes		0	0
Net (loss) income before equity in earnings of subsidiaries		(4.8)	(9.9)
Equity in undistributed earnings of subsidiaries		29.6	46.6
Net income		24.8	36.7
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Premiums and other revenue:
Earned premiums		156.9	130.9
Net investment (expense) income		22.1	18.9
Fee and other income		0.8	2.8
Net realized investment (losses) gains before income taxes		(15.4)	13.8
Total revenue		164.4	166.4
Expenses:
Losses and loss adjustment expenses		95.4	76.1
Underwriting, acquisition and insurance expenses		69.8	58.2
Interest expense		4.3	4.3
Fee and other expense		1.4	3.4
Foreign currency exchange loss (gains)		0.1	0.1
Total expenses		171	142.1
Income before income taxes		(6.6)	24.3
(Benefit) Provision for income taxes		(1.4)	6.4
Net (loss) income before equity in earnings of subsidiaries		(5.2)	17.9
Equity in undistributed earnings of subsidiaries		0	0
Net income		(5.2)	17.9
Other Subsidiaries and Eliminations [Member]
Premiums and other revenue:
Earned premiums	[1]	257.8	248.5
Net investment (expense) income	[1]	14.6	13.3
Fee and other income	[1]	1.2	0.8
Net realized investment (losses) gains before income taxes	[1]	0.1	0.3
Total revenue	[1]	273.7	262.9
Expenses:
Losses and loss adjustment expenses	[1]	141.8	146.4
Underwriting, acquisition and insurance expenses	[1]	88	87
Interest expense	[1]	2.1	1.3
Fee and other expense	[1]	0.6	0.7
Foreign currency exchange loss (gains)	[1]	4.8	(0.8)
Total expenses	[1]	237.3	234.6
Income before income taxes	[1]	36.4	28.3
(Benefit) Provision for income taxes	[1]	1.6	(0.4)
Net (loss) income before equity in earnings of subsidiaries	[1]	34.8	28.7
Equity in undistributed earnings of subsidiaries	[1]	0	0
Net income	[1]	34.8	28.7
Consolidating Adjustments [Member]
Premiums and other revenue:
Earned premiums	[2]	0	0
Net investment (expense) income	[2]	0	0
Fee and other income	[2]	0	0
Net realized investment (losses) gains before income taxes	[2]	0	0
Total revenue	[2]	0	0
Expenses:
Losses and loss adjustment expenses	[2]	0	0
Underwriting, acquisition and insurance expenses	[2]	0	0
Interest expense	[2]	0	0
Fee and other expense	[2]	0	0
Foreign currency exchange loss (gains)	[2]	0	0
Total expenses	[2]	0	0
Income before income taxes	[2]	0	0
(Benefit) Provision for income taxes	[2]	0	0
Net (loss) income before equity in earnings of subsidiaries	[2]	0	0
Equity in undistributed earnings of subsidiaries	[2]	(29.6)	(46.6)
Net income	[2]	$ (29.6)	$ (46.6)
[1]	Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations.
[2]	Includes all Argo Group parent company eliminations.

Senior Unsecured Fixed Rate N95

Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Cash Flows (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018		Mar. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities		$ 153.6		$ (36.9)
Cash flows from investing activities:
Proceeds from sales of investments		555.4		454
Maturities and mandatory calls of fixed maturity investments		144.8		182.1
Purchases of investments		(687.5)		(644.4)
Change in short-term investments and foreign regulatory deposits		(161)		120.3
Settlements of foreign currency exchange forward contracts		1.7		(2.8)
Acquisition of Maybrooke, net of cash acquired				(83.1)
Issuance of intercompany note, net				0
Purchases of fixed assets and other, net				(1.3)
Cash (used in) provided by investing activities		(128.5)		24.8
Cash flows from financing activities:
Additional long-term borrowings		0		125
Borrowing under intercompany note, net				0
Activity under stock incentive plans		0.2		0.2
Repurchase of Company's common shares		(18.6)		0
Payment of cash dividends to common shareholders		(9.3)		(8.3)
Cash (used in) provided by financing activities		(27.7)		116.9
Effect of exchange rate changes on cash		(0.3)		(0.1)
Change in cash		(2.9)		104.7
Cash, beginning of year		176.6	[1]	86
Cash, end of period		173.7		190.7
Argo Group [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities		27.8		(2)
Cash flows from investing activities:
Proceeds from sales of investments		0		0
Maturities and mandatory calls of fixed maturity investments		0		0
Purchases of investments		0		0
Change in short-term investments and foreign regulatory deposits		0		0.5
Settlements of foreign currency exchange forward contracts		(0.6)		0
Acquisition of Maybrooke, net of cash acquired				(235.3)
Issuance of intercompany note, net				0
Purchases of fixed assets and other, net				(0.1)
Cash (used in) provided by investing activities		(0.7)		(234.9)
Cash flows from financing activities:
Additional long-term borrowings				125
Borrowing under intercompany note, net				120
Activity under stock incentive plans		0.2		0.2
Repurchase of Company's common shares		(18.6)
Payment of cash dividends to common shareholders		(9.3)		(8.3)
Cash (used in) provided by financing activities		(27.7)		236.9
Effect of exchange rate changes on cash		0		0
Change in cash		(0.6)		0
Cash, beginning of year		0.9		0
Cash, end of period		0.3		0
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities		53.2		(14.7)
Cash flows from investing activities:
Proceeds from sales of investments		325		228.9
Maturities and mandatory calls of fixed maturity investments		113.1		135.5
Purchases of investments		(475.2)		(446.5)
Change in short-term investments and foreign regulatory deposits		(4.5)		65.5
Settlements of foreign currency exchange forward contracts		(0.7)		0
Acquisition of Maybrooke, net of cash acquired				0
Issuance of intercompany note, net				0
Purchases of fixed assets and other, net				18.7
Cash (used in) provided by investing activities		(41.6)		2.1
Cash flows from financing activities:
Additional long-term borrowings				0
Borrowing under intercompany note, net				0
Activity under stock incentive plans		0		0
Repurchase of Company's common shares		0
Payment of cash dividends to common shareholders		0		0
Cash (used in) provided by financing activities		0		0
Effect of exchange rate changes on cash		0		0
Change in cash		11.6		(12.6)
Cash, beginning of year		47.8		53.7
Cash, end of period		59.4		41.1
Other Subsidiaries and Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	[2]	72.6		(20.2)
Cash flows from investing activities:
Proceeds from sales of investments	[2]	230.4		225.1
Maturities and mandatory calls of fixed maturity investments	[2]	31.7		46.6
Purchases of investments	[2]	(212.3)		(197.9)
Change in short-term investments and foreign regulatory deposits	[2]	(156.5)		54.3
Settlements of foreign currency exchange forward contracts	[2]	3		(2.8)
Acquisition of Maybrooke, net of cash acquired	[2]			152.2
Issuance of intercompany note, net	[2]			(120)
Purchases of fixed assets and other, net	[2]			(19.9)
Cash (used in) provided by investing activities	[2]	(86.2)		137.6
Cash flows from financing activities:
Additional long-term borrowings	[2]			0
Borrowing under intercompany note, net	[2]			0
Activity under stock incentive plans	[2]	0		0
Repurchase of Company's common shares	[2]	0
Payment of cash dividends to common shareholders	[2]	0		0
Cash (used in) provided by financing activities	[2]	0		0
Effect of exchange rate changes on cash	[2]	(0.3)		(0.1)
Change in cash	[2]	(13.9)		117.3
Cash, beginning of year	[2]	127.9		32.3
Cash, end of period	[2]	114		149.6
Ariscom [Member]
Cash flows from investing activities:
Cash acquired with acquisition of Ariscom		15.6		0
Ariscom [Member] | Argo Group [Member]
Cash flows from investing activities:
Cash acquired with acquisition of Ariscom		0
Ariscom [Member] | Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Cash flows from investing activities:
Cash acquired with acquisition of Ariscom		0
Ariscom [Member] | Other Subsidiaries and Eliminations [Member]
Cash flows from investing activities:
Cash acquired with acquisition of Ariscom	[2]	15.6
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	[3]	0		0
Cash flows from investing activities:
Proceeds from sales of investments	[3]	0		0
Maturities and mandatory calls of fixed maturity investments	[3]	0		0
Purchases of investments	[3]	0		0
Change in short-term investments and foreign regulatory deposits	[3]	0		0
Settlements of foreign currency exchange forward contracts	[3]	0		0
Acquisition of Maybrooke, net of cash acquired	[3]			0
Issuance of intercompany note, net	[3]			120
Purchases of fixed assets and other, net	[3]			0
Cash (used in) provided by investing activities	[3]	0		120
Cash flows from financing activities:
Additional long-term borrowings	[3]			0
Borrowing under intercompany note, net	[3]			(120)
Activity under stock incentive plans	[3]	0		0
Repurchase of Company's common shares	[3]	0
Payment of cash dividends to common shareholders	[3]	0		0
Cash (used in) provided by financing activities	[3]	0		(120)
Effect of exchange rate changes on cash	[3]	0		0
Change in cash	[3]	0		0
Cash, beginning of year	[3]	0		0
Cash, end of period	[3]	0		$ 0
Consolidating Adjustments [Member] | Ariscom [Member]
Cash flows from investing activities:
Cash acquired with acquisition of Ariscom	[3]	$ 0
[1]	Derived from audited consolidated financial statements.
[2]	Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations.
[3]	Includes all Argo Group parent company eliminations.