Argo Group International (ARGO-PA) 10-Q 8 May 15 2015 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2015	5-May-15
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	FALSE
Document Period End Date	31-Mar-15
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Trading Symbol	AGII
Entity Registrant Name	Argo Group International Holdings, Ltd.
Entity Central Index Key	1091748
Current Fiscal Year End Date	-19
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		27,932,719

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Assets
Available-for-sale (cost: 2015 - $2,778.6; 2014 - $2,817.2)	$2,788.60	$2,840.70	[1]
Equity securities, at fair value (cost: 2015 - $315.3; 2014 - $307.3)	483.4	486.3	[1]
Other investments (cost: 2015 - $501.8; 2014 - $488.9)	520.9	495.1	[1]
Short-term investments, at fair value (cost: 2015 - $271.8; 2014 - $275.8)	271.5	275.8	[1]
Total investments	4,064.40	4,097.90	[1]
Cash	89.1	81	[1]
Accrued investment income	20.9	22.1	[1]
Premiums receivable	371.7	353.6	[1]
Reinsurance recoverables	987.3	997.2	[1]
Goodwill	152.2	152.2	[1]
Intangible assets, net of accumulated amortization	78.8	78.6	[1]
Current income taxes receivable, net	4.2	14.9	[1]
Deferred acquisition costs, net	129.7	124.6	[1]
Ceded unearned premiums	256.6	207.6	[1]
Other assets	222.2	226.6	[1]
Total assets	6,377.10	6,356.30	[1]
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	3,035.10	3,042.40	[1]
Unearned premiums	826.4	817.2	[1]
Accrued underwriting expenses	125.7	143.1	[1]
Ceded reinsurance payable, net	207.9	178.8	[1]
Funds held	52.6	55	[1]
Senior unsecured fixed rate notes	143.8	143.8	[1]
Other indebtedness	57.5	62	[1]
Junior subordinated debentures	172.7	172.7	[1]
Deferred tax liabilities, net	53.8	53	[1]
Other liabilities	38.8	41.6	[1]
Total liabilities	4,714.30	4,709.60	[1]
Commitments and contingencies (Note 14)			[1]
Shareholders' equity:
Common shares - $1.00 par, 500,000,000 shares authorized; 36,915,795 and 34,318,224 shares issued and outstanding at March 31, 2015 and December 31, 2014, respectively	36.9	34.3	[1]
Additional paid-in capital	960.9	836.3	[1]
Treasury shares (8,959,543 and 8,606,489 shares at March 31, 2015 and December 31, 2014, respectively)	-319.5	-301.4	[1]
Retained earnings	898.4	969.4	[1]
Accumulated other comprehensive income, net of taxes	86.1	108.1	[1]
Total shareholders' equity	1,662.80	1,646.70	[1]
Total liabilities and shareholders' equity	$6,377.10	$6,356.30	[1]
[1]	Derived from audited consolidated financial statements.

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, except Share data, unless otherwise specified
Statement Of Financial Position [Abstract]
Available-for-sale cost	$2,778.60	$2,817.20	[1]
Equity securities cost	315.3	307.3	[1]
Other investments cost	501.8	488.9	[1]
Short-term investments, Cost	$271.80	$275.80	[1]
Common shares, par value	$1	$1	[1]
Common shares, shares authorized	500,000,000	500,000,000	[1]
Common shares, shares issued	36,915,795	34,318,224	[1]
Common shares, shares outstanding	36,915,795	34,318,224	[1]
Treasury shares, shares	8,959,543	8,606,489	[1]
[1]	Derived from audited consolidated financial statements.

Consolidated_Statements_of_Inc

Consolidated Statements of Income (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Premiums and other revenue:
Earned premiums	$334.60	$325.70
Net investment income	20.8	23.3
Net realized investment and other gains	16	11.1
Total revenue	371.4	360.1
Expenses:
Losses and loss adjustment expenses	183.7	182.5
Underwriting, acquisition and insurance expenses	129.6	128.7
Interest expense	4.9	5
Fee expense, net	0.4	1.4
Foreign currency exchange (gain) loss	-9.6	-0.2
Total expenses	309	317.4
Income before income taxes	62.4	42.7
Provision for income taxes	3.6	2.5
Net income	58.8	40.2
Net income per common share:
Basic	$2.09	$1.38
Diluted	$2.05	$1.36
Dividend declared per common share	$0.20	$0.14
Weighted average common shares:
Basic	28,129,692	29,165,911
Diluted	28,677,379	29,663,090
Net realized investment and other gains before other-than-temporary impairment losses	16.5	11.1
Other-than-temporary impairment losses recognized in earnings:
Other-than-temporary impairment losses on fixed maturities	-0.4	0
Other-than-temporary impairment losses on equity securities	-0.1	0
Impairment losses recognized in earnings	-0.5	0
Net realized investment and other gains	$16	$11.10

Consolidated_Statements_of_Com

Consolidated Statements of Comprehensive Income (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Statement Of Income And Comprehensive Income [Abstract]
Net income	$58.80	$40.20
Other comprehensive (loss) income:
Foreign currency translation adjustments	-3.1	-0.2
Unrealized gains on securities:
(Losses) gains arising during the year	-24.3	5.1
Reclassification adjustment for gains included in net income	2.5	-0.1
Other comprehensive (loss) income before tax	-24.9	4.8
Unrealized gains (losses) on securities:
(Losses) gains arising during the year	-2.8	3.8
Reclassification adjustment for gains included in net income	-0.1	-0.1
Income tax (benefit) provision related to other comprehensive income (loss)	-2.9	3.7
Other comprehensive (loss) income, net of tax	-22	1.1
Comprehensive income	$36.80	$41.30

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Cash flows from operating activities:
Net income	$58.80		$40.20
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization and depreciation	8.9		10.4
Share-based payments expense	5.2		4.4
Excess tax benefit from share-based payment arrangements	0		-0.1
Deferred income tax provision, net	3.8		1.1
Net realized investment and other gains	-16		-11.1
Loss on disposals of fixed assets, net	0.2		0
Change in:
Accrued investment income	1.2		1.7
Receivables	-12.5		261
Deferred acquisition costs	-5.4		0
Ceded unearned premiums	-51		-55.2
Reserves for losses and loss adjustment expenses	-2.2		-182.5
Unearned premiums	13.1		3.3
Ceded reinsurance payable and funds held	27.9		-6.4
Income taxes	7.7		-15.5
Accrued underwriting expenses	-18.9		6.7
Other, net	6.8		-14.4
Cash provided by operating activities	27.6		43.6
Cash flows from investing activities:
Sales of fixed maturity investments	399.3		243.4
Maturities and mandatory calls of fixed maturity investments	142		78.4
Sales of equity securities	14.7		14.9
Sales of other investments	9		5
Purchases of fixed maturity investments	-514.5		-290.4
Purchases of equity securities	-19.9		-7.5
Purchases of other investments	-22.8		-9.4
Change in foreign regulatory deposits and voluntary pools	1.1		-9
Change in short-term investments	3.7		-44.9
Settlements of foreign currency exchange forward contracts	-2.2		-1.4
Purchases of fixed assets	-17.8		-6
Other, net	9.5		-18.1
Cash provided (used) by investing activities	2.1		-45
Cash flows from financing activities:
Activity under stock incentive plans	0.9		0.8
Repurchase of Company's common shares	-17		-3.7
Excess tax expense from share-based payment arrangements	0		0.1
Payment of cash dividends to common shareholders	-5.7		-4
Cash used by financing activities	-21.8		-6.8
Effect of exchange rate changes on cash	0.2		0.3
Change in cash	8.1		-7.9
Cash, beginning of period	81	[1]	157.4
Cash, end of period	$89.10		$149.50
[1]	Derived from audited consolidated financial statements.

Basis_of_Presentation

Basis of Presentation	3 Months Ended
	Mar. 31, 2015
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 accounting principles generally accepted in the United States of America (“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, 2014, filed with the Securities and Exchange Commission on February 27, 2015.
	The interim financial information as of, and for the three months ended, March 31, 2015 and 2014 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. Certain amounts in prior years’ financial statements have been reclassified to conform to the current presentation.
	10% Stock Dividend
	On February 17, 2015, our Board of Directors declared a 10% stock dividend, payable on March 16, 2015, to shareholders of record at the close of business on March 2, 2015. As a result of the stock dividend, 2,554,506 additional shares were issued. Cash was paid in lieu of fractional shares of our common shares. All references to share and per share amounts in this document and related disclosures have been adjusted to reflect the stock dividend for all periods presented.

Recently_Issued_Accounting_Pro

Recently Issued Accounting Pronouncements	3 Months Ended
	Mar. 31, 2015
Accounting Changes And Error Corrections [Abstract]
Recently Issued Accounting Pronouncements	2	Recently Issued Accounting Pronouncements
	In May 2014, the FASB issued Accounting Standards Update (“ASU”) 2014-09, “Revenue from Contracts with Customers” (Topic 606). The ASU is a comprehensive new revenue recognition model that requires a company to recognize revenue to depict the transfer of goods or services to a customer at an amount that reflects the consideration it expects to receive in exchange for those goods or services. The ASU provides a five-step analysis of transactions to determine when and how revenue is recognized and requires additional disclosures sufficient to describe the nature, amount, timing and uncertainty of revenue and cash flows for these transactions. This ASU is effective for annual reporting periods beginning after December 15, 2016 and early adoption is not permitted. Accordingly, we will adopt this ASU on January 1, 2017. On April 1, 2015 the FASB voted to propose to defer ASU 2014-09 for one year, which if accepted, would delay our adoption date until January 1, 2018. Companies may use either a “full retrospective” adoption, meaning the update is applied to all periods presented, or a “modified retrospective” adoption, meaning the update is applied only to the most current period presented in the financial statements. While insurance contracts are excluded from this ASU, fee income related to our brokerage operations and management of the third-party capital for our underwriting Syndicate at Lloyd’s will be subject to this updated guidance. We are currently evaluating what impact this ASU will have on our financial results and disclosures and which adoption method to apply.
	In February 2015, the FASB issued ASU 2015-02, "Consolidation (Topic 810): Amendments to the Consolidation Analysis." ASU 2015-02 changes the analysis that a reporting entity must perform to determine whether entities should be consolidated as they are deemed variable interest entities. It is effective for annual reporting periods, and interim periods within those years, beginning after December 15, 2015. Early adoption is permitted, including adoption in an interim period. We are currently in the process of evaluating the impact of the adoption of ASU 2015-02 on our consolidated financial statements.

Investments

Investments	3 Months Ended
	Mar. 31, 2015
Investments Debt And Equity Securities [Abstract]
Investments	3	Investments
	Composition of Invested Assets
	The amortized cost, gross unrealized gains, gross unrealized losses and fair value of investments were as follows:
	31-Mar-15
	(in millions)		Amortized			Gross			Gross			Fair
			Cost			Unrealized			Unrealized			Value
						Gains			Losses
	Fixed maturities
	USD denominated:
	U.S. Governments		$	181.3		$	1.6		$	0.1		$	182.8
	Non-U.S. Governments			81.3			0.7			0.5			81.5
	Obligations of states and political subdivisions			457.7			22.7			0.4			480
	Credit-Financial			472.7			13			1			484.7
	Credit-Industrial			460.5			11			1.6			469.9
	Credit-Utility			146.2			3.8			2.8			147.2
	Structured securities:
	CMO/MBS-agency (1)			160.8			8.2			0.3			168.7
	CMO/MBS-non agency			12.6			0.8			—			13.4
	CMBS (2)			184.4			1.7			0.1			186
	ABS (3)			223.2			1			0.8			223.4
	Foreign denominated:
	Governments			157.1			1.5			17			141.6
	Credit			240.8			3.6			35			209.4
	Total fixed maturities			2,778.60			69.6			59.6			2,788.60
	Equity securities			315.3			177.2			9.1			483.4
	Other investments			501.8			19.2			0.1			520.9
	Short-term investments			271.8			—			0.3			271.5
	Total investments		$	3,867.50		$	266		$	69.1		$	4,064.40
	(1)	Collateralized mortgage obligations/mortgage-backed securities (“CMO/MBS”).
	(2)	Commercial mortgage-backed securities (“CMBS”).
	(3)	Asset-backed securities (“ABS”).
	31-Dec-14
	(in millions)		Amortized			Gross			Gross			Fair
			Cost			Unrealized			Unrealized			Value
						Gains			Losses
	Fixed maturities
	USD denominated:
	U.S. Governments		$	184		$	1.3		$	0.3		$	185
	Non-U.S. Governments			79.9			0.6			0.6			79.9
	Obligations of states and political subdivisions			468.1			22.9			0.3			490.7
	Credit-Financial			508.1			12.3			2.3			518.1
	Credit-Industrial			493.7			9.4			3.5			499.6
	Credit-Utility			142.7			3.2			3.9			142
	Structured securities:
	CMO/MBS-agency (1)			168			8			0.7			175.3
	CMO/MBS-non agency			13.2			0.8			—			14
	CMBS (2)			178.6			1.6			0.2			180
	ABS (3)			221.4			0.6			1			221
	Foreign denominated:
	Governments			148.4			1.2			9.4			140.2
	Credit			211.1			2.7			18.9			194.9
	Total fixed maturities			2,817.20			64.6			41.1			2,840.70
	Equity securities			307.3			184.1			5.1			486.3
	Other investments			488.9			7.5			1.3			495.1
	Short-term investments			275.8			—			—			275.8
	Total investments		$	3,889.20		$	256.2		$	47.5		$	4,097.90
	(1)	Collateralized mortgage obligations/mortgage-backed securities (“CMO/MBS”).
	(2)	Commercial mortgage-backed securities (“CMBS”).
	(3)	Asset-backed securities (“ABS”).
	Included in “Total investments” in our Consolidated Balance Sheets at March 31, 2015 and December 31, 2014 is $70.9 million and $75.2 million, respectively, of assets managed on behalf of the trade capital providers, who are third-party participants that provide underwriting capital to our Syndicate 1200 segment.
	Contractual Maturity
	The amortized cost and fair values of fixed maturity investments as of March 31, 2015, by contractual maturity, were as follows:
	(in millions)		Amortized			Fair
			Cost			Value
Due in one year or less		$	258.8		$	252.6
Due after one year through five years			1,171.10			1,166.90
Due after five years through ten years			582.5			589.5
Thereafter			185.2			188.1
Structured securities			581			591.5
Total		$	2,778.60		$	2,788.60
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, 2015 and December 31, 2014 were as follows:
31-Mar-15
(in millions)		Carrying Value			Unfunded Commitments
Investment Type
Hedge funds		$	145.5		$	—
Private equity			139.6			60.6
Long only funds			115			—
Other investments			120.8			—
Total other invested assets		$	520.9		$	60.6
31-Dec-14
(in millions)		Carrying Value			Unfunded Commitments
Investment Type
Hedge funds		$	153.2		$	—
Private equity			123.6			72.9
Long only funds			104.4			—
Other investments			113.9			—
Total other invested assets		$	495.1		$	72.9
The following describes each investment type:
·	Hedge funds: Hedge funds include equity long/short, multi-strategy credit, relative value credit and distressed event credit funds.
·	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: These funds include a long-only fund that invest in high yield fixed income securities and funds that primarily invest in distressed debt.
·	Other investments: Other investments include assets on deposit with various regulatory authorities to support Syndicate 1200 segment’s insurance and reinsurance operations and foreign exchange currency forward contracts to manage exposure on losses related to global catastrophic events, our Canadian dollar investment portfolio and certain Euro denominated investments.
Unrealized Losses and Other-than-temporary Impairments
An aging of unrealized losses on our investments in fixed maturities, equity securities, other investments and short-term investments is presented below:
31-Mar-15		Less Than One Year						One Year or Greater						Total
(in millions)		Fair			Unrealized			Fair			Unrealized			Fair			Unrealized
		Value			Losses			Value			Losses			Value			Losses
Fixed maturities
USD denominated:
U.S. Governments (2)		$	18.6		$	0.1		$	4.1		$	—		$	22.7		$	0.1
Non-U.S. Governments			9			0.3			3.4			0.2			12.4			0.5
Obligations of states and political			21.1			0.1			9.2			0.3			30.3			0.4
subdivisions
Credit-Financial			103.2			0.9			8.4			0.1			111.6			1
Credit-Industrial (2)			85			1.6			8.2			—			93.2			1.6
Credit-Utility (2)			44.3			2.8			1.5			—			45.8			2.8
Structured securities:
CMO/MBS-agency (1)			5.2			—			8.1			0.3			13.3			0.3
CMBS (2)			28.4			0.1			3.5			—			31.9			0.1
ABS			74.9			0.5			8.5			0.3			83.4			0.8
Foreign denominated:
Governments			125.5			17			—			—			125.5			17
Credit			199.2			34.8			0.9			0.2			200.1			35
Total fixed maturities			714.4			58.2			55.8			1.4			770.2			59.6
Equity securities			73.3			9.1			—			—			73.3			9.1
Other investments			0.2			0.1			—			—			0.2			0.1
Short-term investments			20.7			0.3			—			—			20.7			0.3
Total		$	808.6		$	67.7		$	55.8		$	1.4		$	864.4		$	69.1
(1) Unrealized losses less than one year are less than $0.1 million.
(2) Unrealized losses one year or greater are less than $0.1 million.
31-Dec-14		Less Than One Year						One Year or Greater						Total
(in millions)		Fair			Unrealized			Fair			Unrealized			Fair			Unrealized
		Value			Losses			Value			Losses			Value			Losses
Fixed maturities
USD denominated:
U.S. Governments		$	55		$	0.2		$	15.7		$	0.1		$	70.7		$	0.3
Non-U.S. Governments			36.5			0.4			5.2			0.2			41.7			0.6
Obligations of states and political			10.4			0.1			16.6			0.2			27			0.3
subdivisions
Credit-Financial			195.7			2.2			11.1			0.1			206.8			2.3
Credit-Industrial			240.8			3.3			12.2			0.2			253			3.5
Credit-Utility			63.1			3.8			1.9			0.1			65			3.9
Structured securities:
CMO/MBS-agency			10.1			0.1			19.2			0.6			29.3			0.7
CMBS			49.3			0.1			6			0.1			55.3			0.2
ABS			129.2			0.7			8.1			0.3			137.3			1
Foreign denominated:
Governments			123.7			9.3			11.2			0.1			134.9			9.4
Credit			190.8			18.7			0.9			0.2			191.7			18.9
Total fixed maturities			1,104.60			38.9			108.1			2.2			1,212.70			41.1
Equity securities			53.6			5.1			—			—			53.6			5.1
Other investments			(0.9	)		1.3			—			—			(0.9	)		1.3
Short-term investments			—			—			—			—			—			—
Total		$	1,157.30		$	45.3		$	108.1		$	2.2		$	1,265.40		$	47.5
We regularly evaluate our investments for other than temporary impairment. For fixed maturity securities, the evaluation for a credit loss is generally based on the present value of expected cash flows of the security as compared to the amortized book value. For structured securities, frequency and severity of loss inputs are used in projecting future cash flows of the securities. Loss frequency is measured as the credit default rate, which includes such factors as loan-to-value ratios and credit scores of borrowers. For equity securities and other investments, the length of time and the amount of decline in fair value are the principal factors in determining other-than-temporary impairment. We also recognize other-than-temporary losses on fixed maturity securities that we intend to sell.
We hold a total of 6,406 securities, of which 1,830 were in an unrealized loss position for less than one year and 90 were in an unrealized loss position for a period one year or greater as of March 31, 2015. Unrealized losses greater than twelve months on fixed maturities were the result of a number of factors, including increased credit spreads, foreign currency fluctuations and higher market yields relative to the date the securities were purchased, and for structured securities, by the performance of the underlying collateral, as well. In considering whether an investment is other-then-temporarily impaired or not, we also considered that we do not intend to sell the 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. In situations where we did not recognize other-than-temporary losses on investments in our equity portfolio, we have evaluated the near-term prospects of the investment in relation to the severity and duration of the impairment and based on that evaluation, have the ability and intent to hold these investments until a recovery of the cost basis. We do not consider these investments to be other-than-temporarily impaired at March 31, 2015.
We recognized other-than-temporary losses on our fixed maturities portfolio of $0.4 million for the three months ended March 31, 2015. We recognized other-than-temporary losses on our equity portfolio of $0.1 million for the three months ended March 31, 2015. We did not recognize any other-than-temporary losses on our fixed maturities or equity portfolios for the three months ended March 31, 2014.
Realized Gains and Losses
The following table presents our gross realized investment gains (losses) and other:
		For the Three Months Ended March 31,
(in millions)		2015			2014
Realized gains
Fixed maturities		$	4.1		$	4.6
Equity securities			3.8			6.5
Other investments			20.9			12.7
Gain on sale of real estate holdings			0.3			—
Gross realized investment gains			29.1			23.8
Realized losses
Fixed maturities			(3.7	)		(3.5	)
Equity securities			(0.7	)		(0.1	)
Other investments			(7.1	)		(4.9	)
Short-term investments			(1.1	)		(0.2	)
Other assets			—			(4.0	)
Other-than-temporary impairment losses on fixed			(0.4	)		—
maturities
Other-than-temporary impairment losses on equity			(0.1	)		—
securities
Gross realized investment and other losses			(13.1	)		(12.7	)
Net realized investment gains and other		$	16		$	11.1
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments are summarized as follows:
(in millions)		Fixed			Equity			Other			Other			Real Estate Holdings			Tax			Total
		Maturities			Maturities			Investments									Effects
Three Months Ended March 31, 2015
Realized before impairments		$	0.4		$	3.1		$	13.8		$	(1.1	)	$	0.3		$	(6.1	)	$	10.4
Realized - impairments			(0.4	)		(0.1	)		—			—			—			0.2			(0.3	)
Change in unrealized			(12.3	)		(10.8	)		1.6			(0.3	)		—			2.9			(18.9	)
Three Months Ended March 31, 2014
Realized before impairments		$	1.1		$	6.4		$	7.8		$	(4.2	)	$	—		$	(3.5	)	$	7.6
Realized - impairments			—			—			—			—			—			—			—
Change in unrealized			8.1			(3.1	)		—			—			—			3.7			8.7
We enter into foreign currency exchange forward contracts to manage currency exposure on losses related to certain global catastrophe events. These currency forward contracts are carried at fair value in our Consolidated Balance Sheets in “Other investments.” The realized and unrealized gains and losses on these contracts are included in “Net realized investment and other gains” in our Consolidated Statements of Income. The notional amount of these currency forward contracts was $8.7 million and $46.6 million at March 31, 2015 and 2014, respectively. The fair value of these currency forward contracts was a gain of $0.1 million and $1.2 million at March 31, 2015 and 2014, respectively. For the three months ended March 31, 2015 and 2014, we recognized $0.3 million and $2.3 million in realized gains and $1.2 million and $1.3 million in realized losses, respectively, from these currency forward contracts.
In the third quarter of 2014, we entered into foreign currency exchange forward contracts to manage currency exposure on our Canadian dollar (“CAD”) investment portfolio. The currency forward contracts are carried at fair value in our Consolidated Balance Sheets in “Other investments.” The realized and unrealized gains and losses are included in “Net realized investment and other gains” in our Consolidated Statements of Income. The notional amount of the currency forward contracts was CAD 141.8 million (USD $112.1 million) as of March 31, 2015. The fair value of the currency forward contracts was a gain of $6.4 million as of March 31, 2015. For the three months ended March 31, 2015, we recognized $7.5 million in realized gains and $2.0 million in realized losses, respectively, from the CAD currency forward contracts.
We also enter into foreign currency exchange forward contracts to manage currency exposure on certain Euro denominated investments. The currency forward contracts are carried at fair value in our Consolidated Balance Sheets in “Other investments.” The realized and unrealized gains and losses are included in “Net realized investment and other gains” in our Consolidated Statements of Income. The fair value of the currency forward contracts was $5.8 million as of March 31, 2015. For the three months ended March 31, 2015, we recognized $6.3 million in realized gains from the currency forward contracts. The fair value of the currency forward contracts at March 31, 2014 and the amounts recognized in realized gains for the three months ended March 31, 2014 were not significant.
Regulatory Deposits, Pledged Securities and Letters of Credit
At March 31, 2015, the amortized cost and fair value of investments on deposit with U.S., Canadian and various agencies for regulatory purposes were $184.3 million and $191.8 million, respectively. At December 31, 2014, the amortized cost and fair value of investments on deposit with U.S., Canadian and various agencies for regulatory purposes were $186.6 million and $194.2 million, respectively.
At March 31, 2015, investments with an amortized cost of $48.9 million and fair value of $49.2 million were pledged as collateral in support of irrevocable letters of credit (“LOCs”) in the amount of $36.2 million issued under the terms of certain reinsurance agreements in respect of reported loss and loss expense reserves. At December 31, 2014, investments with an amortized cost of $55.0 million and fair value of $55.3 million were pledged as collateral in support of irrevocable LOCs in the amount of $43.6 million issued under the terms of certain reinsurance agreements in respect of reported loss and loss expense reserves.
Our Corporate member’s capital supporting our Lloyd’s business was $209.3 million and $217.9 million at March 31, 2015 and December 31, 2014, respectively.
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, 2015. 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 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.
—	CMO/MBS agency, CMO/MBS non-agency, CMBS, ABS residential and ABS non-residential 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, 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.
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 equity and mutual funds that are reported at fair value using Level 2 inputs. The valuations are based on the funds’ net asset value per share, determined weekly or at the end of each month. The underlying assets in the funds are valued primarily on the basis of closing market quotations or official closing prices on each valuation day.
Equity Securities Level 3: We own certain equity securities that are reported at fair value using Level 3 inputs. The valuation techniques for these securities include the following:
—	Fair value measurements are 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.
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. Foreign currency future contracts are valued by our counterparty using market driven foreign currency exchange rates and are considered Level 2 investments.
Short-term Investments: Short-term investments are principally reported at fair value using Level 1 inputs, with the exception of short-term corporate 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, 2015.
Based on an analysis of the inputs, our financial assets measured at fair value on a recurring basis have been categorized as follows:
					Fair Value Measurements at Reporting Date Using
(in millions)		31-Mar-15			Level 1 (a)			Level 2 (b)			Level 3 (c)
Fixed maturities
USD denominated:
U.S. Governments		$	182.8		$	102		$	80.8		$	—
Non-U.S. Governments			81.5			—			81.5			—
Obligations of states and political subdivisions			480			—			480			—
Credit-Financial			484.7			—			484.7			—
Credit-Industrial			469.9			—			469.9			—
Credit-Utility			147.2			—			147.2			—
Structured securities:
CMO/MBS-agency			168.7			—			168.7			—
CMO/MBS-non agency			13.4			—			13.4			—
CMBS			186			—			186			—
ABS			223.4			—			223.4			—
Foreign denominated:
Governments			141.6			—			141.6			—
Credit			209.4			—			209.4			—
Total fixed maturities			2,788.60			102			2,686.60			—
Equity securities			483.4			482.6			—			0.8
Other investments			104.9			—			104.9			—
Short-term investments			271.5			271.1			0.4			—
		$	3,648.40		$	855.7		$	2,791.90		$	0.8
(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)		31-Dec-14			Level 1 (a)			Level 2 (b)			Level 3 (c)
Fixed maturities
USD denominated:
U.S. Governments		$	185		$	99.2		$	85.8		$	—
Non-U.S. Governments			79.9			—			79.9			—
Obligations of states and political subdivisions			490.7			—			490.7			—
Credit-Financial			518.1			—			518.1			—
Credit-Industrial			499.6			—			499.6			—
Credit-Utility			142			—			142			—
Structured securities:
CMO/MBS-agency			175.3			—			175.3			—
CMO/MBS-non agency			14			—			14			—
CMBS			180			—			180			—
ABS			221			—			221			—
Foreign denominated:
Governments			140.2			—			140.2			—
Credit			194.9			—			194.9			—
Total fixed maturities			2,840.70			99.2			2,741.50			—
Equity securities			486.3			485.4			—			0.9
Other investments			97.3			—			97.3			—
Short-term investments			275.8			273.9			1.9			—
		$	3,700.10		$	858.5		$	2,840.70		$	0.9
(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)		Equity			Total
		Securities
Beginning balance, January 1, 2015		$	0.9		$	0.9
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			(0.1	)		(0.1	)
Settlements			—			—
Ending balance, March 31, 2015		$	0.8		$	0.8
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 March 31, 2015
(in millions)		Equity			Other			Total
		Securities			Assets
Beginning balance, January 1, 2014		$	1.3		$	2.6		$	3.9
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			—			0.1
Purchases, issuances, sales, and settlements
Purchases			—			—			—
Issuances			—			—			—
Sales			(0.5	)		—			(0.5	)
Settlements			—			(2.6	)		(2.6	)
Ending balance, December 31, 2014		$	0.9		$	—		$	0.9
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 December 31, 2014
At March 31, 2015 and December 31, 2014, 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_A

Reserves for Losses and Loss Adjustment Expenses	3 Months Ended
	Mar. 31, 2015
Insurance [Abstract]
Reserves for Losses and Loss Adjustment Expenses	4	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)		2015			2014
	Net reserves beginning of the year		$	2,137.10		$	2,107.60
	Add:
	Losses and LAE incurred during current calendar
	year, net of reinsurance:
	Current accident year			187.4			191.4
	Prior accident years			(3.7	)		(8.9	)
	Losses and LAE incurred during calendar year, net			183.7			182.5
	of reinsurance
	Deduct:
	Losses and LAE payments made during current
	calendar year, net of reinsurance:
	Current accident year			19.8			27.9
	Prior accident years			141			150.1
	Losses and LAE payments made during current			160.8			178
	calendar year, net of reinsurance:
	Change in participation interest (1)			(1.2	)		24.7
	Foreign exchange adjustments			(6.0	)		1.7
	Net reserves - end of period			2,152.80			2,138.50
	Add:
	Reinsurance recoverables on unpaid losses and LAE,			882.3			903.9
	end of period
	Gross reserves - end of period		$	3,035.10		$	3,042.40
	(1)	Amount represents (decreases) increases in reserves due to change in syndicate participation
	Reserves for losses and LAE represent the estimated indemnity cost and related adjustment expenses necessary to investigate and settle claims. Such estimates are based upon individual case estimates for reported claims, estimates from ceding companies for reinsurance assumed and actuarial estimates for losses that have been incurred but not yet reported to the insurer. Any change in probable ultimate liabilities is reflected in current operating results.
	Impacting losses and LAE for the three months ended March 31, 2015 was $3.7 million in favorable prior years’ loss reserve development comprised of the following: $8.2 million of net favorable development in the Excess and Surplus Lines segment primarily caused by favorable development in the general and products liability lines and commercial automobile, partially offset by unfavorable development in property lines; $7.2 million of net unfavorable development in the Commercial Specialty segment, primarily driven by unfavorable development in general liability due to increases in claim severity, as well as unfavorable development in short tail lines and workers compensation, partially offset by favorable development in auto liability; $2.5 million of net favorable development in the International Specialty segment primarily driven by favorable development in Argo Re and long-tail lines, partially offset by unfavorable development in the Brazil unit; $0.3 million of net favorable development in the Syndicate 1200 segment primarily driven by favorable development in various property classes, partially offset by unfavorable development in general liability; and $0.1 million of unfavorable development in the Run-off Lines segment primarily caused by unfavorable development in workers compensation lines and assumed asbestos and environmental liability, offset in part by favorable development in run-off reinsurance claims.
	Impacting losses and LAE for the three months ended March 31, 2014 was $8.9 million in favorable prior years’ loss reserve development comprised of the following: $8.0 million of net favorable development in the Excess and Surplus Lines segment primarily caused by favorable development of $21.6 million in the general and products liability lines, partially offset by $9.4 million unfavorable development in commercial automobile and $4.2 million of unfavorable development in property lines; $2.0 million of net unfavorable development in the Commercial Specialty segment, primarily driven by $6.7 million of unfavorable development in general liability due to increases in claim severity, partially offset by $4.3 million of favorable development in workers compensation and $1.1 million of favorable development in short-tail lines; $0.4 million of net unfavorable development in the International Specialty segment; $8.8 million of net favorable development in the Syndicate 1200 segment primarily driven by favorable development in marine and energy lines, as well as favorable development in professional indemnity and general liability; and $5.5 million of unfavorable development in the Run-off Lines segment primarily caused by $4.5 million of unfavorable development in workers compensation lines driven by increasing medical costs on older claims, as well as $1.0 million unfavorable development in asbestos liability on assumed business.
	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 loss development, favorable or unfavorable, will not occur.

Debt

Debt	3 Months Ended
	Mar. 31, 2015
Debt Disclosure [Abstract]
Debt	5	Debt
	Borrowing Under Revolving Credit Facility
	On March 7, 2014, each of Argo Group, Argo Group US, Inc., Argo International Holdings Limited and Argo Underwriting Agency Limited (the “Borrowers”) entered into a $175.0 million credit agreement (“Credit Agreement”) with JPMorgan Chase Bank, N.A., as administrative agent. The Credit Agreement provides for a $175.0 million revolving credit facility with a maturity date of March 7, 2018 unless extended in accordance with the terms of the Credit Agreement. Borrowings under the Credit Agreement may be used for general corporate purposes, including working capital and permitted acquisitions, and each of the Borrowers has agreed to be jointly and severally liable for the obligations of the other Borrowers under the Credit Agreement.
	The Credit Agreement contains customary events of default. If an event of default occurs and is continuing, the Borrowers might be required immediately to repay all amounts outstanding under the Credit Agreement. Lenders holding at least a majority of the loans and commitments under the Credit Agreement may elect to accelerate the maturity of the loans and/or terminate the commitments under the Credit Agreement upon the occurrence and during the continuation of an event of default.
	Included in the Credit Agreement is a provision that allows up to $17.5 million of the revolving credit facility to be used for LOCs, subject to availability. As of March 31, 2015 and December 31, 2014, there were no borrowings outstanding and $0.2 million in LOCs against the Credit Facility.
	Junior Subordinated Debentures
	On July 16, 2014, Argo Group US, Inc. purchased the outstanding PXRE Capital Trust V $20,000,000 Junior Subordinated Debt Securities (“Capital Trust V”) at a discount equal to 90.0% of the principal amount plus accrued and unpaid interest through the date of purchase for a total price of $18.2 million, resulting in the recognition of a $2.0 million pre-tax realized gain. In accordance with ASC Topic 405, “Liabilities,” we have eliminated the outstanding debt, associated interest, and intercompany balances related to the purchase of the Capital Trust V since the purchase by Argo Group US, Inc., in essence, reduces our outstanding debt related to the Junior Subordinated Debentures. As of March 31, 2015, the Capital Trust V had not yet been redeemed and continued to accrue interest. The Capital Trust V was redeemed on April 29, 2015.

Disclosures_about_Fair_Value_o

Disclosures about Fair Value of Financial Instruments	3 Months Ended
	Mar. 31, 2015
Fair Value Disclosures [Abstract]
Disclosures about Fair Value of Financial Instruments	6	Disclosures about Fair Value of Financial Instruments
	Cash. The carrying amount approximates fair value.
	Investment securities and short-term investments. See Note 3, “Investments,” for additional information.
	Premiums receivable and reinsurance recoverables on paid losses. The carrying value of current receivables approximates fair value. At March 31, 2015 and December 31, 2014, the carrying values of premiums receivable over 90 days were $12.8 million and $12.4 million, respectively. Included in “Reinsurance recoverables” in our Consolidated Balance Sheets at March 31, 2015 and December 31, 2014, are amounts that are due from trade capital providers associated with the operations of ArgoGlobal Syndicate 1200. Upon settlement, the receivable is offset against the liability also reflected in our accompanying Consolidated Balance Sheets. At March 31, 2015 and December 31, 2014, the payable was in excess of the receivable. Of our reinsurance recoverables on paid losses, excluding amounts attributable to ArgoGlobal Syndicate 1200’s trade capital providers, at March 31, 2015 and December 31, 2014, the carrying values over 90 days were $6.8 million and $9.9 million, respectively. Our methodology for establishing our allowances for doubtful accounts includes specifically identifying all potential uncollectible balances regardless of aging. At March 31, 2015 and December 31, 2014, the allowance for doubtful accounts for premiums receivable was $4.5 million and $5.2 million, respectively, and the allowance for doubtful accounts for reinsurance recoverables on paid losses was $2.1 million and $1.8 million, respectively. Premiums receivable over 90 days were secured by collateral in the amount of $0.3 million and $0.3 million at March 31, 2015 and December 31, 2014, respectively. Reinsurance recoverables on paid losses over 90 days were secured by collateral in the amount of $0.7 million and $0.4 at March 31, 2015 and December 31, 2014, respectively.
	At March 31, 2015 and December 31, 2014, 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.
	A summary of our financial instruments whose carrying value did not equal fair value is shown below:
			31-Mar-15				31-Dec-14
	(in millions)		Carrying		Fair		Carrying		Fair
			Amount		Value		Amount		Value
	Junior subordinated debentures		$	172.7	$	155.5	$	172.7	$	155.5
	Senior unsecured fixed rate notes			143.8		132.3		143.8		132.3
	Other indebtedness:
	Floating rate loan stock			56.9		51.2		61.3		55.2
	Note payable			0.6		0.6		0.7		0.6
			$	374	$	339.6	$	378.5	$	343.6

Shareholders_Equity

Shareholders' Equity	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Shareholders' Equity	7	Shareholders’ Equity
	On May 5, 2015, our Board of Directors declared a quarterly cash dividend in the amount of $0.20 on each share of common stock outstanding. The dividend will be paid on June 15, 2015 to shareholders of record at the close of business on June 1, 2015.
	On February 17, 2015, our Board of Directors declared a 10% stock dividend, payable on March 16, 2015, to shareholders of record at the close of business on March 2, 2015. As a result of the stock dividend, 2,554,506 additional shares were issued. Cash was paid in lieu of fractional shares of our common shares. All references to share and per share amounts in this document and related disclosures have been adjusted to reflect the stock dividend for all periods presented.
	In addition, on February 17, 2015, our Board of Directors declared a quarterly cash dividend in the amount of $0.20 on each share of common stock outstanding, on a post-stock dividend basis. On March 19, 2015, we paid $5.7 million to our shareholders of record on March 2, 2015.
	On February 18, 2014, our Board of Directors declared a quarterly cash dividend in the amount of $0.15 on each share of common stock outstanding or $0.14 on each share outstanding adjusted for the 10% stock dividend. On March 17, 2014, we paid $4.0 million to our shareholders of record on March 3, 2014.
	On November 5, 2013, our Board of Directors authorized the repurchase of up to $150.0 million of our common shares (“2013 Repurchase Authorization”). The 2013 Repurchase Authorization supersedes all the previous Repurchase Authorizations. As of March 31, 2015, availability under the 2013 Repurchase Authorization for future repurchases of our common shares was $74.7 million.
	For the three months ended March 31, 2015, we repurchased a total of 353,054 common shares for $18.1 million. A summary of activity for shares repurchased on the open market and under Security Exchange Act of 1934 Rule 10b5-1 from January 1, 2015 through March 31, 2015 is described below.
	For the three months ended March 31, 2015, we purchased 196,864 common shares on the open market for $9.9 million. In 2015, we repurchased shares under Securities Exchange Act of 1934 Rule 10b5-1 trading plans as follows:
	Date		2015	Number of		Average Price		Total Cost		Repurchase
	Trading Plan		Purchase	Shares		of Shares		(in millions)		Authorization
	Initiated		Period	Repurchased		Repurchased				Year
	12/15/14		01/05/2015-02/12/2015		117,482	$	53.5	$	6.3	2013
	3/16/15		03/18/2015-03/31/2015		38,708	$	49.65	$	1.9	2013

Accumulated_Other_Comprehensiv

Accumulated Other Comprehensive Income (Loss)	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Accumulated Other Comprehensive Income (Loss)	8	Accumulated Other Comprehensive Income (Loss)
	A summary of changes in accumulated other comprehensive income (loss), net of taxes (where applicable) by component for the three months ended March 31, 2015 and 2014 is presented below:
	(in millions)		Foreign Currency Translation Adjustments			Unrealized			Defined Benefit Pension Plans			Total
						Holding Gains
						on Securities
	Balance, January 1, 2015		$	(15.6	)	$	130.7		$	(7.0	)	$	108.1
	Other comprehensive loss before			(3.1	)		(21.5	)		—			(24.6	)
	reclassifications
	Amounts reclassified from accumulated other			—			2.6			—			2.6
	comprehensive income (loss)
	Net current-period other comprehensive loss			(3.1	)		(18.9	)		—			(22.0	)
	Balance at March 31, 2015		$	(18.7	)	$	111.8		$	(7.0	)	$	86.1
	(in millions)		Foreign Currency Translation Adjustments			Unrealized			Defined Benefit Pension Plans			Total
						Holding Gains
						on Securities
	Balance, January 1, 2014		$	(11.5	)	$	163.9		$	(4.6	)	$	147.8
	Other comprehensive (loss) income before			(0.2	)		1.3			—			1.1
	reclassifications
	Amounts reclassified from accumulated other			—			—			—			—
	comprehensive income (loss)
	Net current-period other comprehensive (loss)			(0.2	)		1.3			-			1.1
	income
	Balance at March 31, 2014		$	(11.7	)	$	165.2		$	(4.6	)	$	148.9
	The amounts reclassified from accumulated other comprehensive income (loss) shown in the above table have been included in the following captions in our Consolidated Statements of Income:
			For the Three Months Ended March 31,
	(in millions)		2015			2014
	Unrealized gains and losses on securities:
	Net realized investment gains		$	2.5		$	(0.1	)
	Provision for income taxes			0.1			0.1
	Net of taxes		$	2.6		$	—

Net_Income_Per_Common_Share

Net Income Per Common Share	3 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Net Income Per Common Share	9	Net Income Per Common Share
	The following table presents the calculation of net income per common share on a basic and diluted basis (all balances have been adjusted to reflect the 10% stock dividend):
			For the Three Months Ended March 31,
	(in millions, except number of shares and per share amounts)		2015		2014
	Net income		$	58.8	$	40.2
	Weighted average common shares outstanding - basic			28,129,692		29,165,911
	Effect of dilutive securities:
	Equity compensation awards			547,687		497,179
	Weighted average common shares outstanding - diluted			28,677,379		29,663,090
	Net income per common share:
	Basic		$	2.09	$	1.38
	Diluted		$	2.05	$	1.36
	Excluded from the weighted average common shares outstanding calculation at March 31, 2015, and 2014 are 8,959,543 shares and 7,723,529 shares, respectively, which are held as treasury shares. The shares are excluded as of their repurchase date. For the three months ended March 31, 2015, equity compensation awards to purchase 1,760 shares of common stock were excluded from the computation of diluted net income per common share as these instruments were anti-dilutive. These instruments expired during the second quarter 2015. For the three months ended March 31, 2014, equity compensation awards to purchase 3,300 shares of common stock were excluded from the computation of diluted net income per common share as these instruments were anti-dilutive. These instruments expired or will expire at varying times from 2014 through 2020.

Supplemental_Cash_Flow_Informa

Supplemental Cash Flow Information	3 Months Ended
	Mar. 31, 2015
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information	10	Supplemental Cash Flow Information
	Income taxes paid. We paid income taxes of $0.5 million and $15.6 million during the three months ended March 31, 2015 and 2014, respectively.
	Income taxes recovered. During the three months ended March 31, 2015, $11.6 million of income taxes was recovered. No income taxes were recovered during the three months ended March 31, 2014.
	Interest paid.
			For the Three Months Ended March 31,
	(in millions)		2015		2014
	Senior unsecured fixed rate notes		$	2.3	$	2.3
	Junior subordinated debentures			1.7		2
	Other indebtedness			0.8		0.9
	Total interest paid		$	4.8	$	5.2

Sharebased_Compensation

Share-based Compensation	3 Months Ended
	Mar. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Share-based Compensation	11	Share-based Compensation
	The fair value method of accounting is used for equity-based compensation plans. Under the fair value method, compensation cost is measured based on the fair value of the award at the measurement date and recognized over the requisite service period. We use the Black-Scholes model to estimate the fair values on the measurement date for share options and share appreciation rights (“SARs”). The Black-Scholes model uses several assumptions to value a share award. The volatility assumption is based on the historical change in our stock price over the previous five years preceding the measurement date. The risk-free rate of return assumption is based on the five-year U.S. Treasury constant maturity rate on the measurement date. The expected award life is based upon the average holding period over the history of the incentive plan. The expected dividend yield is based on our history and expected dividend payouts.
	The following table summarizes the assumptions we used for the three months ended March 31, 2015 and 2014:
			For the Three Months Ended March 31,
			2015			2014
	Risk-free rate of return			1.47%			1.74%
	Expected dividend yields			1.63%			1.49%
	Expected award life (years)			4.71		4.85
	Expected volatility			22.09%			24.91%
	All outstanding awards were adjusted to reflect the 10% stock dividend, resulting in a 10% increase to the number of awards outstanding and a 9.09% 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, 1,457,800 shares remained available for grant under the 2007 Plan. In May 2014, our shareholders approved the 2014 Long-Term Incentive Plan (the “2014 Plan”), which provides for an additional 2.8 million shares of our common stock to be available for issuance to executives, non-employee directors and other key employees. The share awards may be in the form of share options, SARs, restricted shares, restricted share awards, restricted share units awards, performance awards, other share-based awards and other cash-based awards. Shares issued under this plan may be shares that are authorized and unissued or shares that we reacquired, including shares purchased on the open market. Share options and SARs will count as one share for the purposes of the limits under the incentive plans; restricted shares, restricted share units, performance units, performance shares or other share-based incentive awards which settle in common shares will count as 2.75 shares for purpose of the limits under the 2014 Plan.
	Share options may be in the form of incentive share options, non-qualified share options and restorative options. Share options are required to have an exercise price that is not less than the market value on the date of grant. We are prohibited from repricing the options. The term of the share options cannot exceed seven years from the grant date.
	A summary of restricted share activity as of March 31, 2015 and changes during the three months then ended is as follows:
			Shares			Weighted-Average
						Grant Date
						Fair Value
	Outstanding at January 1, 2015			313,716		$	35.17
	Granted			51,873		$	46.57
	Vested and issued			(110,982	)	$	29.88
	Expired or forfeited			(3,569	)	$	36.38
	Outstanding at March 31, 2015			251,038		$	39.85
	The restricted shares vest over two to four years. Expense recognized under this plan for the restricted shares was $1.5 million and $0.7 million for the three months ended March 31, 2015 and 2014, respectively. Compensation expense for all share-based compensation awards is included in “Underwriting, acquisition and insurance expense” in the accompanying Consolidated Statements of Income. As of March 31, 2015, there was $8.0 million of total unrecognized compensation cost related to restricted share compensation arrangements granted by Argo Group.
	A summary of stock-settled SARs activity as of March 31, 2015 and changes during the three months then ended is as follows:
			Shares			Weighted-Average
						Exercise Price
	Outstanding at January 1, 2015			1,313,901		$	28.65
	Granted			243,306		$	37.33
	Exercised			(24,622	)	$	28.11
	Expired or forfeited			(9,210	)	$	30.68
	Outstanding at March 31, 2015			1,523,375		$	29.26
	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 $0.6 million for the three months ended March 31, 2015 and 2014. As of March 31, 2015, there was $3.9 million of total unrecognized compensation cost related to stock-settled SARs outstanding.
	A summary of cash-settled SARs activity as of March 31, 2015 and changes during the three months then ended is as follows:
			Shares			Weighted-Average
						Exercise Price
	Outstanding at January 1, 2015			2,001,451		$	32.76
	Granted			866,673		$	47.83
	Exercised			(31,461	)	$	29.77
	Expired or forfeited			(70,547	)	$	36.28
	Outstanding at March 31, 2015			2,766,116		$	37.42
	The cash-settled SARs vest over a one to four year period. 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-settled SARs was $2.7 million for the three months ended March 31, 2015 and 2014. As of March 31, 2015, there was $14.5 million of total unrecognized compensation cost related to cash-settled SARs outstanding.

Underwriting_Acquisition_and_I

Underwriting, Acquisition and Insurance Expenses	3 Months Ended
	Mar. 31, 2015
Text Block [Abstract]
Underwriting, Acquisition and Insurance Expenses	12	Underwriting, Acquisition and Insurance Expenses
	Underwriting, acquisition and insurance expenses were as follows:
			For the Three Months Ended March 31,
	(in millions)		2015			2014
	Commissions		$	58.7		$	54.9
	General expenses			72.9			68.8
	Premium taxes, boards and bureaus			0.6			6.6
				132.2			130.3
	Net deferral of policy acquisition costs			(2.6	)		(1.6	)
	Total underwriting, acquisition and insurance expenses		$	129.6		$	128.7
	Included in general expenses for the three months ended March 31, 2015 and 2014 is $4.9 million and $4.3 million, respectively, of expense for our total equity compensation.

Income_Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2015
Income Tax Disclosure [Abstract]
Income Taxes	13	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. Six 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. Corporate income tax losses incurred in the United Kingdom can be carried forward, for application against future income, indefinitely. 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 file a consolidated United States federal income tax return.
	We also have operations in Belgium, Switzerland, Brazil, France, Malta, Spain and Ireland, which also are subject to income taxes imposed by the jurisdiction in which they operate. We have operations in the United Arab Emirates, which are not subject to income tax under the laws of that country.
	Our income tax provision includes the following components:
			For the Three Months Ended March 31,
	(in millions)		2015				2014
	Current tax (benefit) provision		$	(0.2	)		$	1.4
	Deferred tax provision related to:
	Future tax deductions			5.5				30.2
	Valuation allowance change			(1.7	)			(29.1	)
	Income tax provision		$	3.6			$	2.5
	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, 2015 and 2014, 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)		2015							2014
			Pre-Tax				Effective			Pre-Tax				Effective
			Income (Loss)				Tax			Income (Loss)				Tax
							Rate							Rate
	Bermuda		$	25.7				0	%	$	21.9				0	%
	United States			28.5				20.2	%		14.8				20.6	%
	United Kingdom			10.1				-21.9	%		7.3				-7.7	%
	Belgium			(0.2	)			-4.8	%		—		-1		18.1	%
	Brazil			(1.9	)			0	%		—				0	%
	United Arab Emirates			0.1				0	%		(0.9	)			0	%
	Ireland			(0.2	)			0	%		—				0	%
	Malta			0.3				0	%		(0.4	)			0	%
	Switzerland			—		-1		21	%		—		-1		-19.8	%
	Pre-tax income		$	62.4						$	42.7
	(1)	Pre-tax income for the respective year was less than $0.1 million.
	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)		2015				2014
	Income tax provision at expected rate		$	11.2			$	4.7
	Tax effect of:
	Tax-exempt interest			(0.9	)			(1.2	)
	Dividends received deduction			(0.6	)			(0.6	)
	Valuation allowance change			(1.7	)			(29.1	)
	Other permanent adjustments, net			0.2				0.1
	Adjustment for annualized rate			(0.5	)			0.8
	United States state tax (benefit) expense			(2.5	)			0.1
	Capital loss carryforward from prior merger			—				29.8
	Other foreign adjustments			(0.3	)			—
	Prior period adjustments to deferred			2.2				—
	Prior year foreign taxes recovered			(1.2	)			—
	Foreign exchange adjustments			(2.4	)			(2.2	)
	Foreign withholding taxes			0.1				0.1
	Income tax provision		$	3.6			$	2.5
	Income tax benefit - Foreign		$	(2.2	)		$	(0.6	)
	Income tax provision - United States, Federal			9.5				2.8
	Income tax (benefit) provision  - United States, state			(3.8	)			0.2
	Foreign withholding tax - United States			0.1				0.1
	Income tax provision		$	3.6			$	2.5
	We recognize potential accrued interest and penalties within our global operations in “Interest expense” and “Underwriting, acquisition and insurance expenses,” respectively, in our Consolidated Statements of Income.
	Our net deferred tax assets (liabilities) are supported by taxes paid in previous periods, reversal of taxable temporary differences and recognition of future 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 sufficient taxable income in the future to recover tax benefits that cannot be recovered from taxes paid in the carryback period, which is generally two years for net operating losses and three years for capital losses for our United States operations. At March 31, 2015, we had a total net deferred tax liability of $30.0 million prior to any valuation allowance. Management has concluded that a valuation allowance is required for a portion of the tax effected net capital loss carryforward of $17.8 million generated from a prior merger and for the tax effected net operating loss carryforward of $1.0 million from ARIS. Of the capital loss carryforwards from a prior merger, $16.3 million will expire if not used by December 31, 2025 and $1.5 million will expire if not used by December 31, 2027. Of the ARIS loss carryforward, $0.2 million will expire if not used by December 31, 2027, $0.4 million will expire if not used by December 31, 2028 and $0.4 million will expire if not used by December 31, 2029. The valuation allowances have been established as Internal Revenue Code Section 382 limits the application of net operating loss and net capital loss carryforwards following an ownership change. The loss carryforwards available per year are $2.8 million as required by Internal Revenue Code Section 382. Further, due to cumulative losses 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. Accordingly, a valuation allowance of $23.8 million is required as of March 31, 2015 of which $15.9 million relates to the prior merger and ARIS loss carryforwards, $6.1 million relates to Brazil operations, and $1.8 million relates to Malta operations. For the three months ended March 31, 2015, the valuation allowance was reduced by $0.1 million pertaining to the prior merger and ARIS loss carryforwards, decreased by $1.3 million pertaining to our Brazil operations and decreased by $0.3 million pertaining to our Malta operations.
	We had no material unrecognized tax benefits as of March 31, 2015 and 2014. Our United States subsidiaries are no longer subject to U.S. federal and state income tax examinations by tax authorities for years before 2011. Our United Kingdom subsidiaries are no longer subject to United Kingdom income tax examinations by Her Majesty’s Revenue and Customs for years before 2012.

Commitments_and_Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2015
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	14	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 $60.6 million related to our limited partnership investments at March 31, 2015. These commitments will be funded as required by the partnership agreements.

Segment_Information

Segment Information	3 Months Ended
	Mar. 31, 2015
Segment Reporting [Abstract]
Segment Information	15	Segment Information
	We are primarily engaged in underwriting property and casualty insurance and reinsurance. We have four ongoing reporting segments: Excess and Surplus Lines, Commercial Specialty, International Specialty and Syndicate 1200. 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.
	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 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)		2015			2014
	Revenue:
	Earned premiums
	Excess and Surplus Lines		$	123.1		$	117.4
	Commercial Specialty			72.6			71.7
	International Specialty			37.3			37
	Syndicate 1200			101.9			99.3
	Run-off Lines			(0.3	)		0.3
	Total earned premiums			334.6			325.7
	Net investment income
	Excess and Surplus Lines			8.5			9.3
	Commercial Specialty			4.5			4.7
	International Specialty			2.9			1.8
	Syndicate 1200			2.2			3.7
	Run-off Lines			2			2.5
	Corporate and Other			0.7			1.3
	Total net investment income			20.8			23.3
	Net realized investment and other gains			16			11.1
	Total revenue		$	371.4		$	360.1
			For the Three Months
			Ended March 31,
	(in millions)		2015			2014
	Income (loss) before income taxes
	Excess and Surplus Lines		$	24.7		$	20.1
	Commercial Specialty			3.8			1.7
	International Specialty			9.1			6.8
	Syndicate 1200			11			15.8
	Run-off Lines			1.2			(5.9	)
	Total segment income before taxes			49.8			38.5
	Corporate and Other			(3.4	)		(6.9	)
	Net realized investment and other gains			16			11.1
	Total income before income taxes		$	62.4		$	42.7
	The table below presents earned premiums by geographic location for the three months ended March 31, 2015 and 2014. 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)		2015			2014
	Bermuda		$	23.9		$	24.4
	Brazil			12.5			12.6
	Malta			0.5			0.5
	United Kingdom			101.9			98.9
	United States			195.8			189.3
	Total earned premiums		$	334.6		$	325.7
	The following table represents identifiable assets:
			March 31,			December 31,
	(in millions)		2015			2014
	Excess and Surplus Lines		$	2,352.80		$	2,350.70
	Commercial Specialty			1,384.00			1,358.50
	International Specialty			799.5			776.8
	Syndicate 1200			1,232.50			1,258.50
	Run-off Lines			557.8			552.8
	Corporate and Other			50.5			59
	Total		$	6,377.10		$	6,356.30
	Included in total assets at March 31, 2015 and December 31, 2014 are $294.2 million and $315.4 million, respectively, in assets associated with trade capital providers.

Information_Provided_in_Connec

Information Provided in Connection with Outstanding Debt of Subsidiaries	3 Months Ended
	Mar. 31, 2015
Debt Disclosure [Abstract]
Information Provided in Connection with Outstanding Debt of Subsidiaries	16	Information Provided in Connection with Outstanding Debt of Subsidiaries
	The following tables present condensed consolidating financial information at March  31, 2015 and December 31, 2014 and for the three months ended March 31, 2015 and 2014, for Argo Group (the “Parent Guarantor”) and Argo Group US (the “Subsidiary Issuer”). The Subsidiary Issuer is an indirect 100 percent-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
	31-Mar-15
	(in millions)
			Argo Group			Argo Group US, Inc.			Other Subsidiaries			Consolidating			Total
			International			and Subsidiaries			and Eliminations (1)			Adjustments (2)
			Holdings, Ltd			(Subsidiary Issuer)
			(Parent Guarantor)
	Assets
	Investments		$	7.2		$	2,851.40		$	1,205.80		$	—		$	4,064.40
	Cash			—			54.8			34.3			—			89.1
	Accrued investment income			—			17.1			3.8			—			20.9
	Premiums receivable			—			160			211.7			—			371.7
	Reinsurance recoverables			—			1,152.00			(164.7	)		—			987.3
	Goodwill and other intangible assets, net			—			132.7			98.3			—			231
	Current income taxes receivable, net			—			—			4.2			—			4.2
	Deferred acquisition costs, net			—			60			69.7			—			129.7
	Ceded unearned premiums			—			112.2			144.4			—			256.6
	Other assets			12.7			175.4			59.1			(25.0	)		222.2
	Due from (to) affiliates			(16.0	)		(19.3	)		19.3			16			—
	Intercompany note receivable			—			55.6			(55.6	)		—			—
	Investments in subsidiaries			1,722.80			—			—			(1,722.8	)		—
	Total assets		$	1,726.70		$	4,751.90		$	1,630.30		$	(1,731.8	)	$	6,377.10
	Liabilities and Shareholders' Equity
	Reserves for losses and loss adjustment expenses		$	—		$	2,138.80		$	896.3		$	—		$	3,035.10
	Unearned premiums			—			458.8			367.6			—			826.4
	Funds held and ceded reinsurance payable, net			—			662.8			(402.3	)		—			260.5
	Long-term debt			49			288.7			56.9			(20.6	)		374
	Current income taxes payable, net			—			0.5			(0.5	)		—			—
	Deferred tax liabilities, net			—			44.7			9.1			—			53.8
	Accrued underwriting expenses and other liabilities			14.9			91.7			57.9			—			164.5
	Total liabilities			63.9			3,686.00			985			(20.6	)		4,714.30
	Total shareholders' equity			1,662.80			1,065.90			645.3			(1,711.2	)		1,662.80
	Total liabilities and shareholders' equity		$	1,726.70		$	4,751.90		$	1,630.30		$	(1,731.8	)	$	6,377.10
	CONDENSED CONSOLIDATING BALANCE SHEET
	31-Dec-14
	(in millions)
			Argo Group			Argo Group US, Inc.			Other Subsidiaries			Consolidating			Total
			International			and Subsidiaries			and Eliminations (1)			Adjustments (2)
			Holdings, Ltd			(Subsidiary Issuer)
			(Parent Guarantor)
	Assets
	Investments		$	0.7		$	2,841.50		$	1,255.70		$	—		$	4,097.90
	Cash			—			49.3			31.7			—			81
	Accrued investment income			—			17.8			4.3			—			22.1
	Premiums receivable			—			154.6			199			—			353.6
	Reinsurance recoverables			—			1,173.60			(176.4	)		—			997.2
	Goodwill and other intangible assets, net			—			131.7			99.1			—			230.8
	Current income taxes receivable, net			—			10.1			4.8			—			14.9
	Deferred acquisition costs, net			—			58			66.6			—			124.6
	Ceded unearned premiums			—			98.5			109.1			—			207.6
	Other assets			9.6			174.1			67.9			(25.0	)		226.6
	Due from (to) affiliates			2.9			(19.8	)		19.8			(2.9	)		—
	Intercompany note receivable			—			72			(72.0	)		—			—
	Investments in subsidiaries			1,698.00			—			—			(1,698.0	)		—
	Total assets		$	1,711.20		$	4,761.40		$	1,609.60		$	(1,725.9	)	$	6,356.30
	Liabilities and Shareholders' Equity
	Reserves for losses and loss adjustment expenses		$	—		$	2,136.40		$	906		$	—		$	3,042.40
	Unearned premiums			—			448.9			368.3			—			817.2
	Funds held and ceded reinsurance payable, net			—			675.1			(441.3	)		—			233.8
	Long-term debt			49			288.7			61.4			(20.6	)		378.5
	Deferred tax liabilities, net			—			41.2			11.8			—			53
Accrued underwriting expenses and other liabilities			15.5			104.2			65			—			184.7
Total liabilities			64.5			3,694.50			971.2			(20.6	)		4,709.60
Total shareholders' equity			1,646.70			1,066.90			638.4			(1,705.3	)		1,646.70
Total liabilities and shareholders' equity		$	1,711.20		$	4,761.40		$	1,609.60		$	(1,725.9	)	$	6,356.30
(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, 2015
(in millions)
		Argo Group			Argo Group US, Inc.			Other Subsidiaries			Consolidating			Total
		International			and Subsidiaries			and Eliminations (1)			Adjustments (2)
		Holdings, Ltd			(Subsidiary Issuer)
		(Parent Guarantor)
Premiums and other revenue:
Earned premiums		$	—		$	135.1		$	199.5		$	—		$	334.6
Net investment (expense) income			(0.2	)		14.5			6.5			—			20.8
Net realized investment gains and other			—			15.9			0.1			—			16
Total revenue			(0.2	)		165.5			206.1			—			371.4
Expenses:
Losses and loss adjustment expenses			—			80.2			103.5			—			183.7
Underwriting, acquisition and insurance			4.7			51.9			73			—			129.6
expenses
Interest expense			0.5			3.8			0.6			—			4.9
Fee expense (income), net			—			0.8			(0.4	)		—			0.4
Foreign currency exchange loss (gain)			—			0.8			(10.4	)		—			(9.6	)
Total expenses			5.2			137.5			166.3			—			309
Income before income taxes			(5.4	)		28			39.8			—			62.4
Provision for income taxes			—			5.8			(2.2	)		—			3.6
Net income before equity in earnings of			(5.4	)		22.2			42			—			58.8
subsidiaries
Equity in undistributed earnings of			64.2			—			—			(64.2	)		—
subsidiaries
Net income		$	58.8		$	22.2		$	42		$	(64.2	)	$	58.8
(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, 2014
(in millions)
		Argo Group			Argo Group US, Inc.			Other Subsidiaries			Consolidating			Total
		International			and Subsidiaries			and Eliminations (1)			Adjustments (2)
		Holdings, Ltd			(Subsidiary Issuer)
		(Parent Guarantor)
Premiums and other revenue:
Earned premiums		$	—		$	112		$	213.7		$	—		$	325.7
Net investment (expense) income			(0.1	)		15			8.4			—			23.3
Net realized investment and other gains (loss)			—			11.5			(0.4	)		—			11.1
Total revenue			(0.1	)		138.5			221.7			—			360.1
Expenses:
Losses and loss adjustment expenses			—			72.5			110			—			182.5
Underwriting, acquisition and insurance			6			46.3			76.4			—			128.7
expenses
Interest expense			0.6			3.8			0.7			(0.1	)		5
Fee expense, net			—			1.3			0.1			—			1.4
Foreign currency exchange (gain) loss			—			(0.1	)		(0.1	)		—			(0.2	)
Total expenses			6.6			123.8			187.1			(0.1	)		317.4
Income (loss) before income taxes			(6.7	)		14.7			34.6			0.1			42.7
Provision (benefit) for income taxes			—			3			(0.5	)		—			2.5
Net income (loss) before equity in earnings of			(6.7	)		11.7			35.1			0.1			40.2
subsidiaries
Equity in undistributed earnings of			46.9			—			—			(46.9	)		—
subsidiaries
Net income (loss)		$	40.2		$	11.7		$	35.1		$	(46.8	)	$	40.2
(1)	Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations.
(2)	Includes all Argo Group parent company eliminations.
CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS
FOR THE THREE MONTHS ENDED MARCH 31, 2015
(in millions)
		Argo Group			Argo Group US, Inc.			Other Subsidiaries			Consolidating			Total
		International			and Subsidiaries			and Eliminations (1)			Adjustments (2)
		Holdings, Ltd			(Subsidiary Issuer)
		(Parent Guarantor)
Net cash flows from operating activities		$	2.9		$	43.6		$	(18.9	)	$	—		$	27.6
Cash flows from investing activities:
Proceeds from sales of investments			—			340.1			82.9			—			423
Maturities and mandatory calls of fixed			—			114.5			27.5			—			142
maturity investments
Purchases of investments			—			(471.4	)		(85.8	)		—			(557.2	)
Change in short-term investments and			1.1			7			(3.3	)		—			4.8
foreign regulatory deposits
Settlements of foreign currency exchange			0.7			—			(2.9	)		—			(2.2	)
forward contracts
Issuance of intercompany note, net			—			—			—			—			—
Purchases of fixed assets and other, net			0.1			(11.3	)		2.9			—			(8.3	)
Cash (used) provided by investing activities			1.9			(21.1	)		21.3			—			2.1
Cash flows from financing activities:
Borrowings under intercompany note, net			—			—			—			—			—
Activity under stock incentive plans			0.9			—			—			—			0.9
Repurchase of Company's common shares			—			(17.0	)		—			—			(17.0	)
Excess tax expense from share-based			—			—			—			—			—
payment arrangements
Payment of cash dividend to common			(5.7	)		—			—			—			(5.7	)
shareholders
Cash (used) provided by financing activities			(4.8	)		(17.0	)		—			—			(21.8	)
Effect of exchange rate changes on cash			—			—			0.2			—			0.2
Change in cash			—			5.5			2.6			—			8.1
Cash, beginning of the period			—			49.3			31.7			—			81
Cash, end of period		$	—		$	54.8		$	34.3		$	—		$	89.1
(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, 2014
(in millions)
		Argo Group			Argo Group US, Inc.			Other Subsidiaries			Consolidating			Total
		International			and Subsidiaries			and Eliminations (1)			Adjustments (2)
		Holdings, Ltd			(Subsidiary Issuer)
		(Parent Guarantor)
Net cash flows from operating activities		$	(9.4	)	$	(11.2	)	$	64.2		$	—		$	43.6
Cash flows from investing activities:
Proceeds from sales of investments			—			163.3			100			—			263.3
Maturities and mandatory calls of fixed			—			51.3			27.1			—			78.4
maturity investments
Purchases of investments			—			(178.6	)		(128.7	)		—			(307.3	)
Change in short-term investments and			(0.9	)		(29.8	)		(23.2	)		—			(53.9	)
foreign regulatory deposits
Settlements of foreign currency exchange			0.9			—			(2.3	)		—			(1.4	)
forward contracts
Issuance of intercompany note, net			—			—			(12.5	)		12.5			—
Purchases of fixed assets and other, net			0.1			(5.1	)		(19.1	)		—			(24.1	)
Cash (used) provided by investing activities			0.1			1.1			(58.7	)		12.5			(45.0	)
Cash flows from financing activities:
Borrowings under intercompany note, net			12.5			—			—			(12.5	)		—
Activity under stock incentive plans			0.8			—			—			—			0.8
Repurchase of Company's common shares			—			(3.7	)		—			—			(3.7	)
Excess tax expense from share-based			—			0.1			—			—			0.1
payment arrangements
Payment of cash dividend to common			(4.0	)		—			—			—			(4.0	)
shareholders
Cash (used) provided by financing activities			9.3			(3.6	)		—			(12.5	)		(6.8	)
Effect of exchange rate changes on cash			—			—			0.3			—			0.3
Change in cash			—			(13.7	)		5.8			—			(7.9	)
Cash, beginning of period			—			132.1			25.3			—			157.4
Cash, end of period		$	—		$	118.4		$	31.1		$	—		$	149.5
(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, 2015
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 accounting principles generally accepted in the United States of America (“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, 2014, filed with the Securities and Exchange Commission on February 27, 2015.
	The interim financial information as of, and for the three months ended, March 31, 2015 and 2014 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. Certain amounts in prior years’ financial statements have been reclassified to conform to the current presentation.
10% Stock Dividend	10% Stock Dividend
	On February 17, 2015, our Board of Directors declared a 10% stock dividend, payable on March 16, 2015, to shareholders of record at the close of business on March 2, 2015. As a result of the stock dividend, 2,554,506 additional shares were issued. Cash was paid in lieu of fractional shares of our common shares. All references to share and per share amounts in this document and related disclosures have been adjusted to reflect the stock dividend for all periods presented.

Investments_Tables

Investments (Tables)	3 Months Ended
	Mar. 31, 2015
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:
	31-Mar-15
	(in millions)		Amortized			Gross			Gross			Fair
			Cost			Unrealized			Unrealized			Value
						Gains			Losses
	Fixed maturities
	USD denominated:
	U.S. Governments		$	181.3		$	1.6		$	0.1		$	182.8
	Non-U.S. Governments			81.3			0.7			0.5			81.5
	Obligations of states and political subdivisions			457.7			22.7			0.4			480
	Credit-Financial			472.7			13			1			484.7
	Credit-Industrial			460.5			11			1.6			469.9
	Credit-Utility			146.2			3.8			2.8			147.2
	Structured securities:
	CMO/MBS-agency (1)			160.8			8.2			0.3			168.7
	CMO/MBS-non agency			12.6			0.8			—			13.4
	CMBS (2)			184.4			1.7			0.1			186
	ABS (3)			223.2			1			0.8			223.4
	Foreign denominated:
	Governments			157.1			1.5			17			141.6
	Credit			240.8			3.6			35			209.4
	Total fixed maturities			2,778.60			69.6			59.6			2,788.60
	Equity securities			315.3			177.2			9.1			483.4
	Other investments			501.8			19.2			0.1			520.9
	Short-term investments			271.8			—			0.3			271.5
	Total investments		$	3,867.50		$	266		$	69.1		$	4,064.40
	(1)	Collateralized mortgage obligations/mortgage-backed securities (“CMO/MBS”).
	(2)	Commercial mortgage-backed securities (“CMBS”).
	(3)	Asset-backed securities (“ABS”).
	31-Dec-14
	(in millions)		Amortized			Gross			Gross			Fair
			Cost			Unrealized			Unrealized			Value
						Gains			Losses
	Fixed maturities
	USD denominated:
	U.S. Governments		$	184		$	1.3		$	0.3		$	185
	Non-U.S. Governments			79.9			0.6			0.6			79.9
	Obligations of states and political subdivisions			468.1			22.9			0.3			490.7
	Credit-Financial			508.1			12.3			2.3			518.1
	Credit-Industrial			493.7			9.4			3.5			499.6
	Credit-Utility			142.7			3.2			3.9			142
	Structured securities:
	CMO/MBS-agency (1)			168			8			0.7			175.3
	CMO/MBS-non agency			13.2			0.8			—			14
	CMBS (2)			178.6			1.6			0.2			180
	ABS (3)			221.4			0.6			1			221
	Foreign denominated:
	Governments			148.4			1.2			9.4			140.2
	Credit			211.1			2.7			18.9			194.9
	Total fixed maturities			2,817.20			64.6			41.1			2,840.70
	Equity securities			307.3			184.1			5.1			486.3
	Other investments			488.9			7.5			1.3			495.1
	Short-term investments			275.8			—			—			275.8
	Total investments		$	3,889.20		$	256.2		$	47.5		$	4,097.90
	(1)	Collateralized mortgage obligations/mortgage-backed securities (“CMO/MBS”).
	(2)	Commercial mortgage-backed securities (“CMBS”).
	(3)	Asset-backed securities (“ABS”).
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, 2015, by contractual maturity, were as follows:
	(in millions)		Amortized			Fair
			Cost			Value
	Due in one year or less		$	258.8		$	252.6
	Due after one year through five years			1,171.10			1,166.90
	Due after five years through ten years			582.5			589.5
	Thereafter			185.2			188.1
	Structured securities			581			591.5
	Total		$	2,778.60		$	2,788.60
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, 2015 and December 31, 2014 were as follows:
	31-Mar-15
	(in millions)		Carrying Value			Unfunded Commitments
	Investment Type
	Hedge funds		$	145.5		$	—
	Private equity			139.6			60.6
	Long only funds			115			—
	Other investments			120.8			—
	Total other invested assets		$	520.9		$	60.6
	31-Dec-14
	(in millions)		Carrying Value			Unfunded Commitments
	Investment Type
	Hedge funds		$	153.2		$	—
	Private equity			123.6			72.9
	Long only funds			104.4			—
	Other investments			113.9			—
	Total other invested assets		$	495.1		$	72.9
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, other investments and short-term investments is presented below:
	31-Mar-15		Less Than One Year						One Year or Greater						Total
	(in millions)		Fair			Unrealized			Fair			Unrealized			Fair			Unrealized
			Value			Losses			Value			Losses			Value			Losses
	Fixed maturities
	USD denominated:
	U.S. Governments (2)		$	18.6		$	0.1		$	4.1		$	—		$	22.7		$	0.1
	Non-U.S. Governments			9			0.3			3.4			0.2			12.4			0.5
	Obligations of states and political			21.1			0.1			9.2			0.3			30.3			0.4
	subdivisions
	Credit-Financial			103.2			0.9			8.4			0.1			111.6			1
	Credit-Industrial (2)			85			1.6			8.2			—			93.2			1.6
	Credit-Utility (2)			44.3			2.8			1.5			—			45.8			2.8
	Structured securities:
	CMO/MBS-agency (1)			5.2			—			8.1			0.3			13.3			0.3
	CMBS (2)			28.4			0.1			3.5			—			31.9			0.1
	ABS			74.9			0.5			8.5			0.3			83.4			0.8
	Foreign denominated:
	Governments			125.5			17			—			—			125.5			17
	Credit			199.2			34.8			0.9			0.2			200.1			35
	Total fixed maturities			714.4			58.2			55.8			1.4			770.2			59.6
	Equity securities			73.3			9.1			—			—			73.3			9.1
	Other investments			0.2			0.1			—			—			0.2			0.1
	Short-term investments			20.7			0.3			—			—			20.7			0.3
	Total		$	808.6		$	67.7		$	55.8		$	1.4		$	864.4		$	69.1
	(1) Unrealized losses less than one year are less than $0.1 million.
	(2) Unrealized losses one year or greater are less than $0.1 million.
	31-Dec-14		Less Than One Year						One Year or Greater						Total
	(in millions)		Fair			Unrealized			Fair			Unrealized			Fair			Unrealized
			Value			Losses			Value			Losses			Value			Losses
	Fixed maturities
	USD denominated:
	U.S. Governments		$	55		$	0.2		$	15.7		$	0.1		$	70.7		$	0.3
	Non-U.S. Governments			36.5			0.4			5.2			0.2			41.7			0.6
	Obligations of states and political			10.4			0.1			16.6			0.2			27			0.3
	subdivisions
	Credit-Financial			195.7			2.2			11.1			0.1			206.8			2.3
	Credit-Industrial			240.8			3.3			12.2			0.2			253			3.5
	Credit-Utility			63.1			3.8			1.9			0.1			65			3.9
	Structured securities:
	CMO/MBS-agency			10.1			0.1			19.2			0.6			29.3			0.7
	CMBS			49.3			0.1			6			0.1			55.3			0.2
	ABS			129.2			0.7			8.1			0.3			137.3			1
	Foreign denominated:
	Governments			123.7			9.3			11.2			0.1			134.9			9.4
	Credit			190.8			18.7			0.9			0.2			191.7			18.9
	Total fixed maturities			1,104.60			38.9			108.1			2.2			1,212.70			41.1
	Equity securities			53.6			5.1			—			—			53.6			5.1
	Other investments			(0.9	)		1.3			—			—			(0.9	)		1.3
	Short-term investments			—			—			—			—			—			—
	Total		$	1,157.30		$	45.3		$	108.1		$	2.2		$	1,265.40		$	47.5
Schedule of Company's Gross Realized Investment Gains (Losses)	The following table presents our gross realized investment gains (losses) and other:
			For the Three Months Ended March 31,
	(in millions)		2015			2014
	Realized gains
	Fixed maturities		$	4.1		$	4.6
	Equity securities			3.8			6.5
	Other investments			20.9			12.7
	Gain on sale of real estate holdings			0.3			—
	Gross realized investment gains			29.1			23.8
	Realized losses
	Fixed maturities			(3.7	)		(3.5	)
	Equity securities			(0.7	)		(0.1	)
	Other investments			(7.1	)		(4.9	)
	Short-term investments			(1.1	)		(0.2	)
	Other assets			—			(4.0	)
	Other-than-temporary impairment losses on fixed			(0.4	)		—
	maturities
	Other-than-temporary impairment losses on equity			(0.1	)		—
	securities
	Gross realized investment and other losses			(13.1	)		(12.7	)
	Net realized investment gains and other		$	16		$	11.1
Schedule of Realized Gains (Losses) and Changes in Unrealized Appreciation (Depreciation)	Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments are summarized as follows:
	(in millions)		Fixed			Equity			Other			Other			Real Estate Holdings			Tax			Total
			Maturities			Maturities			Investments									Effects
	Three Months Ended March 31, 2015
	Realized before impairments		$	0.4		$	3.1		$	13.8		$	(1.1	)	$	0.3		$	(6.1	)	$	10.4
	Realized - impairments			(0.4	)		(0.1	)		—			—			—			0.2			(0.3	)
	Change in unrealized			(12.3	)		(10.8	)		1.6			(0.3	)		—			2.9			(18.9	)
	Three Months Ended March 31, 2014
	Realized before impairments		$	1.1		$	6.4		$	7.8		$	(4.2	)	$	—		$	(3.5	)	$	7.6
	Realized - impairments			—			—			—			—			—			—			—
	Change in unrealized			8.1			(3.1	)		—			—			—			3.7			8.7
Financial Assets Measured at Fair Value on Recurring Basis	Based on an analysis of the inputs, our financial assets measured at fair value on a recurring basis have been categorized as follows:
						Fair Value Measurements at Reporting Date Using
	(in millions)		31-Mar-15			Level 1 (a)			Level 2 (b)			Level 3 (c)
	Fixed maturities
	USD denominated:
	U.S. Governments		$	182.8		$	102		$	80.8		$	—
	Non-U.S. Governments			81.5			—			81.5			—
	Obligations of states and political subdivisions			480			—			480			—
	Credit-Financial			484.7			—			484.7			—
	Credit-Industrial			469.9			—			469.9			—
	Credit-Utility			147.2			—			147.2			—
	Structured securities:
	CMO/MBS-agency			168.7			—			168.7			—
	CMO/MBS-non agency			13.4			—			13.4			—
	CMBS			186			—			186			—
	ABS			223.4			—			223.4			—
	Foreign denominated:
	Governments			141.6			—			141.6			—
	Credit			209.4			—			209.4			—
	Total fixed maturities			2,788.60			102			2,686.60			—
	Equity securities			483.4			482.6			—			0.8
	Other investments			104.9			—			104.9			—
	Short-term investments			271.5			271.1			0.4			—
			$	3,648.40		$	855.7		$	2,791.90		$	0.8
	(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)		31-Dec-14			Level 1 (a)			Level 2 (b)			Level 3 (c)
	Fixed maturities
	USD denominated:
	U.S. Governments		$	185		$	99.2		$	85.8		$	—
	Non-U.S. Governments			79.9			—			79.9			—
	Obligations of states and political subdivisions			490.7			—			490.7			—
	Credit-Financial			518.1			—			518.1			—
	Credit-Industrial			499.6			—			499.6			—
	Credit-Utility			142			—			142			—
	Structured securities:
	CMO/MBS-agency			175.3			—			175.3			—
	CMO/MBS-non agency			14			—			14			—
	CMBS			180			—			180			—
	ABS			221			—			221			—
	Foreign denominated:
	Governments			140.2			—			140.2			—
	Credit			194.9			—			194.9			—
	Total fixed maturities			2,840.70			99.2			2,741.50			—
	Equity securities			486.3			485.4			—			0.9
	Other investments			97.3			—			97.3			—
	Short-term investments			275.8			273.9			1.9			—
			$	3,700.10		$	858.5		$	2,840.70		$	0.9
	(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)		Equity			Total
			Securities
	Beginning balance, January 1, 2015		$	0.9		$	0.9
	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			(0.1	)		(0.1	)
	Settlements			—			—
	Ending balance, March 31, 2015		$	0.8		$	0.8
	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 March 31, 2015
	(in millions)		Equity			Other			Total
			Securities			Assets
	Beginning balance, January 1, 2014		$	1.3		$	2.6		$	3.9
	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			—			0.1
	Purchases, issuances, sales, and settlements
	Purchases			—			—			—
	Issuances			—			—			—
	Sales			(0.5	)		—			(0.5	)
	Settlements			—			(2.6	)		(2.6	)
	Ending balance, December 31, 2014		$	0.9		$	—		$	0.9
	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 December 31, 2014

Reserves_for_Losses_and_Loss_A1

Reserves for Losses and Loss Adjustment Expenses (Tables)	3 Months Ended
	Mar. 31, 2015
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)	2015			2014
	Net reserves beginning of the year	$	2,137.10		$	2,107.60
	Add:
	Losses and LAE incurred during current calendar
	year, net of reinsurance:
	Current accident year		187.4			191.4
	Prior accident years		(3.7	)		(8.9	)
	Losses and LAE incurred during calendar year, net		183.7			182.5
	of reinsurance
	Deduct:
	Losses and LAE payments made during current
	calendar year, net of reinsurance:
	Current accident year		19.8			27.9
	Prior accident years		141			150.1
	Losses and LAE payments made during current		160.8			178
	calendar year, net of reinsurance:
	Change in participation interest (1)		(1.2	)		24.7
	Foreign exchange adjustments		(6.0	)		1.7
	Net reserves - end of period		2,152.80			2,138.50
	Add:
	Reinsurance recoverables on unpaid losses and LAE,		882.3			903.9
	end of period
	Gross reserves - end of period	$	3,035.10		$	3,042.40

Disclosures_about_Fair_Value_o1

Disclosures about Fair Value of Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2015
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:
		31-Mar-15				31-Dec-14
	(in millions)	Carrying		Fair		Carrying		Fair
		Amount		Value		Amount		Value
	Junior subordinated debentures	$	172.7	$	155.5	$	172.7	$	155.5
	Senior unsecured fixed rate notes		143.8		132.3		143.8		132.3
	Other indebtedness:
	Floating rate loan stock		56.9		51.2		61.3		55.2
	Note payable		0.6		0.6		0.7		0.6
		$	374	$	339.6	$	378.5	$	343.6

Shareholders_Equity_Tables

Shareholders' Equity (Tables)	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Schedule of Repurchase of Shares	In 2015, we repurchased shares under Securities Exchange Act of 1934 Rule 10b5-1 trading plans as follows:
	Date	2015	Number of		Average Price		Total Cost		Repurchase
	Trading Plan	Purchase	Shares		of Shares		(in millions)		Authorization
	Initiated	Period	Repurchased		Repurchased				Year
	12/15/14	01/05/2015-02/12/2015		117,482	$	53.5	$	6.3	2013
	3/16/15	03/18/2015-03/31/2015		38,708	$	49.65	$	1.9	2013

Accumulated_Other_Comprehensiv1

Accumulated Other Comprehensive Income (Loss) (Tables)	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Changes in Accumulated Other Comprehensive Income (Loss)	A summary of changes in accumulated other comprehensive income (loss), net of taxes (where applicable) by component for the three months ended March 31, 2015 and 2014 is presented below:
	(in millions)	Foreign Currency Translation Adjustments			Unrealized			Defined Benefit Pension Plans			Total
					Holding Gains
					on Securities
	Balance, January 1, 2015	$	(15.6	)	$	130.7		$	(7.0	)	$	108.1
	Other comprehensive loss before		(3.1	)		(21.5	)		—			(24.6	)
	reclassifications
	Amounts reclassified from accumulated other		—			2.6			—			2.6
	comprehensive income (loss)
	Net current-period other comprehensive loss		(3.1	)		(18.9	)		—			(22.0	)
	Balance at March 31, 2015	$	(18.7	)	$	111.8		$	(7.0	)	$	86.1
	(in millions)	Foreign Currency Translation Adjustments			Unrealized			Defined Benefit Pension Plans			Total
					Holding Gains
					on Securities
	Balance, January 1, 2014	$	(11.5	)	$	163.9		$	(4.6	)	$	147.8
	Other comprehensive (loss) income before		(0.2	)		1.3			—			1.1
	reclassifications
	Amounts reclassified from accumulated other		—			—			—			—
	comprehensive income (loss)
	Net current-period other comprehensive (loss)		(0.2	)		1.3			-			1.1
	income
	Balance at March 31, 2014	$	(11.7	)	$	165.2		$	(4.6	)	$	148.9
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	The amounts reclassified from accumulated other comprehensive income (loss) shown in the above table have been included in the following captions in our Consolidated Statements of Income:
		For the Three Months Ended March 31,
	(in millions)	2015			2014
	Unrealized gains and losses on securities:
	Net realized investment gains	$	2.5		$	(0.1	)
	Provision for income taxes		0.1			0.1
	Net of taxes	$	2.6		$	—

Net_Income_Per_Common_Share_Ta

Net Income Per Common Share (Tables)	3 Months Ended
	Mar. 31, 2015
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 (all balances have been adjusted to reflect the 10% stock dividend):
		For the Three Months Ended March 31,
	(in millions, except number of shares and per share amounts)	2015		2014
	Net income	$	58.8	$	40.2
	Weighted average common shares outstanding - basic		28,129,692		29,165,911
	Effect of dilutive securities:
	Equity compensation awards		547,687		497,179
	Weighted average common shares outstanding - diluted		28,677,379		29,663,090
	Net income per common share:
	Basic	$	2.09	$	1.38
	Diluted	$	2.05	$	1.36

Supplemental_Cash_Flow_Informa1

Supplemental Cash Flow Information (Tables)	3 Months Ended
	Mar. 31, 2015
Supplemental Cash Flow Elements [Abstract]
Schedule of Interest Paid	Interest paid.
		For the Three Months Ended March 31,
	(in millions)	2015		2014
	Senior unsecured fixed rate notes	$	2.3	$	2.3
	Junior subordinated debentures		1.7		2
	Other indebtedness		0.8		0.9
	Total interest paid	$	4.8	$	5.2

Sharebased_Compensation_Tables

Share-based Compensation (Tables)	3 Months Ended
	Mar. 31, 2015
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, 2015 and 2014:
		For the Three Months Ended March 31,
		2015			2014
	Risk-free rate of return		1.47%			1.74%
	Expected dividend yields		1.63%			1.49%
	Expected award life (years)		4.71		4.85
	Expected volatility		22.09%			24.91%
Summary of Restricted Share Activity	A summary of restricted share activity as of March 31, 2015 and changes during the three months then ended is as follows:
		Shares			Weighted-Average
					Grant Date
					Fair Value
	Outstanding at January 1, 2015		313,716		$	35.17
	Granted		51,873		$	46.57
	Vested and issued		(110,982	)	$	29.88
	Expired or forfeited		(3,569	)	$	36.38
	Outstanding at March 31, 2015		251,038		$	39.85
Summary of Stock-Settled SARs Activity	A summary of stock-settled SARs activity as of March 31, 2015 and changes during the three months then ended is as follows:
		Shares			Weighted-Average
					Exercise Price
	Outstanding at January 1, 2015		1,313,901		$	28.65
	Granted		243,306		$	37.33
	Exercised		(24,622	)	$	28.11
	Expired or forfeited		(9,210	)	$	30.68
	Outstanding at March 31, 2015		1,523,375		$	29.26
Summary of Cash-Settled SARs Activity	A summary of cash-settled SARs activity as of March 31, 2015 and changes during the three months then ended is as follows:
		Shares			Weighted-Average
					Exercise Price
	Outstanding at January 1, 2015		2,001,451		$	32.76
	Granted		866,673		$	47.83
	Exercised		(31,461	)	$	29.77
	Expired or forfeited		(70,547	)	$	36.28
	Outstanding at March 31, 2015		2,766,116		$	37.42

Underwriting_Acquisition_and_I1

Underwriting, Acquisition and Insurance Expenses (Tables)	3 Months Ended
	Mar. 31, 2015
Text Block [Abstract]
Underwriting, Acquisition and Insurance Expenses	Underwriting, acquisition and insurance expenses were as follows:
		For the Three Months Ended March 31,
	(in millions)	2015			2014
	Commissions	$	58.7		$	54.9
	General expenses		72.9			68.8
	Premium taxes, boards and bureaus		0.6			6.6
			132.2			130.3
	Net deferral of policy acquisition costs		(2.6	)		(1.6	)
	Total underwriting, acquisition and insurance expenses	$	129.6		$	128.7

Income_Taxes_Tables

Income Taxes (Tables)	3 Months Ended
	Mar. 31, 2015
Income Tax Disclosure [Abstract]
Income Tax Provision (Benefit)	Our income tax provision includes the following components:
		For the Three Months Ended March 31,
	(in millions)	2015				2014
	Current tax (benefit) provision	$	(0.2	)		$	1.4
	Deferred tax provision related to:
	Future tax deductions		5.5				30.2
	Valuation allowance change		(1.7	)			(29.1	)
	Income tax provision	$	3.6			$	2.5
Schedule of Pre-Tax Income (Loss) and Effective Income Tax Rates	For the three months ended March 31, 2015 and 2014, 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)	2015							2014
		Pre-Tax				Effective			Pre-Tax				Effective
		Income (Loss)				Tax			Income (Loss)				Tax
						Rate							Rate
	Bermuda	$	25.7				0	%	$	21.9				0	%
	United States		28.5				20.2	%		14.8				20.6	%
	United Kingdom		10.1				-21.9	%		7.3				-7.7	%
	Belgium		(0.2	)			-4.8	%		—		-1		18.1	%
	Brazil		(1.9	)			0	%		—				0	%
	United Arab Emirates		0.1				0	%		(0.9	)			0	%
	Ireland		(0.2	)			0	%		—				0	%
	Malta		0.3				0	%		(0.4	)			0	%
	Switzerland		—		-1		21	%		—		-1		-19.8	%
	Pre-tax income	$	62.4						$	42.7
Reconciliation of Difference Between Provision for Income Taxes and Expected Tax Provision at Weighted Average Tax Rate	A reconciliation of the difference between the provision for income taxes and the expected tax provision at the weighted average tax rate is as follows:
		For the Three Months Ended March 31,
	(in millions)	2015				2014
	Income tax provision at expected rate	$	11.2			$	4.7
	Tax effect of:
	Tax-exempt interest		(0.9	)			(1.2	)
	Dividends received deduction		(0.6	)			(0.6	)
	Valuation allowance change		(1.7	)			(29.1	)
	Other permanent adjustments, net		0.2				0.1
	Adjustment for annualized rate		(0.5	)			0.8
	United States state tax (benefit) expense		(2.5	)			0.1
	Capital loss carryforward from prior merger		—				29.8
	Other foreign adjustments		(0.3	)			—
	Prior period adjustments to deferred		2.2				—
	Prior year foreign taxes recovered		(1.2	)			—
	Foreign exchange adjustments		(2.4	)			(2.2	)
	Foreign withholding taxes		0.1				0.1
	Income tax provision	$	3.6			$	2.5
	Income tax benefit - Foreign	$	(2.2	)		$	(0.6	)
	Income tax provision - United States, Federal		9.5				2.8
	Income tax (benefit) provision  - United States, state		(3.8	)			0.2
	Foreign withholding tax - United States		0.1				0.1
	Income tax provision	$	3.6			$	2.5

Segment_Information_Tables

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2015
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)	2015			2014
	Revenue:
	Earned premiums
	Excess and Surplus Lines	$	123.1		$	117.4
	Commercial Specialty		72.6			71.7
	International Specialty		37.3			37
	Syndicate 1200		101.9			99.3
	Run-off Lines		(0.3	)		0.3
	Total earned premiums		334.6			325.7
	Net investment income
	Excess and Surplus Lines		8.5			9.3
	Commercial Specialty		4.5			4.7
	International Specialty		2.9			1.8
	Syndicate 1200		2.2			3.7
	Run-off Lines		2			2.5
	Corporate and Other		0.7			1.3
	Total net investment income		20.8			23.3
	Net realized investment and other gains		16			11.1
	Total revenue	$	371.4		$	360.1
		For the Three Months
		Ended March 31,
	(in millions)	2015			2014
	Income (loss) before income taxes
	Excess and Surplus Lines	$	24.7		$	20.1
	Commercial Specialty		3.8			1.7
	International Specialty		9.1			6.8
	Syndicate 1200		11			15.8
	Run-off Lines		1.2			(5.9	)
	Total segment income before taxes		49.8			38.5
	Corporate and Other		(3.4	)		(6.9	)
	Net realized investment and other gains		16			11.1
	Total income before income taxes	$	62.4		$	42.7
Schedule of Earned Premiums by Geographic Location	The table below presents earned premiums by geographic location for the three months ended March 31, 2015 and 2014. 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)	2015			2014
	Bermuda	$	23.9		$	24.4
	Brazil		12.5			12.6
	Malta		0.5			0.5
	United Kingdom		101.9			98.9
	United States		195.8			189.3
	Total earned premiums	$	334.6		$	325.7
Identifiable Assets	The following table represents identifiable assets:
		March 31,			December 31,
	(in millions)	2015			2014
	Excess and Surplus Lines	$	2,352.80		$	2,350.70
	Commercial Specialty		1,384.00			1,358.50
	International Specialty		799.5			776.8
	Syndicate 1200		1,232.50			1,258.50
	Run-off Lines		557.8			552.8
	Corporate and Other		50.5			59
	Total	$	6,377.10		$	6,356.30

Information_Provided_in_Connec1

Information Provided in Connection with Outstanding Debt of Subsidiaries (Tables)	3 Months Ended
	Mar. 31, 2015
Debt Disclosure [Abstract]
Condensed Consolidating Balance Sheet	CONDENSED CONSOLIDATING BALANCE SHEET
	31-Mar-15
	(in millions)
			Argo Group			Argo Group US, Inc.			Other Subsidiaries			Consolidating			Total
			International			and Subsidiaries			and Eliminations (1)			Adjustments (2)
			Holdings, Ltd			(Subsidiary Issuer)
			(Parent Guarantor)
	Assets
	Investments		$	7.2		$	2,851.40		$	1,205.80		$	—		$	4,064.40
	Cash			—			54.8			34.3			—			89.1
	Accrued investment income			—			17.1			3.8			—			20.9
	Premiums receivable			—			160			211.7			—			371.7
	Reinsurance recoverables			—			1,152.00			(164.7	)		—			987.3
	Goodwill and other intangible assets, net			—			132.7			98.3			—			231
	Current income taxes receivable, net			—			—			4.2			—			4.2
	Deferred acquisition costs, net			—			60			69.7			—			129.7
	Ceded unearned premiums			—			112.2			144.4			—			256.6
	Other assets			12.7			175.4			59.1			(25.0	)		222.2
	Due from (to) affiliates			(16.0	)		(19.3	)		19.3			16			—
	Intercompany note receivable			—			55.6			(55.6	)		—			—
	Investments in subsidiaries			1,722.80			—			—			(1,722.8	)		—
	Total assets		$	1,726.70		$	4,751.90		$	1,630.30		$	(1,731.8	)	$	6,377.10
	Liabilities and Shareholders' Equity
	Reserves for losses and loss adjustment expenses		$	—		$	2,138.80		$	896.3		$	—		$	3,035.10
	Unearned premiums			—			458.8			367.6			—			826.4
	Funds held and ceded reinsurance payable, net			—			662.8			(402.3	)		—			260.5
	Long-term debt			49			288.7			56.9			(20.6	)		374
	Current income taxes payable, net			—			0.5			(0.5	)		—			—
	Deferred tax liabilities, net			—			44.7			9.1			—			53.8
	Accrued underwriting expenses and other liabilities			14.9			91.7			57.9			—			164.5
	Total liabilities			63.9			3,686.00			985			(20.6	)		4,714.30
	Total shareholders' equity			1,662.80			1,065.90			645.3			(1,711.2	)		1,662.80
	Total liabilities and shareholders' equity		$	1,726.70		$	4,751.90		$	1,630.30		$	(1,731.8	)	$	6,377.10
	CONDENSED CONSOLIDATING BALANCE SHEET
	31-Dec-14
	(in millions)
			Argo Group			Argo Group US, Inc.			Other Subsidiaries			Consolidating			Total
			International			and Subsidiaries			and Eliminations (1)			Adjustments (2)
			Holdings, Ltd			(Subsidiary Issuer)
			(Parent Guarantor)
	Assets
	Investments		$	0.7		$	2,841.50		$	1,255.70		$	—		$	4,097.90
	Cash			—			49.3			31.7			—			81
	Accrued investment income			—			17.8			4.3			—			22.1
	Premiums receivable			—			154.6			199			—			353.6
	Reinsurance recoverables			—			1,173.60			(176.4	)		—			997.2
	Goodwill and other intangible assets, net			—			131.7			99.1			—			230.8
	Current income taxes receivable, net			—			10.1			4.8			—			14.9
	Deferred acquisition costs, net			—			58			66.6			—			124.6
	Ceded unearned premiums			—			98.5			109.1			—			207.6
	Other assets			9.6			174.1			67.9			(25.0	)		226.6
	Due from (to) affiliates			2.9			(19.8	)		19.8			(2.9	)		—
	Intercompany note receivable			—			72			(72.0	)		—			—
	Investments in subsidiaries			1,698.00			—			—			(1,698.0	)		—
	Total assets		$	1,711.20		$	4,761.40		$	1,609.60		$	(1,725.9	)	$	6,356.30
	Liabilities and Shareholders' Equity
	Reserves for losses and loss adjustment expenses		$	—		$	2,136.40		$	906		$	—		$	3,042.40
	Unearned premiums			—			448.9			368.3			—			817.2
	Funds held and ceded reinsurance payable, net			—			675.1			(441.3	)		—			233.8
	Long-term debt			49			288.7			61.4			(20.6	)		378.5
	Deferred tax liabilities, net			—			41.2			11.8			—			53
	Accrued underwriting expenses and other liabilities			15.5			104.2			65			—			184.7
	Total liabilities			64.5			3,694.50			971.2			(20.6	)		4,709.60
	Total shareholders' equity			1,646.70			1,066.90			638.4			(1,705.3	)		1,646.70
Total liabilities and shareholders' equity		$	1,711.20		$	4,761.40		$	1,609.60		$	(1,725.9	)	$	6,356.30
(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, 2015
	(in millions)
			Argo Group			Argo Group US, Inc.			Other Subsidiaries			Consolidating			Total
			International			and Subsidiaries			and Eliminations (1)			Adjustments (2)
			Holdings, Ltd			(Subsidiary Issuer)
			(Parent Guarantor)
	Premiums and other revenue:
	Earned premiums		$	—		$	135.1		$	199.5		$	—		$	334.6
	Net investment (expense) income			(0.2	)		14.5			6.5			—			20.8
	Net realized investment gains and other			—			15.9			0.1			—			16
	Total revenue			(0.2	)		165.5			206.1			—			371.4
	Expenses:
	Losses and loss adjustment expenses			—			80.2			103.5			—			183.7
	Underwriting, acquisition and insurance			4.7			51.9			73			—			129.6
	expenses
	Interest expense			0.5			3.8			0.6			—			4.9
	Fee expense (income), net			—			0.8			(0.4	)		—			0.4
	Foreign currency exchange loss (gain)			—			0.8			(10.4	)		—			(9.6	)
	Total expenses			5.2			137.5			166.3			—			309
	Income before income taxes			(5.4	)		28			39.8			—			62.4
	Provision for income taxes			—			5.8			(2.2	)		—			3.6
	Net income before equity in earnings of			(5.4	)		22.2			42			—			58.8
	subsidiaries
	Equity in undistributed earnings of			64.2			—			—			(64.2	)		—
	subsidiaries
	Net income		$	58.8		$	22.2		$	42		$	(64.2	)	$	58.8
	(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, 2014
	(in millions)
			Argo Group			Argo Group US, Inc.			Other Subsidiaries			Consolidating			Total
			International			and Subsidiaries			and Eliminations (1)			Adjustments (2)
			Holdings, Ltd			(Subsidiary Issuer)
			(Parent Guarantor)
	Premiums and other revenue:
	Earned premiums		$	—		$	112		$	213.7		$	—		$	325.7
	Net investment (expense) income			(0.1	)		15			8.4			—			23.3
	Net realized investment and other gains (loss)			—			11.5			(0.4	)		—			11.1
	Total revenue			(0.1	)		138.5			221.7			—			360.1
	Expenses:
	Losses and loss adjustment expenses			—			72.5			110			—			182.5
	Underwriting, acquisition and insurance			6			46.3			76.4			—			128.7
	expenses
	Interest expense			0.6			3.8			0.7			(0.1	)		5
	Fee expense, net			—			1.3			0.1			—			1.4
	Foreign currency exchange (gain) loss			—			(0.1	)		(0.1	)		—			(0.2	)
	Total expenses			6.6			123.8			187.1			(0.1	)		317.4
	Income (loss) before income taxes			(6.7	)		14.7			34.6			0.1			42.7
	Provision (benefit) for income taxes			—			3			(0.5	)		—			2.5
	Net income (loss) before equity in earnings of			(6.7	)		11.7			35.1			0.1			40.2
	subsidiaries
	Equity in undistributed earnings of			46.9			—			—			(46.9	)		—
	subsidiaries
	Net income (loss)		$	40.2		$	11.7		$	35.1		$	(46.8	)	$	40.2
	(1)	Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations.
	(2)	Includes all Argo Group parent company eliminations.
Condensed Consolidating Statement of Cash Flows	CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS
	FOR THE THREE MONTHS ENDED MARCH 31, 2015
	(in millions)
			Argo Group			Argo Group US, Inc.			Other Subsidiaries			Consolidating			Total
			International			and Subsidiaries			and Eliminations (1)			Adjustments (2)
			Holdings, Ltd			(Subsidiary Issuer)
			(Parent Guarantor)
	Net cash flows from operating activities		$	2.9		$	43.6		$	(18.9	)	$	—		$	27.6
	Cash flows from investing activities:
	Proceeds from sales of investments			—			340.1			82.9			—			423
	Maturities and mandatory calls of fixed			—			114.5			27.5			—			142
	maturity investments
	Purchases of investments			—			(471.4	)		(85.8	)		—			(557.2	)
	Change in short-term investments and			1.1			7			(3.3	)		—			4.8
	foreign regulatory deposits
	Settlements of foreign currency exchange			0.7			—			(2.9	)		—			(2.2	)
	forward contracts
	Issuance of intercompany note, net			—			—			—			—			—
	Purchases of fixed assets and other, net			0.1			(11.3	)		2.9			—			(8.3	)
	Cash (used) provided by investing activities			1.9			(21.1	)		21.3			—			2.1
	Cash flows from financing activities:
	Borrowings under intercompany note, net			—			—			—			—			—
	Activity under stock incentive plans			0.9			—			—			—			0.9
	Repurchase of Company's common shares			—			(17.0	)		—			—			(17.0	)
	Excess tax expense from share-based			—			—			—			—			—
	payment arrangements
	Payment of cash dividend to common			(5.7	)		—			—			—			(5.7	)
	shareholders
	Cash (used) provided by financing activities			(4.8	)		(17.0	)		—			—			(21.8	)
	Effect of exchange rate changes on cash			—			—			0.2			—			0.2
	Change in cash			—			5.5			2.6			—			8.1
	Cash, beginning of the period			—			49.3			31.7			—			81
	Cash, end of period		$	—		$	54.8		$	34.3		$	—		$	89.1
	(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, 2014
	(in millions)
			Argo Group			Argo Group US, Inc.			Other Subsidiaries			Consolidating			Total
			International			and Subsidiaries			and Eliminations (1)			Adjustments (2)
			Holdings, Ltd			(Subsidiary Issuer)
			(Parent Guarantor)
	Net cash flows from operating activities		$	(9.4	)	$	(11.2	)	$	64.2		$	—		$	43.6
	Cash flows from investing activities:
	Proceeds from sales of investments			—			163.3			100			—			263.3
	Maturities and mandatory calls of fixed			—			51.3			27.1			—			78.4
	maturity investments
	Purchases of investments			—			(178.6	)		(128.7	)		—			(307.3	)
	Change in short-term investments and			(0.9	)		(29.8	)		(23.2	)		—			(53.9	)
	foreign regulatory deposits
	Settlements of foreign currency exchange			0.9			—			(2.3	)		—			(1.4	)
	forward contracts
	Issuance of intercompany note, net			—			—			(12.5	)		12.5			—
	Purchases of fixed assets and other, net			0.1			(5.1	)		(19.1	)		—			(24.1	)
	Cash (used) provided by investing activities			0.1			1.1			(58.7	)		12.5			(45.0	)
	Cash flows from financing activities:
	Borrowings under intercompany note, net			12.5			—			—			(12.5	)		—
	Activity under stock incentive plans			0.8			—			—			—			0.8
	Repurchase of Company's common shares			—			(3.7	)		—			—			(3.7	)
	Excess tax expense from share-based			—			0.1			—			—			0.1
	payment arrangements
	Payment of cash dividend to common			(4.0	)		—			—			—			(4.0	)
	shareholders
	Cash (used) provided by financing activities			9.3			(3.6	)		—			(12.5	)		(6.8	)
Effect of exchange rate changes on cash			—			—			0.3			—			0.3
Change in cash			—			(13.7	)		5.8			—			(7.9	)
Cash, beginning of period			—			132.1			25.3			—			157.4
Cash, end of period		$	—		$	118.4		$	31.1		$	—		$	149.5
(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_Addition

Basis of Presentation - Additional Information (Detail)	0 Months Ended
	Feb. 17, 2015	Feb. 18, 2014	Mar. 31, 2015
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Stock dividend declared	10.00%	10.00%	10.00%
Additional stock issued as dividend	2,554,506
Dividends declaration date	17-Feb-15	18-Feb-14
Dividends payable date	16-Mar-15	17-Mar-14
Dividend payable, date of record	2-Mar-15	3-Mar-14

Investments_Schedule_of_Amorti

Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Detail) (USD $)	3 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$3,867.50		$3,889.20
Gross Unrealized Gains	266		256.2
Gross Unrealized Losses	69.1		47.5
Fair Value	4,064.40		4,097.90
Other Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	501.8		488.9
Gross Unrealized Gains	19.2		7.5
Gross Unrealized Losses	0.1		1.3
Fair Value	520.9		495.1
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,778.60		2,817.20
Gross Unrealized Gains	69.6		64.6
Gross Unrealized Losses	59.6		41.1
Fair Value	2,788.60		2,840.70
Fixed Maturities [Member] | U.S. Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	181.3		184
Gross Unrealized Gains	1.6		1.3
Gross Unrealized Losses	0.1	[1]	0.3
Fair Value	182.8		185
Fixed Maturities [Member] | Non-U.S. Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	81.3		79.9
Gross Unrealized Gains	0.7		0.6
Gross Unrealized Losses	0.5		0.6
Fair Value	81.5		79.9
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	457.7		468.1
Gross Unrealized Gains	22.7		22.9
Gross Unrealized Losses	0.4		0.3
Fair Value	480		490.7
Fixed Maturities [Member] | Credit-Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	472.7		508.1
Gross Unrealized Gains	13		12.3
Gross Unrealized Losses	1		2.3
Fair Value	484.7		518.1
Fixed Maturities [Member] | Credit-Industrial [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	460.5		493.7
Gross Unrealized Gains	11		9.4
Gross Unrealized Losses	1.6	[1]	3.5
Fair Value	469.9		499.6
Fixed Maturities [Member] | Credit-Utility [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	146.2		142.7
Gross Unrealized Gains	3.8		3.2
Gross Unrealized Losses	2.8	[1]	3.9
Fair Value	147.2		142
Fixed Maturities [Member] | CMO/MBS-Agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	160.8	[2]	168	[2]
Gross Unrealized Gains	8.2	[2]	8	[2]
Gross Unrealized Losses	0.3	[2],[3]	0.7	[2]
Fair Value	168.7	[2]	175.3	[2]
Fixed Maturities [Member] | CMO/MBS-Non Agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12.6		13.2
Gross Unrealized Gains	0.8		0.8
Gross Unrealized Losses	0		0
Fair Value	13.4		14
Fixed Maturities [Member] | CMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	184.4	[4]	178.6	[4]
Gross Unrealized Gains	1.7	[4]	1.6	[4]
Gross Unrealized Losses	0.1	[1],[4]	0.2	[4]
Fair Value	186	[4]	180	[4]
Fixed Maturities [Member] | ABS [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	223.2	[5]	221.4	[5]
Gross Unrealized Gains	1	[5]	0.6	[5]
Gross Unrealized Losses	0.8	[5]	1	[5]
Fair Value	223.4	[5]	221	[5]
Fixed Maturities [Member] | Foreign Denominated: Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	157.1		148.4
Gross Unrealized Gains	1.5		1.2
Gross Unrealized Losses	17		9.4
Fair Value	141.6		140.2
Fixed Maturities [Member] | Foreign Denominated: Credit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	240.8		211.1
Gross Unrealized Gains	3.6		2.7
Gross Unrealized Losses	35		18.9
Fair Value	209.4		194.9
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	315.3		307.3
Gross Unrealized Gains	177.2		184.1
Gross Unrealized Losses	9.1		5.1
Fair Value	483.4		486.3
Short-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	271.8		275.8
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0.3		0
Fair Value	$271.50		$275.80
[1]	Unrealized losses one year or greater are less than $0.1 million.
[2]	Collateralized mortgage obligations/mortgage-backed securities (bCMO/MBSb).
[3]	Unrealized losses less than one year are less than $0.1 million.
[4]	Commercial mortgage-backed securities (bCMBSb).
[5]	Asset-backed securities (bABSb).

Investments_Additional_Informa

Investments - Additional Information (Detail)

3 Months Ended

3 Months Ended

3 Months Ended

3 Months Ended

Mar. 31, 2015

Mar. 31, 2014

Dec. 31, 2014

Mar. 31, 2015

Dec. 31, 2014

Mar. 31, 2015

Mar. 31, 2015

Mar. 31, 2015

Mar. 31, 2014

Mar. 31, 2015

Mar. 31, 2015

Mar. 31, 2015

Mar. 31, 2014

Dec. 31, 2014

Mar. 31, 2015

Mar. 31, 2014

Dec. 31, 2014

Mar. 31, 2015

Dec. 31, 2014

USD ($)

USD ($)

USD ($)

Securities Pledged as Collateral [Member]

Securities Pledged as Collateral [Member]

Canadian Dollar [Member]

Euro [Member]

Currency Forward Contracts [Member]

Currency Forward Contracts [Member]

Currency Forward Contracts [Member]

Currency Forward Contracts [Member]

Fixed Maturities [Member]

Fixed Maturities [Member]

Fixed Maturities [Member]

Equity Securities [Member]

Equity Securities [Member]

Equity Securities [Member]

Deposit with Various U.S. Canadian Insurance and Other Agencies [Member]

Deposit with Various U.S. Insurance and Other Agencies [Member]

Security

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

Canadian Dollar [Member]

Canadian Dollar [Member]

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

United States [Member]

United States [Member]

USD ($)

CAD

USD ($)

USD ($)

Schedule Of Investments [Line Items]

Fair value of investments, assets managed on behalf of the trade capital providers

$70,900,000

$75,200,000

Number of securities in an unrealized loss position, total

6,406

Number of securities in an unrealized loss position for less than one year

1,830

Number of securities in an unrealized loss position for a period of one year or greater

90

Other-than-temporary losses

400,000

0

100,000

0

Notional amount of the currency forward contracts

8,700,000

46,600,000

112,100,000

141,800,000

Fair value of the currency forward contracts

100,000

1,200,000

6,400,000

5,800,000

Realized gains from currency forward contracts

300,000

2,300,000

7,500,000

6,300,000

Realized losses from currency forward contracts

1,200,000

1,300,000

2,000,000

Amortized Cost

3,867,500,000

3,889,200,000

2,778,600,000

2,817,200,000

315,300,000

307,300,000

184,300,000

186,600,000

Fair Value

4,064,400,000

4,097,900,000

2,788,600,000

2,840,700,000

483,400,000

486,300,000

191,800,000

194,200,000

Pledged assets separately reported, securities pledged under letters of credit facilities, at amortized cost

48,900,000

55,000,000

Pledged assets separately reported, securities pledged under letters of credit facilities, at fair value

49,200,000

55,300,000

Reported loss expense reserve

36,200,000

43,600,000

Securities pledged to Lloyd's

209,300,000

217,900,000

Transfers between Level 1 and Level 2 securities

0

Transfers between Level 2 and Level 1 securities

$0

Investments_Schedule_of_Amorti1

Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) (USD $)	Mar. 31, 2015
In Millions, unless otherwise specified
Investments Debt And Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$258.80
Amortized Cost, Due after one year through five years	1,171.10
Amortized Cost, Due after five years through ten years	582.5
Amortized Cost, Thereafter	185.2
Amortized Cost, Structured securities	581
Amortized Cost, Total	2,778.60
Fair Value, Due in one year or less	252.6
Fair Value, Due after one year through five years	1,166.90
Fair Value, Due after five years through ten years	589.5
Fair Value, Thereafter	188.1
Fair Value, Structured securities	591.5
Fair Value, Total	$2,788.60

Investments_Schedule_of_Carryi

Investments - Schedule of Carrying Value and Unfunded Investment Commitments of Other Invested Assets Portfolio (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Schedule Of Investments [Line Items]
Carrying Value	$4,064.40	$4,097.90	[1]
Other Investments [Member]
Schedule Of Investments [Line Items]
Carrying Value	120.8	113.9
Unfunded Commitments	0	0
Hedge funds [Member]
Schedule Of Investments [Line Items]
Carrying Value	145.5	153.2
Unfunded Commitments	0	0
Private equity [Member]
Schedule Of Investments [Line Items]
Carrying Value	139.6	123.6
Unfunded Commitments	60.6	72.9
Long only funds [Member]
Schedule Of Investments [Line Items]
Carrying Value	115	104.4
Unfunded Commitments	0	0
Other invested assets [Member]
Schedule Of Investments [Line Items]
Carrying Value	520.9	495.1
Unfunded Commitments	$60.60	$72.90
[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 $)	3 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year	$808.60		$1,157.30
Unrealized Losses, Less Than One Year	67.7		45.3
Fair Value, One Year or Greater	55.8		108.1
Unrealized Losses, One Year or Greater	1.4		2.2
Fair Value, Total	864.4		1,265.40
Unrealized Losses, Total	69.1		47.5
Other Investments [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year	0.2		-0.9
Unrealized Losses, Less Than One Year	0.1		1.3
Fair Value, One Year or Greater	0		0
Unrealized Losses, One Year or Greater	0		0
Fair Value, Total	0.2		-0.9
Unrealized Losses, Total	0.1		1.3
Fixed Maturities [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year	714.4		1,104.60
Unrealized Losses, Less Than One Year	58.2		38.9
Fair Value, One Year or Greater	55.8		108.1
Unrealized Losses, One Year or Greater	1.4		2.2
Fair Value, Total	770.2		1,212.70
Unrealized Losses, Total	59.6		41.1
Fixed Maturities [Member] | U.S. Governments [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year	18.6	[1]	55
Unrealized Losses, Less Than One Year	0.1	[1]	0.2
Fair Value, One Year or Greater	4.1	[1]	15.7
Unrealized Losses, One Year or Greater	0	[1]	0.1
Fair Value, Total	22.7	[1]	70.7
Unrealized Losses, Total	0.1	[1]	0.3
Fixed Maturities [Member] | Non-U.S. Governments [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year	9		36.5
Unrealized Losses, Less Than One Year	0.3		0.4
Fair Value, One Year or Greater	3.4		5.2
Unrealized Losses, One Year or Greater	0.2		0.2
Fair Value, Total	12.4		41.7
Unrealized Losses, Total	0.5		0.6
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year	21.1		10.4
Unrealized Losses, Less Than One Year	0.1		0.1
Fair Value, One Year or Greater	9.2		16.6
Unrealized Losses, One Year or Greater	0.3		0.2
Fair Value, Total	30.3		27
Unrealized Losses, Total	0.4		0.3
Fixed Maturities [Member] | Credit-Financial [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year	103.2		195.7
Unrealized Losses, Less Than One Year	0.9		2.2
Fair Value, One Year or Greater	8.4		11.1
Unrealized Losses, One Year or Greater	0.1		0.1
Fair Value, Total	111.6		206.8
Unrealized Losses, Total	1		2.3
Fixed Maturities [Member] | Credit-Industrial [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year	85	[1]	240.8
Unrealized Losses, Less Than One Year	1.6	[1]	3.3
Fair Value, One Year or Greater	8.2	[1]	12.2
Unrealized Losses, One Year or Greater	0	[1]	0.2
Fair Value, Total	93.2	[1]	253
Unrealized Losses, Total	1.6	[1]	3.5
Fixed Maturities [Member] | Credit-Utility [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year	44.3	[1]	63.1
Unrealized Losses, Less Than One Year	2.8	[1]	3.8
Fair Value, One Year or Greater	1.5	[1]	1.9
Unrealized Losses, One Year or Greater	0	[1]	0.1
Fair Value, Total	45.8	[1]	65
Unrealized Losses, Total	2.8	[1]	3.9
Fixed Maturities [Member] | CMO/MBS-Agency [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year	5.2	[2]	10.1
Unrealized Losses, Less Than One Year	0	[2]	0.1
Fair Value, One Year or Greater	8.1	[2]	19.2
Unrealized Losses, One Year or Greater	0.3	[2]	0.6
Fair Value, Total	13.3	[2]	29.3
Unrealized Losses, Total	0.3	[2],[3]	0.7	[3]
Fixed Maturities [Member] | CMBS [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year	28.4	[1]	49.3
Unrealized Losses, Less Than One Year	0.1	[1]	0.1
Fair Value, One Year or Greater	3.5	[1]	6
Unrealized Losses, One Year or Greater	0	[1]	0.1
Fair Value, Total	31.9	[1]	55.3
Unrealized Losses, Total	0.1	[1],[4]	0.2	[4]
Fixed Maturities [Member] | ABS [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year	74.9		129.2
Unrealized Losses, Less Than One Year	0.5		0.7
Fair Value, One Year or Greater	8.5		8.1
Unrealized Losses, One Year or Greater	0.3		0.3
Fair Value, Total	83.4		137.3
Unrealized Losses, Total	0.8	[5]	1	[5]
Fixed Maturities [Member] | Foreign Denominated: Governments [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year	125.5		123.7
Unrealized Losses, Less Than One Year	17		9.3
Fair Value, One Year or Greater	0		11.2
Unrealized Losses, One Year or Greater	0		0.1
Fair Value, Total	125.5		134.9
Unrealized Losses, Total	17		9.4
Fixed Maturities [Member] | Foreign Denominated: Credit [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year	199.2		190.8
Unrealized Losses, Less Than One Year	34.8		18.7
Fair Value, One Year or Greater	0.9		0.9
Unrealized Losses, One Year or Greater	0.2		0.2
Fair Value, Total	200.1		191.7
Unrealized Losses, Total	35		18.9
Equity Securities [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year	73.3		53.6
Unrealized Losses, Less Than One Year	9.1		5.1
Fair Value, One Year or Greater	0		0
Unrealized Losses, One Year or Greater	0		0
Fair Value, Total	73.3		53.6
Unrealized Losses, Total	9.1		5.1
Short-Term Investments [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year	20.7		0
Unrealized Losses, Less Than One Year	0.3		0
Fair Value, One Year or Greater	0		0
Unrealized Losses, One Year or Greater	0		0
Fair Value, Total	20.7		0
Unrealized Losses, Total	$0.30		$0
[1]	Unrealized losses one year or greater are less than $0.1 million.
[2]	Unrealized losses less than one year are less than $0.1 million.
[3]	Collateralized mortgage obligations/mortgage-backed securities (bCMO/MBSb).
[4]	Commercial mortgage-backed securities (bCMBSb).
[5]	Asset-backed securities (bABSb).

Investments_Schedule_of_Aging_1

Investments - Schedule of Aging of Unrealized Losses on Company's Investments in Fixed Maturities, Equity Securities and Other Investments (Parenthetical) (Detail) (USD $)	Mar. 31, 2015
In Millions, unless otherwise specified
Investments Debt And Equity Securities [Abstract]
Unrealized losses less than one year	$0.10
Unrealized losses one year or greater	$0.10

Investments_Schedule_of_Compan

Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Schedule Of Investments [Line Items]
Gross realized investment gains and other	$29.10	$23.80
Gross realized investment and other losses	-13.1	-12.7
Net realized investment and other gains	16	11.1
Other Investments [Member]
Schedule Of Investments [Line Items]
Gross realized investment gains and other	20.9	12.7
Gross realized investment and other losses	-7.1	-4.9
Other Assets [Member]
Schedule Of Investments [Line Items]
Gross realized investment and other losses	0	-4
Fixed Maturities [Member]
Schedule Of Investments [Line Items]
Gross realized investment gains and other	4.1	4.6
Gross realized investment and other losses	-3.7	-3.5
Equity Securities [Member]
Schedule Of Investments [Line Items]
Gross realized investment gains and other	3.8	6.5
Gross realized investment and other losses	-0.7	-0.1
Real Estate Holdings [Member]
Schedule Of Investments [Line Items]
Gross realized investment gains and other	0.3	0
Short-Term Investments [Member]
Schedule Of Investments [Line Items]
Gross realized investment and other losses	-1.1	-0.2
Other-Than-Temporary Impairment Losses on Fixed Maturities [Member]
Schedule Of Investments [Line Items]
Gross realized investment and other losses	-0.4	0
Other-Than-Temporary Impairment Losses on Equity Securities [Member]
Schedule Of Investments [Line Items]
Gross realized investment and other losses	($0.10)	$0

Investments_Schedule_of_Realiz

Investments - Schedule of Realized Gains (Losses) and Changes in Unrealized of Appreciation(Depreciation) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Schedule Of Investments [Line Items]
Realized before impairments	$10.40	$7.60
Realized - impairments	-0.3	0
Change in unrealized	-18.9	8.7
Other Investments [Member]
Schedule Of Investments [Line Items]
Realized before impairments	13.8	7.8
Realized - impairments	0	0
Change in unrealized	1.6	0
Other Assets [Member]
Schedule Of Investments [Line Items]
Realized before impairments	-1.1	-4.2
Realized - impairments	0	0
Change in unrealized	-0.3	0
Fixed Maturities [Member]
Schedule Of Investments [Line Items]
Realized before impairments	0.4	1.1
Realized - impairments	-0.4	0
Change in unrealized	-12.3	8.1
Equity Securities [Member]
Schedule Of Investments [Line Items]
Realized before impairments	3.1	6.4
Realized - impairments	-0.1	0
Change in unrealized	-10.8	-3.1
Real Estate Holdings [Member]
Schedule Of Investments [Line Items]
Realized before impairments	0.3	0
Realized - impairments	0	0
Change in unrealized	0	0
Tax Effects [Member]
Schedule Of Investments [Line Items]
Realized before impairments	-6.1	-3.5
Realized - impairments	0.2	0
Change in unrealized	$2.90	$3.70

Investments_Financial_Assets_M

Investments - Financial Assets Measured at Fair Value on Recurring Basis (Detail) (USD $)	Mar. 31, 2015		Dec. 31, 2014
In Millions, unless otherwise specified
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	$4,064.40		$4,097.90
Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	3,648.40		3,700.10
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	855.7	[1]	858.5	[1]
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	2,791.90	[2]	2,840.70	[2]
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0.8	[3]	0.9	[3]
Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	520.9		495.1
Other Investments [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	104.9		97.3
Other Investments [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[1]	0	[1]
Other Investments [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	104.9	[2]	97.3	[2]
Other Investments [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[3]	0	[3]
Fixed Maturities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	2,788.60		2,840.70
Fixed Maturities [Member] | U.S. Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	182.8		185
Fixed Maturities [Member] | Non-U.S. Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	81.5		79.9
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	480		490.7
Fixed Maturities [Member] | Credit-Financial [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	484.7		518.1
Fixed Maturities [Member] | Credit-Industrial [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	469.9		499.6
Fixed Maturities [Member] | Credit-Utility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	147.2		142
Fixed Maturities [Member] | CMO/MBS-Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	168.7	[4]	175.3	[4]
Fixed Maturities [Member] | CMO/MBS-Non Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	13.4		14
Fixed Maturities [Member] | CMBS [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	186	[5]	180	[5]
Fixed Maturities [Member] | ABS [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	223.4	[6]	221	[6]
Fixed Maturities [Member] | Foreign Denominated: Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	141.6		140.2
Fixed Maturities [Member] | Foreign Denominated: Credit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	209.4		194.9
Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	2,788.60		2,840.70
Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	182.8		185
Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | Non-U.S. Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	81.5		79.9
Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	480		490.7
Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | Credit-Financial [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	484.7		518.1
Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | Credit-Industrial [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	469.9		499.6
Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | Credit-Utility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	147.2		142
Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | CMO/MBS-Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	168.7		175.3
Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | CMO/MBS-Non Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	13.4		14
Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | CMBS [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	186		180
Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | ABS [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	223.4		221
Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | Foreign Denominated: Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	141.6		140.2
Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | Foreign Denominated: Credit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	209.4		194.9
Fixed Maturities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	102	[1]	99.2	[1]
Fixed Maturities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	102	[1]	99.2	[1]
Fixed Maturities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] | Non-U.S. Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[1]	0	[1]
Fixed Maturities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[1]	0	[1]
Fixed Maturities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] | Credit-Financial [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[1]	0	[1]
Fixed Maturities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] | Credit-Industrial [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[1]	0	[1]
Fixed Maturities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] | Credit-Utility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[1]	0	[1]
Fixed Maturities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] | CMO/MBS-Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[1]	0	[1]
Fixed Maturities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] | CMO/MBS-Non Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[1]	0	[1]
Fixed Maturities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] | CMBS [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[1]	0	[1]
Fixed Maturities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] | ABS [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[1]	0	[1]
Fixed Maturities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] | Foreign Denominated: Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[1]	0	[1]
Fixed Maturities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] | Foreign Denominated: Credit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[1]	0	[1]
Fixed Maturities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	2,686.60	[2]	2,741.50	[2]
Fixed Maturities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	80.8	[2]	85.8	[2]
Fixed Maturities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | Non-U.S. Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	81.5	[2]	79.9	[2]
Fixed Maturities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	480	[2]	490.7	[2]
Fixed Maturities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | Credit-Financial [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	484.7	[2]	518.1	[2]
Fixed Maturities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | Credit-Industrial [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	469.9	[2]	499.6	[2]
Fixed Maturities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | Credit-Utility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	147.2	[2]	142	[2]
Fixed Maturities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | CMO/MBS-Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	168.7	[2]	175.3	[2]
Fixed Maturities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | CMO/MBS-Non Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	13.4	[2]	14	[2]
Fixed Maturities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | CMBS [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	186	[2]	180	[2]
Fixed Maturities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | ABS [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	223.4	[2]	221	[2]
Fixed Maturities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | Foreign Denominated: Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	141.6	[2]	140.2	[2]
Fixed Maturities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | Foreign Denominated: Credit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	209.4	[2]	194.9	[2]
Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[3]	0	[3]
Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[3]	0	[3]
Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | Non-U.S. Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[3]	0	[3]
Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[3]	0	[3]
Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | Credit-Financial [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[3]	0	[3]
Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | Credit-Industrial [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[3]	0	[3]
Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | Credit-Utility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[3]	0	[3]
Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | CMO/MBS-Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[3]	0	[3]
Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | CMO/MBS-Non Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[3]	0	[3]
Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | CMBS [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[3]	0	[3]
Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | ABS [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[3]	0	[3]
Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | Foreign Denominated: Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[3]	0	[3]
Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | Foreign Denominated: Credit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[3]	0	[3]
Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	483.4		486.3
Equity Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	483.4		486.3
Equity Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	482.6	[1]	485.4	[1]
Equity Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	[2]	0	[2]
Equity Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0.8	[3]	0.9	[3]
Short-Term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	271.5		275.8
Short-Term Investments [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	271.5		275.8
Short-Term Investments [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	271.1	[1]	273.9	[1]
Short-Term Investments [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0.4	[2]	1.9	[2]
Short-Term Investments [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	$0	[3]	$0	[3]
[1]	Quoted prices in active markets for identical assets
[2]	Significant other observable inputs
[3]	Significant unobservable inputs
[4]	Collateralized mortgage obligations/mortgage-backed securities (bCMO/MBSb).
[5]	Commercial mortgage-backed securities (bCMBSb).
[6]	Asset-backed securities (bABSb).

Investments_Schedule_of_Reconc

Investments - Schedule of Reconciliation of Beginning and Ending Balances for Investments Categorized as Level 3 (Detail) (USD $)	3 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$0.90	$3.90
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	-0.1	-0.5
Settlements	0	-2.6
Ending balance	0.8	0.9
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
Other Assets [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance		2.6
Transfers into Level 3		0
Transfers out of Level 3		0
Included in net income (loss)		0
Included in other comprehensive income (loss)		0
Purchases		0
Issuances		0
Sales		0
Settlements		-2.6
Ending balance		0
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
Equity Securities [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	0.9	1.3
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	-0.1	-0.5
Settlements	0	0
Ending balance	0.8	0.9
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

Reserves_for_Losses_and_Loss_A2

Reserves for Losses and Loss Adjustment Expenses - Reinsurance Recoverables (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Liability for Claims and Claims Adjustment Expense [Abstract]
Net reserves beginning of the year	$2,137.10	$2,107.60
Losses and LAE incurred during current calendar year, net of reinsurance:
Current accident year	187.4	191.4
Prior accident years	-3.7	-8.9
Losses and LAE incurred during calendar year, net of reinsurance	183.7	182.5
Losses and LAE payments made during current calendar year, net of reinsurance:
Current accident year	19.8	27.9
Prior accident years	141	150.1
Losses and LAE payments made during current calendar year, net of reinsurance:	160.8	178
Change in participation interest	-1.2	24.7
Foreign exchange adjustments	-6	1.7
Net reserves - end of period	2,152.80	2,138.50
Reinsurance recoverables on unpaid losses and LAE, end of period	882.3	903.9
Gross reserves - end of period	$3,035.10	$3,042.40	$3,042.40	[1]
[1]	Derived from audited consolidated financial statements.

Reserves_for_Losses_and_Loss_A3

Reserves for Losses and Loss Adjustment Expenses - Additional Information (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	$3.70	$8.90
Unfavorable Development In Property Lines [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years		4.2
Unfavorable Development [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years		4.5
Excess and Surplus Lines [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	8.2	8
Excess and Surplus Lines [Member] | General and Products Liability Lines of Business [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years		21.6
Commercial Specialty [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	7.2	2
International Specialty [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	2.5	0.4
Syndicate 1200 Segment [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	0.3	8.8
Run Off Lines [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	0.1	5.5
Commercial Automobile [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years		9.4
General Liability Claim Severity [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years		6.7
Workers Compensation [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years		4.3
Short Tail Lines [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years		1.1
Medical Malpractice Liability [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years		$1

Debt_Additional_Information_De

Debt - Additional Information (Detail) (USD $)	0 Months Ended	3 Months Ended
	Jul. 16, 2014	Mar. 31, 2015	Dec. 31, 2014		Mar. 07, 2014
Debt Instrument [Line Items]
Junior subordinated debentures		$172,700,000	$172,700,000	[1]
Junior subordinated debentures, Redemption Date	29-Apr-15
Junior Subordinated Debentures [Member]
Debt Instrument [Line Items]
Junior subordinated debentures	20,000,000
Percentage of debt securities redeemed as compare to principal amount	90.00%
Redemption received on debt securities	18,200,000
Recognition of pre-tax gain	2,000,000
Credit Agreement [Member]
Debt Instrument [Line Items]
Credit agreement with major money center banks, amount					175,000,000
Revolving credit facility maturity date		7-Mar-18
Letter of credit facility amount		0	200,000
Credit Agreement [Member] | Revolving Credit Facility
Debt Instrument [Line Items]
Letter of credit facility amount		$17,500,000
[1]	Derived from audited consolidated financial statements.

Disclosures_about_Fair_Value_o2

Disclosures about Fair Value of Financial Instruments - Additional Information (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Financial Instruments Owned and Pledged as Collateral [Line Items]
Premiums receivable	$371.70	$353.60	[1]
Reinsurance recoverables	987.3	997.2	[1]
Premiums receivable, allowance for doubtful accounts	4.5	5.2
Over 90 Days [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Premiums receivable	12.8	12.4
Reinsurance recoverables	6.8	9.9
Reinsurance recoverables, allowance for doubtful accounts	2.1	1.8
Premiums receivables, secured by collateral	0.3	0.3
Reinsurance recoverables, secured by collateral	$0.70	$0.40
[1]	Derived from audited consolidated financial statements.

Disclosures_about_Fair_Value_o3

Disclosures about Fair Value of Financial Instruments - Summary of Company's Financial Instruments Whose Carrying Amount Did Not Equal Fair Value (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	$374	$378.50
Fair Value	339.6	343.6
Junior Subordinated Debentures [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	172.7	172.7
Fair Value	155.5	155.5
Senior Unsecured Fixed Rate Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	143.8	143.8
Fair Value	132.3	132.3
Other Indebtedness [Member] | Floating Rate Loan Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	56.9	61.3
Fair Value	51.2	55.2
Other Indebtedness [Member] | Notes Payable [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	0.6	0.7
Fair Value	$0.60	$0.60

Shareholders_Equity_Additional

Shareholders Equity - Additional Information (Detail) (USD $)	0 Months Ended				3 Months Ended		0 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 19, 2015	Feb. 17, 2015	Mar. 17, 2014	Feb. 18, 2014	Mar. 31, 2015	Mar. 31, 2014	Nov. 05, 2013	5-May-15
Class Of Stock [Line Items]
Dividend declared per common share		$0.20		$0.15	$0.20	$0.14
Dividends payable date		16-Mar-15		17-Mar-14
Dividend payable, date of record		2-Mar-15		3-Mar-14
Stock dividend declared		10.00%		10.00%	10.00%
Dividends declaration date		17-Feb-15		18-Feb-14
Additional stock issued as dividend		2,554,506
Cash dividends paid - common shares, total	$5.70		$4
2013 Repurchase Authorization [Member]
Class Of Stock [Line Items]
Total number of shares authorized for purchase, Value					74.7		150
Common shares repurchased on open market					353,054
Common shares repurchased on open market, total cost					18.1
Common shares purchased					196,864
Common shares purchased, total cost					$9.90
10% Stock Dividend [Member]
Class Of Stock [Line Items]
Dividend declared per common share				$0.14
Subsequent Event [Member]
Class Of Stock [Line Items]
Dividend declared per common share								$0.20
Dividends payable date								15-Jun-15
Dividend payable, date of record								1-Jun-15

Shareholders_Equity_Schedule_o

Shareholders Equity - Schedule of Repurchase of Shares (Detail) (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2015
12/15/2014 [Member]
Equity Class Of Treasury Stock [Line Items]
Date Trading Plan Initiated	15-Dec-14
2015 Purchase Period	01/05/2015-02/12/2015
Number of Shares Repurchased	117,482
Average Price of Shares Repurchased	$53.50
Common shares purchased, total cost	$6.30
Repurchase Authorization Year	2013
12/15/2014 [Member] | Treasury Shares Period Start Date [Member]
Equity Class Of Treasury Stock [Line Items]
Trading Plans Purchase Period Date Range Start	1/5/15
12/15/2014 [Member] | Treasury Shares Period End Date [Member]
Equity Class Of Treasury Stock [Line Items]
Trading Plans Purchase Period Date Range Start	2/12/15
1/1/2022 [Member]
Equity Class Of Treasury Stock [Line Items]
Date Trading Plan Initiated	16-Mar-15
2015 Purchase Period	03/18/2015-03/31/2015
Number of Shares Repurchased	38,708
Average Price of Shares Repurchased	$49.65
Common shares purchased, total cost	$1.90
Repurchase Authorization Year	2013
1/1/2022 [Member] | Treasury Shares Period Start Date [Member]
Equity Class Of Treasury Stock [Line Items]
Trading Plans Purchase Period Date Range Start	3/18/15
1/1/2022 [Member] | Treasury Shares Period End Date [Member]
Equity Class Of Treasury Stock [Line Items]
Trading Plans Purchase Period Date Range Start	3/31/15

Accumulated_Other_Comprehensiv2

Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Income (Loss) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Beginning Balance	$108.10	[1]	$147.80
Other comprehensive (loss) income before reclassifications	-24.6		1.1
Amounts reclassified from accumulated other comprehensive income (loss)	2.6		0
Other comprehensive (loss) income, net of tax	-22		1.1
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Ending Balance	86.1		148.9
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Beginning Balance	-15.6		-11.5
Other comprehensive (loss) income before reclassifications	-3.1		-0.2
Amounts reclassified from accumulated other comprehensive income (loss)	0		0
Other comprehensive (loss) income, net of tax	-3.1		-0.2
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Ending Balance	-18.7		-11.7
Unrealized Holding Gains on Securities
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Beginning Balance	130.7		163.9
Other comprehensive (loss) income before reclassifications	-21.5		1.3
Amounts reclassified from accumulated other comprehensive income (loss)	2.6		0
Other comprehensive (loss) income, net of tax	-18.9		1.3
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Ending Balance	111.8		165.2
Defined Benefit Pension Plans
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Beginning Balance	-7		-4.6
Other comprehensive (loss) income before reclassifications	0		0
Amounts reclassified from accumulated other comprehensive income (loss)	0		0
Other comprehensive (loss) income, net of tax	0		0
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Ending Balance	($7)		($4.60)
[1]	Derived from audited consolidated financial statements.

Accumulated_Other_Comprehensiv3

Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Unrealized gains and losses on securities:
Net realized investment gains	$2.50	($0.10)
Provision for income taxes	0.1	0.1
Net of taxes	2.6	0
Unrealized Holding Gains on Securities
Unrealized gains and losses on securities:
Net of taxes	2.6	0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized Holding Gains on Securities
Unrealized gains and losses on securities:
Net realized investment gains	2.5	-0.1
Provision for income taxes	0.1	0.1
Net of taxes	$2.60	$0

Net_Income_Per_Common_Share_Ad

Net Income Per Common Share - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Feb. 17, 2015	Dec. 31, 2014		Feb. 18, 2014
Earnings Per Share [Abstract]
Stock dividend declared	10.00%		10.00%			10.00%
Treasury shares, shares	8,959,543	7,723,529		8,606,489	[1]
Antidilutive securities excluded from computation of diluted net income per share, shares	1,760	3,300
[1]	Derived from audited consolidated financial statements.

Net_Income_Per_Common_Share_Ne

Net Income Per Common Share - Net Income Per Common Share on Basic and Diluted Basis (Detail) (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Earnings Per Share [Abstract]
Net income	$58.80	$40.20
Weighted average common shares outstanding - basic	28,129,692	29,165,911
Effect of dilutive securities:
Equity compensation awards	547,687	497,179
Weighted average common shares outstanding - diluted	28,677,379	29,663,090
Net income per common share - basic	$2.09	$1.38
Net income per common share - diluted	$2.05	$1.36

Supplemental_Cash_Flow_Informa2

Supplemental Cash Flow Information - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Income Taxes Paid Net [Abstract]
Income taxes paid	$500,000	$15,600,000
Income taxes recovered	$11,600,000	$0

Supplemental_Cash_Flow_Informa3

Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Supplemental Cash Flow [Line Items]
Total interest paid	$4.80	$5.20
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	1.7	2
Other Indebtedness [Member]
Supplemental Cash Flow [Line Items]
Total interest paid	$0.80	$0.90

Sharebased_Compensation_Additi

Share-based Compensation - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Feb. 17, 2015	Feb. 18, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Share-based compensation arrangement by share-based payment award, award vesting period, years	5 years	5 years
Stock dividend declared	10.00%		10.00%	10.00%
Percentage increase in number of awards outstanding due to stock dividend	10.00%
Reduction in exercise price of awards outstanding due to stock dividend	9.09%

Sharebased_Compensation_Fair_V

Share-based Compensation - Fair Value Assumptions (Detail)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Risk-free rate of return	1.47%	1.74%
Expected dividend yields	1.63%	1.49%
Expected award life (years)	4 years 8 months 16 days	4 years 10 months 6 days
Expected volatility	22.09%	24.91%

ShareBased_Compensation_2014_L

Share-Based Compensation - 2014 Long-Term Incentive Plan - Additional Information (Detail) (USD $)	3 Months Ended		1 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	31-May-14	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 Share Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, compensation expense	1.5	0.7
Employee service share-based compensation, unrecognized compensation costs on non-vested awards	8
Restricted Share Units [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	2 years
Restricted Share Units [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
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	0.6	0.6
Employee service share-based compensation, unrecognized compensation costs on non-vested awards	3.9
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	2.7	2.7
Employee service share-based compensation, unrecognized compensation costs on non-vested awards	14.5
Cash-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
Cash-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
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			1,457,800
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

Sharebased_Compensation_Summar

Share-based Compensation - Summary of Restricted Share Activity (Detail) (Restricted Stock Units (RSUs) [Member], USD $)	3 Months Ended
	Mar. 31, 2015
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares, Outstanding at January 1, 2015	313,716
Granted, Shares	51,873
Vested and issued, Shares	-110,982
Expired or forfeited, Shares	-3,569
Shares, Outstanding at March 31, 2015	251,038
Weighted-Average Grant Date Fair Value, Outstanding at January 1, 2015	$35.17
Granted, Weighted-Average Grant Date Fair Value	$46.57
Vested and issued, Weighted-Average Grant Date Fair Value	$29.88
Expired or forfeited, Weighted-Average Grant Date Fair Value	$36.38
Weighted-Average Grant Date Fair Value, Outstanding at March 31, 2015	$39.85

Sharebased_Compensation_Summar1

Share-based Compensation - Summary of Stock-Settled SARs Activity (Detail) (Stock-Settled SARs Activity [Member], USD $)	3 Months Ended
	Mar. 31, 2015
Stock-Settled SARs Activity [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares, Outstanding at January 1, 2015	1,313,901
Granted, Shares	243,306
Exercised, Shares	-24,622
Expired or forfeited, Shares	-9,210
Shares, Outstanding at March 31, 2015	1,523,375
Weighted-Average Exercise Price, Outstanding at January 1, 2015	$28.65
Granted, Weighted-Average Exercise Price	$37.33
Exercised, Weighted-Average Exercise Price	$28.11
Expired or forfeited, Weighted-Average Exercise Price	$30.68
Weighted-Average Exercise Price, Outstanding at March 31, 2015	$29.26

Sharebased_Compensation_Summar2

Share-based Compensation - Summary of Cash-Settled SARs Activity (Detail) (Cash-Settled SARs Activity [Member], USD $)	3 Months Ended
	Mar. 31, 2015
Cash-Settled SARs Activity [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares, Outstanding at January 1, 2015	2,001,451
Granted, Shares	866,673
Exercised, Shares	-31,461
Expired or forfeited, Shares	-70,547
Shares, Outstanding at March 31, 2015	2,766,116
Weighted-Average Exercise Price, Outstanding at January 1, 2015	$32.76
Granted, Weighted-Average Exercise Price	$47.83
Exercised, Weighted-Average Exercise Price	$29.77
Expired or forfeited, Weighted-Average Exercise Price	$36.28
Weighted-Average Exercise Price, Outstanding at March 31, 2015	$37.42

Underwriting_Acquisition_and_I2

Underwriting, Acquisition and Insurance Expenses - Underwriting, Acquisition and Insurance Expenses (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Other Income And Expenses [Abstract]
Commissions	$58.70	$54.90
General expenses	72.9	68.8
Premium taxes, boards and bureaus	0.6	6.6
Underwriting, acquisition and insurance expenses, total	132.2	130.3
Net deferral of policy acquisition costs	-2.6	-1.6
Total underwriting, acquisition and insurance expenses	$129.60	$128.70

Underwriting_Acquisition_and_I3

Underwriting, Acquisition and Insurance Expenses - Additional Information (Detail) (Stock Compensation Plan [Member], USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Stock Compensation Plan [Member]
Underwriting Acquisition And Insurance Expense [Line Items]
Expense associated with cash settled stock appreciation rights	$4.90	$4.30

Income_Taxes_Income_Tax_Provis

Income Taxes - Income Tax Provision (Benefit) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Income Tax Disclosure [Abstract]
Current tax (benefit) provision	($0.20)	$1.40
Future tax deductions	5.5	30.2
Valuation allowance change	-1.7	-29.1
Income tax provision	$3.60	$2.50

Income_Taxes_Schedule_of_PreTa

Income Taxes - Schedule of Pre-Tax Income (Loss) and Effective Income Tax Rates (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$62.40	$42.70
Bermuda [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	25.7	21.9
Effective Tax Rate	0.00%	0.00%
United States [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	28.5	14.8
Effective Tax Rate	20.20%	20.60%
United Kingdom [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	10.1	7.3
Effective Tax Rate	-21.90%	-7.70%
Belgium [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	-0.2	0	[1]
Effective Tax Rate	-4.80%	18.10%
Brazil [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	-1.9	0
Effective Tax Rate	0.00%	0.00%
United Arab Emirates [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	0.1	-0.9
Effective Tax Rate	0.00%	0.00%
Ireland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	-0.2	0
Effective Tax Rate	0.00%	0.00%
Malta [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	0.3	-0.4
Effective Tax Rate	0.00%	0.00%
Switzerland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$0	$0	[1]
Effective Tax Rate	21.00%	-19.80%
[1]	Pre-tax income for the respective year was less than $0.1 million.

Income_Taxes_Schedule_of_PreTa1

Income Taxes - Schedule of Pre-Tax Income (Loss) and Effective Income Tax Rates (Parenthetical) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	$62.40	$42.70
Belgium [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	-0.2	0	[1]
Switzerland [Member]
Income Tax Examination [Line Items]
Pre-Tax Income (Loss)	0	0	[1]
Maximum [Member] | Belgium [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)	$0.10	$0.10
[1]	Pre-tax income for the respective year was less than $0.1 million.

Income_Taxes_Reconciliation_of

Income Taxes - Reconciliation of Difference Between Provision for Income Taxes and Expected Tax Provision at Weighted Average Tax Rate (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Income Tax Disclosure [Abstract]
Income tax provision at expected rate	$11.20	$4.70
Tax-exempt interest	-0.9	-1.2
Dividends received deduction	-0.6	-0.6
Valuation allowance change	-1.7	-29.1
Other permanent adjustments, net	0.2	0.1
Adjustment for annualized rate	-0.5	0.8
United States state tax (benefit) expense	-2.5	0.1
Capital loss carryforward from prior merger	0	29.8
Other foreign adjustments	-0.3	0
Prior period adjustments to deferred	2.2	0
Prior year foreign taxes recovered	-1.2	0
Foreign exchange adjustments	-2.4	-2.2
Foreign withholding taxes	0.1	0.1
Income tax provision	3.6	2.5
Income tax benefit - Foreign	-2.2	-0.6
Income tax provision - United States, Federal	9.5	2.8
Income tax (benefit) provision - United States, state	-3.8	0.2
Foreign withholding tax - United States	$0.10	$0.10

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Income Tax Contingency [Line Items]
Total net deferred tax liabilities	$30,000,000
Net operating loss carryforward	17,800,000
Valuation allowance	23,800,000
Loss carryforwards per year	2,800,000
Unrecognized tax benefits	0	0
ARIS [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforward	1,000,000
Malta [Member]
Income Tax Contingency [Line Items]
Valuation allowance	1,800,000
Decreased valuation allowance	300,000
Brazil [Member]
Income Tax Contingency [Line Items]
Valuation allowance	6,100,000
Decreased valuation allowance	1,300,000
December 31, 2025 [Member]
Income Tax Contingency [Line Items]
Loss carryforwards, amounts expiring if not utilized	16,300,000
December 31, 2027 [Member]
Income Tax Contingency [Line Items]
Loss carryforwards, amounts expiring if not utilized	1,500,000
December 31, 2027 [Member] | ARIS [Member]
Income Tax Contingency [Line Items]
Loss carryforwards, amounts expiring if not utilized	200,000
December 31, 2028 [Member] | ARIS [Member]
Income Tax Contingency [Line Items]
Loss carryforwards, amounts expiring if not utilized	400,000
December 31, 2029 [Member] | ARIS [Member]
Income Tax Contingency [Line Items]
Loss carryforwards, amounts expiring if not utilized	400,000
PXRE Reinsurance Company [Member]
Income Tax Contingency [Line Items]
Valuation allowance	15,900,000
Reduced valuation allowance	$100,000

Commitments_and_Contingencies_

Commitments and Contingencies - Additional Information (Detail) (USD $)	Mar. 31, 2015
In Millions, unless otherwise specified
Commitments And Contingencies Disclosure [Abstract]
Contractual commitments related to its limited partnership investments	$60.60

Segment_Information_Additional

Segment Information - Additional Information (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
	Segment
Segment Reporting [Abstract]
Number of reportable segments	4
Assets associated with trade capital providers	$294.20	$315.40

Segment_Information_Revenue_an

Segment Information - Revenue and Income (Loss) Before Income Taxes for Each Segment (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Earned premiums	$334.60	$325.70
Net investment income	20.8	23.3
Net realized investment and other gains	16	11.1
Total revenue	371.4	360.1
Income before income taxes	62.4	42.7
Operating Segments [Member]
Segment Reporting Information [Line Items]
Income before income taxes	49.8	38.5
Excess and Surplus Lines [Member]
Segment Reporting Information [Line Items]
Earned premiums	123.1	117.4
Net investment income	8.5	9.3
Excess and Surplus Lines [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Income before income taxes	24.7	20.1
Commercial Specialty [Member]
Segment Reporting Information [Line Items]
Earned premiums	72.6	71.7
Net investment income	4.5	4.7
Commercial Specialty [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Income before income taxes	3.8	1.7
International Specialty [Member]
Segment Reporting Information [Line Items]
Earned premiums	37.3	37
Net investment income	2.9	1.8
International Specialty [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Income before income taxes	9.1	6.8
Syndicate 1200 [Member]
Segment Reporting Information [Line Items]
Earned premiums	101.9	99.3
Net investment income	2.2	3.7
Syndicate 1200 [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Income before income taxes	11	15.8
Run Off Lines [Member]
Segment Reporting Information [Line Items]
Earned premiums	-0.3	0.3
Net investment income	2	2.5
Run Off Lines [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Income before income taxes	1.2	-5.9
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Net investment income	0.7	1.3
Net realized investment and other gains	($3.40)	($6.90)

Segment_Information_Schedule_o

Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$334.60	$325.70
Bermuda [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	23.9	24.4
Brazil [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	12.5	12.6
Malta [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	0.5	0.5
United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	101.9	98.9
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$195.80	$189.30

Segment_Information_Identifiab

Segment Information - Identifiable Assets (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$6,377.10	$6,356.30	[1]
Excess and Surplus Lines [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	2,352.80	2,350.70
Commercial Specialty [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	1,384	1,358.50
International Specialty [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	799.5	776.8
Syndicate 1200 [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	1,232.50	1,258.50
Run Off Lines [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	557.8	552.8
Corporate and Other [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$50.50	$59
[1]	Derived from audited consolidated financial statements.

Information_Provided_in_Connec2

Information Provided in Connection With Outstanding Debt of Subsidiaries - Additional Information (Detail)	Mar. 31, 2015
Debt Disclosure [Abstract]
Ownership Percentage	100.00%

Information_Provided_in_Connec3

Information Provided in Connection with Outstanding Debt of Subsidiaries - Condensed Consolidating Balance Sheet (Detail) (USD $)	Mar. 31, 2015		Dec. 31, 2014		Mar. 31, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Assets
Investments	$4,064.40		$4,097.90	[1]
Cash	89.1		81	[1]	149.5		157.4
Accrued investment income	20.9		22.1	[1]
Premiums receivable	371.7		353.6	[1]
Reinsurance recoverables	987.3		997.2	[1]
Goodwill and other intangible assets, net	231		230.8
Current income taxes receivable, net	4.2		14.9	[1]
Deferred acquisition costs, net	129.7		124.6	[1]
Ceded unearned premiums	256.6		207.6	[1]
Other assets	222.2		226.6	[1]
Due from (to) affiliates	0		0
Intercompany note receivable	0		0
Investments in subsidiaries	0		0
Total assets	6,377.10		6,356.30	[1]
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	3,035.10		3,042.40	[1]	3,042.40
Unearned premiums	826.4		817.2	[1]
Funds held and ceded reinsurance payable, net	260.5		233.8
Long-term debt	374		378.5
Current income taxes payable, net	0
Deferred tax liabilities, net	53.8		53	[1]
Accrued underwriting expenses and other liabilities	164.5		184.7
Total liabilities	4,714.30		4,709.60	[1]
Total shareholders' equity	1,662.80		1,646.70	[1]
Total liabilities and shareholders' equity	6,377.10		6,356.30	[1]
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Assets
Investments	7.2		0.7
Cash	0		0		0		0
Accrued investment income	0		0
Premiums receivable	0		0
Reinsurance recoverables	0		0
Goodwill and other intangible assets, net	0		0
Current income taxes receivable, net	0		0
Deferred acquisition costs, net	0		0
Ceded unearned premiums	0		0
Other assets	12.7		9.6
Due from (to) affiliates	-16		2.9
Intercompany note receivable	0		0
Investments in subsidiaries	1,722.80		1,698
Total assets	1,726.70		1,711.20
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	49		49
Current income taxes payable, net	0
Deferred tax liabilities, net	0		0
Accrued underwriting expenses and other liabilities	14.9		15.5
Total liabilities	63.9		64.5
Total shareholders' equity	1,662.80		1,646.70
Total liabilities and shareholders' equity	1,726.70		1,711.20
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Assets
Investments	2,851.40		2,841.50
Cash	54.8		49.3		118.4		132.1
Accrued investment income	17.1		17.8
Premiums receivable	160		154.6
Reinsurance recoverables	1,152		1,173.60
Goodwill and other intangible assets, net	132.7		131.7
Current income taxes receivable, net	0		10.1
Deferred acquisition costs, net	60		58
Ceded unearned premiums	112.2		98.5
Other assets	175.4		174.1
Due from (to) affiliates	-19.3		-19.8
Intercompany note receivable	55.6		72
Investments in subsidiaries	0		0
Total assets	4,751.90		4,761.40
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	2,138.80		2,136.40
Unearned premiums	458.8		448.9
Funds held and ceded reinsurance payable, net	662.8		675.1
Long-term debt	288.7		288.7
Current income taxes payable, net	0.5
Deferred tax liabilities, net	44.7		41.2
Accrued underwriting expenses and other liabilities	91.7		104.2
Total liabilities	3,686		3,694.50
Total shareholders' equity	1,065.90		1,066.90
Total liabilities and shareholders' equity	4,751.90		4,761.40
Other Subsidiaries and Eliminations [Member]
Assets
Investments	1,205.80	[2]	1,255.70	[2]
Cash	34.3	[2]	31.7	[2]	31.1	[2]	25.3	[2]
Accrued investment income	3.8	[2]	4.3	[2]
Premiums receivable	211.7	[2]	199	[2]
Reinsurance recoverables	-164.7	[2]	-176.4	[2]
Goodwill and other intangible assets, net	98.3	[2]	99.1	[2]
Current income taxes receivable, net	4.2	[2]	4.8	[2]
Deferred acquisition costs, net	69.7	[2]	66.6	[2]
Ceded unearned premiums	144.4	[2]	109.1	[2]
Other assets	59.1	[2]	67.9	[2]
Due from (to) affiliates	19.3	[2]	19.8	[2]
Intercompany note receivable	-55.6	[2]	-72	[2]
Investments in subsidiaries	0	[2]	0	[2]
Total assets	1,630.30	[2]	1,609.60	[2]
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	896.3	[2]	906	[2]
Unearned premiums	367.6	[2]	368.3	[2]
Funds held and ceded reinsurance payable, net	-402.3	[2]	-441.3	[2]
Long-term debt	56.9	[2]	61.4	[2]
Current income taxes payable, net	-0.5	[2]
Deferred tax liabilities, net	9.1	[2]	11.8	[2]
Accrued underwriting expenses and other liabilities	57.9	[2]	65	[2]
Total liabilities	985	[2]	971.2	[2]
Total shareholders' equity	645.3	[2]	638.4	[2]
Total liabilities and shareholders' equity	1,630.30	[2]	1,609.60	[2]
Consolidating Adjustments [Member]
Assets
Investments	0	[3]	0	[3]
Cash	0	[3]	0	[3]	0	[3]	0	[3]
Accrued investment income	0	[3]	0	[3]
Premiums receivable	0	[3]	0	[3]
Reinsurance recoverables	0	[3]	0	[3]
Goodwill and other intangible assets, net	0	[3]	0	[3]
Current income taxes receivable, net	0	[3]	0	[3]
Deferred acquisition costs, net	0	[3]	0	[3]
Ceded unearned premiums	0	[3]	0	[3]
Other assets	-25	[3]	-25	[3]
Due from (to) affiliates	16	[3]	-2.9	[3]
Intercompany note receivable	0	[3]	0	[3]
Investments in subsidiaries	-1,722.80	[3]	-1,698	[3]
Total assets	-1,731.80	[3]	-1,725.90	[3]
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	0	[3]	0	[3]
Unearned premiums	0	[3]	0	[3]
Funds held and ceded reinsurance payable, net	0	[3]	0	[3]
Long-term debt	-20.6	[3]	-20.6	[3]
Current income taxes payable, net	0	[3]
Deferred tax liabilities, net	0	[3]	0	[3]
Accrued underwriting expenses and other liabilities	0	[3]	0	[3]
Total liabilities	-20.6	[3]	-20.6	[3]
Total shareholders' equity	-1,711.20	[3]	-1,705.30	[3]
Total liabilities and shareholders' equity	($1,731.80)	[3]	($1,725.90)	[3]
[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.

Information_Provided_in_Connec4

Information Provided in Connection with Outstanding Debt of Subsidiaries - Condensed Consolidating Statement of Income (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Premiums and other revenue:
Earned premiums	$334.60		$325.70
Net investment (expense) income	20.8		23.3
Net realized investment and other gains (loss)	16		11.1
Total revenue	371.4		360.1
Expenses:
Losses and loss adjustment expenses	183.7		182.5
Underwriting, acquisition and insurance expenses	129.6		128.7
Interest expense	4.9		5
Fee expense, net	0.4		1.4
Foreign currency exchange (gain) loss	-9.6		-0.2
Total expenses	309		317.4
Income before income taxes	62.4		42.7
Provision for income taxes	3.6		2.5
Net income (loss) before equity in earnings of subsidiaries	58.8		40.2
Equity in undistributed earnings of subsidiaries	0		0
Net income	58.8		40.2
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Premiums and other revenue:
Earned premiums	0		0
Net investment (expense) income	-0.2		-0.1
Net realized investment and other gains (loss)	0		0
Total revenue	-0.2		-0.1
Expenses:
Losses and loss adjustment expenses	0		0
Underwriting, acquisition and insurance expenses	4.7		6
Interest expense	0.5		0.6
Fee expense, net	0		0
Foreign currency exchange (gain) loss	0		0
Total expenses	5.2		6.6
Income before income taxes	-5.4		-6.7
Provision for income taxes	0		0
Net income (loss) before equity in earnings of subsidiaries	-5.4		-6.7
Equity in undistributed earnings of subsidiaries	64.2		46.9
Net income	58.8		40.2
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Premiums and other revenue:
Earned premiums	135.1		112
Net investment (expense) income	14.5		15
Net realized investment and other gains (loss)	15.9		11.5
Total revenue	165.5		138.5
Expenses:
Losses and loss adjustment expenses	80.2		72.5
Underwriting, acquisition and insurance expenses	51.9		46.3
Interest expense	3.8		3.8
Fee expense, net	0.8		1.3
Foreign currency exchange (gain) loss	0.8		-0.1
Total expenses	137.5		123.8
Income before income taxes	28		14.7
Provision for income taxes	5.8		3
Net income (loss) before equity in earnings of subsidiaries	22.2		11.7
Equity in undistributed earnings of subsidiaries	0		0
Net income	22.2		11.7
Other Subsidiaries and Eliminations [Member]
Premiums and other revenue:
Earned premiums	199.5	[1]	213.7	[1]
Net investment (expense) income	6.5	[1]	8.4	[1]
Net realized investment and other gains (loss)	0.1	[1]	-0.4	[1]
Total revenue	206.1	[1]	221.7	[1]
Expenses:
Losses and loss adjustment expenses	103.5	[1]	110	[1]
Underwriting, acquisition and insurance expenses	73	[1]	76.4	[1]
Interest expense	0.6	[1]	0.7	[1]
Fee expense, net	-0.4	[1]	0.1	[1]
Foreign currency exchange (gain) loss	-10.4	[1]	-0.1	[1]
Total expenses	166.3	[1]	187.1	[1]
Income before income taxes	39.8	[1]	34.6	[1]
Provision for income taxes	-2.2	[1]	-0.5	[1]
Net income (loss) before equity in earnings of subsidiaries	42	[1]	35.1	[1]
Equity in undistributed earnings of subsidiaries	0	[1]	0	[1]
Net income	42	[1]	35.1	[1]
Consolidating Adjustments [Member]
Premiums and other revenue:
Earned premiums	0	[2]	0	[2]
Net investment (expense) income	0	[2]	0	[2]
Net realized investment and other gains (loss)	0	[2]	0	[2]
Total revenue	0	[2]	0	[2]
Expenses:
Losses and loss adjustment expenses	0	[2]	0	[2]
Underwriting, acquisition and insurance expenses	0	[2]	0	[2]
Interest expense	0	[2]	-0.1	[2]
Fee expense, net	0	[2]	0	[2]
Foreign currency exchange (gain) loss	0	[2]	0	[2]
Total expenses	0	[2]	-0.1	[2]
Income before income taxes	0	[2]	0.1	[2]
Provision for income taxes	0	[2]	0	[2]
Net income (loss) before equity in earnings of subsidiaries	0	[2]	0.1	[2]
Equity in undistributed earnings of subsidiaries	-64.2	[2]	-46.9	[2]
Net income	($64.20)	[2]	($46.80)	[2]
[1]	Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations.
[2]	Includes all Argo Group parent company eliminations.

Information_Provided_in_Connec5

Information Provided in Connection with Outstanding Debt of Subsidiaries - Condensed Consolidating Statement of Cash Flows (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Condensed Financial Statements Captions [Line Items]
Net cash flows from operating activities	$27.60		$43.60
Cash flows from investing activities:
Proceeds from sales of investments	423		263.3
Maturities and mandatory calls of fixed maturity investments	142		78.4
Purchases of investments	-557.2		-307.3
Change in short-term investments and foreign regulatory deposits	4.8		-53.9
Settlements of foreign currency exchange forward contracts	-2.2		-1.4
Issuance of intercompany note, net	0		0
Purchases of fixed assets and other, net	-8.3		-24.1
Cash provided (used) by investing activities	2.1		-45
Cash flows from financing activities:
Borrowings under intercompany note, net	0		0
Activity under stock incentive plans	0.9		0.8
Repurchase of Company's common shares	-17		-3.7
Excess tax expense from share-based payment arrangements	0		0.1
Payment of cash dividends to common shareholders	-5.7		-4
Cash used by financing activities	-21.8		-6.8
Effect of exchange rate changes on cash	0.2		0.3
Change in cash	8.1		-7.9
Cash, beginning of period	81	[1]	157.4
Cash, end of period	89.1		149.5
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Condensed Financial Statements Captions [Line Items]
Net cash flows from operating activities	2.9		-9.4
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	1.1		-0.9
Settlements of foreign currency exchange forward contracts	0.7		0.9
Issuance of intercompany note, net	0		0
Purchases of fixed assets and other, net	0.1		0.1
Cash provided (used) by investing activities	1.9		0.1
Cash flows from financing activities:
Borrowings under intercompany note, net	0		12.5
Activity under stock incentive plans	0.9		0.8
Repurchase of Company's common shares	0		0
Excess tax expense from share-based payment arrangements	0		0
Payment of cash dividends to common shareholders	-5.7		-4
Cash used by financing activities	-4.8		9.3
Effect of exchange rate changes on cash	0		0
Change in cash	0		0
Cash, beginning of period	0		0
Cash, end of period	0		0
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Condensed Financial Statements Captions [Line Items]
Net cash flows from operating activities	43.6		-11.2
Cash flows from investing activities:
Proceeds from sales of investments	340.1		163.3
Maturities and mandatory calls of fixed maturity investments	114.5		51.3
Purchases of investments	-471.4		-178.6
Change in short-term investments and foreign regulatory deposits	7		-29.8
Settlements of foreign currency exchange forward contracts	0		0
Issuance of intercompany note, net	0		0
Purchases of fixed assets and other, net	-11.3		-5.1
Cash provided (used) by investing activities	-21.1		1.1
Cash flows from financing activities:
Borrowings under intercompany note, net	0		0
Activity under stock incentive plans	0		0
Repurchase of Company's common shares	-17		-3.7
Excess tax expense from share-based payment arrangements	0		0.1
Payment of cash dividends to common shareholders	0		0
Cash used by financing activities	-17		-3.6
Effect of exchange rate changes on cash	0		0
Change in cash	5.5		-13.7
Cash, beginning of period	49.3		132.1
Cash, end of period	54.8		118.4
Other Subsidiaries and Eliminations [Member]
Condensed Financial Statements Captions [Line Items]
Net cash flows from operating activities	-18.9	[2]	64.2	[2]
Cash flows from investing activities:
Proceeds from sales of investments	82.9	[2]	100	[2]
Maturities and mandatory calls of fixed maturity investments	27.5	[2]	27.1	[2]
Purchases of investments	-85.8	[2]	-128.7	[2]
Change in short-term investments and foreign regulatory deposits	-3.3	[2]	-23.2	[2]
Settlements of foreign currency exchange forward contracts	-2.9	[2]	-2.3	[2]
Issuance of intercompany note, net	0	[2]	-12.5	[2]
Purchases of fixed assets and other, net	2.9	[2]	-19.1	[2]
Cash provided (used) by investing activities	21.3	[2]	-58.7	[2]
Cash flows from financing activities:
Borrowings under intercompany note, net	0	[2]	0	[2]
Activity under stock incentive plans	0	[2]	0	[2]
Repurchase of Company's common shares	0	[2]	0	[2]
Excess tax expense from share-based payment arrangements	0	[2]	0	[2]
Payment of cash dividends to common shareholders	0	[2]	0	[2]
Cash used by financing activities	0	[2]	0	[2]
Effect of exchange rate changes on cash	0.2	[2]	0.3	[2]
Change in cash	2.6	[2]	5.8	[2]
Cash, beginning of period	31.7	[2]	25.3	[2]
Cash, end of period	34.3	[2]	31.1	[2]
Consolidating Adjustments [Member]
Condensed Financial Statements Captions [Line Items]
Net cash flows from operating activities	0	[3]	0	[3]
Cash flows from investing activities:
Proceeds from sales of investments	0	[3]	0	[3]
Maturities and mandatory calls of fixed maturity investments	0	[3]	0	[3]
Purchases of investments	0	[3]	0	[3]
Change in short-term investments and foreign regulatory deposits	0	[3]	0	[3]
Settlements of foreign currency exchange forward contracts	0	[3]	0	[3]
Issuance of intercompany note, net	0	[3]	12.5	[3]
Purchases of fixed assets and other, net	0	[3]	0	[3]
Cash provided (used) by investing activities	0	[3]	12.5	[3]
Cash flows from financing activities:
Borrowings under intercompany note, net	0	[3]	-12.5	[3]
Activity under stock incentive plans	0	[3]	0	[3]
Repurchase of Company's common shares	0	[3]	0	[3]
Excess tax expense from share-based payment arrangements	0	[3]	0	[3]
Payment of cash dividends to common shareholders	0	[3]	0	[3]
Cash used by financing activities	0	[3]	-12.5	[3]
Effect of exchange rate changes on cash	0	[3]	0	[3]
Change in cash	0	[3]	0	[3]
Cash, beginning of period	0	[3]	0	[3]
Cash, end of period	$0	[3]	$0	[3]
[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.