Argo Group International (ARGO-PA) 10-Q 12 Nov 13 2013 Q3 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
	Sep. 30, 2013	Nov. 05, 2013
Document And Entity Information [Abstract]	'	'
Document Type	'10-Q	'
Amendment Flag	'false	'
Document Period End Date	30-Sep-13	'
Document Fiscal Year Focus	'2013	'
Document Fiscal Period Focus	'Q3	'
Trading Symbol	'AGII	'
Entity Registrant Name	'Argo Group International Holdings, Ltd.	'
Entity Central Index Key	'0001091748	'
Current Fiscal Year End Date	'--12-31	'
Entity Filer Category	'Large Accelerated Filer	'
Entity Common Stock, Shares Outstanding	'	26,625,705

CONSOLIDATED_BALANCE_SHEETS

CONSOLIDATED BALANCE SHEETS (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Assets	'	'
Available-for-sale (cost: 2013 - $2,757.9; 2012 - $2,993.1)	$2,811.90	$3,154
Equity securities, at fair value (cost: 2013 - $386.5; 2012 - $373.5)	580.8	521.4
Other investments (cost: 2013 - $366.9; 2012 - $294.8)	371.2	291
Short-term investments, at fair value (cost: 2013 - $286.3; 2012 - $234.3)	286.3	234.3
Total investments	4,050.20	4,200.70
Cash	141.6	95.8
Accrued investment income	25.3	30.3
Premiums receivable	426.4	361
Reinsurance recoverables	1,257.20	1,320.90
Goodwill	153.8	153.8
Intangible assets, net of accumulated amortization	87.3	91.5
Current income taxes receivable, net	15.3	12.9
Deferred acquisition costs, net	115.3	99.4
Ceded unearned premiums	231	193.6
Other assets	147.2	129
Total assets	6,650.60	6,688.90
Liabilities and Shareholders' Equity	'	'
Reserves for losses and loss adjustment expenses	3,222.90	3,223.50
Unearned premiums	847	730.2
Accrued underwriting expenses	126	105
Ceded reinsurance payable, net	412.2	612.1
Funds held	44.4	33.7
Senior unsecured fixed rate notes	143.8	143.8
Other indebtedness	64.9	63.8
Junior subordinated debentures	193.3	193.3
Deferred tax liabilities, net	30.7	43.8
Other liabilities	38	25.6
Total liabilities	5,123.20	5,174.80
Shareholders' equity:	'	'
Common shares - $1.00 par, 500,000,000 shares authorized; 34,028,008 and 31,384,271 shares issued at September 30, 2013 and December 31, 2012, respectively	34	31.4
Additional paid-in capital	825.3	722.7
Treasury shares (7,408,601 and 6,459,613 shares at September 30, 2013 and December 31, 2012, respectively)	-244.2	-205.5
Retained earnings	760.5	776
Accumulated other comprehensive income, net of taxes	151.8	189.5
Total shareholders' equity	1,527.40	1,514.10
Total liabilities and shareholders' equity	$6,650.60	$6,688.90

CONSOLIDATED_BALANCE_SHEETS_Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, except Share data, unless otherwise specified
Statement Of Financial Position [Abstract]	'	'
Available-for-sale cost	$2,757.90	$2,993.10
Equity securities cost	386.5	373.5
Other investments cost	366.9	294.8
Short-term investments, Cost	$286.30	$234.30
Common shares, par value	$1	$1
Common shares, shares authorized	500,000,000	500,000,000
Common shares, shares issued	34,028,008	31,384,271
Treasury shares, shares	7,408,601	6,459,613

CONSOLIDATED_STATEMENTS_OF_INC

CONSOLIDATED STATEMENTS OF INCOME (USD $)	3 Months Ended		9 Months Ended
In Millions, except Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Premiums and other revenue:	'	'	'	'
Earned premiums	$330.50	$304.30	$962.20	$871.80
Net investment income	24.1	28.9	77.3	90.3
Fee income, net	0.3	3	0.5	4.8
Net realized investment gains	9.1	8.3	29.6	18.7
Total revenue	364	344.5	1,069.60	985.6
Expenses:	'	'	'	'
Losses and loss adjustment expenses	190.9	186.3	554.1	527.9
Other reinsurance-related expenses	4.6	7	14.4	20.8
Underwriting, acquisition and insurance expenses	126.7	118	378	346.3
Interest expense	5.1	5.8	15.1	17
Foreign currency exchange gain (loss)	3.8	9.7	-5.2	2.8
Total expenses	331.1	326.8	956.4	914.8
Income before income taxes	32.9	17.7	113.2	70.8
Provision for income taxes	1.9	4.3	17.8	13.8
Net income	31	13.4	95.4	57
Net income per common share:	'	'	'	'
Basic	$1.16	$0.48	$3.54	$2.02
Diluted	$1.12	$0.47	$3.41	$1.98
Dividend declared per common share:	$0.15	$0.12	$0.45	$0.36
Weighted average common shares:	'	'	'	'
Basic	26,682,176	27,751,140	26,961,792	28,265,456
Diluted	27,819,550	28,333,796	27,993,335	28,713,124
Net realized investment gains before other-than-temporary impairment losses	9.2	9.4	34.9	20.7
Other-than-temporary impairment losses recognized in earnings	'	'	'	'
Other-than-temporary impairment losses on fixed maturities	0	-1.1	-0.3	-1.4
Other-than-temporary impairment losses on equity securities	-0.1	0	-1.6	-0.6
Other-than-temporary impairment losses on other investments	0	0	-3.4	0
Non-credit portion of losses recognized in other comprehensive income	0	0	0	0
Impairment losses recognized in earnings	-0.1	-1.1	-5.3	-2
Net realized investment gains	$9.10	$8.30	$29.60	$18.70

CONSOLIDATED_STATEMENTS_OF_COM

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Statement Of Income And Comprehensive Income [Abstract]	'	'	'	'
Net income	$31	$13.40	$95.40	$57
Other comprehensive income (loss):	'	'	'	'
Foreign currency translation adjustments	-0.3	0.9	-2.2	-2.4
Defined benefit pension plan:	'	'	'	'
Net loss arising during the period	0	0	0	-0.3
Net loss arising during the period	0	0	0	-0.9
Unrealized gains (losses) on securities:	'	'	'	'
Gains (losses) arising during the period	23.2	64.6	-34.8	106.9
Reclassification adjustment for gains included in net income	-3.3	-1.7	-18.3	-11.1
Other comprehensive income (loss) before tax	19.6	63.8	-55.3	92.5
Defined benefit pension plan:	'	'	'	'
Net loss arising during the period	0	0	0	-0.3
Net loss arising during the period	0	0	0	-0.9
Unrealized gains (losses) on securities:	'	'	'	'
Gains (losses) arising during the period	5.4	17.6	-10.9	31.7
Reclassification adjustment for gains included in net income	-2	-0.4	-6.7	-3.2
Income tax provision related to other comprehensive income	3.4	17.2	-17.6	28.2
Other comprehensive income (loss), net of tax	16.2	46.6	-37.7	64.3
Comprehensive income	$47.20	$60	$57.70	$121.30

CONSOLIDATED_STATEMENTS_OF_CAS

CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Cash flows from operating activities:	'	'
Net income	$95.40	$57
Adjustments to reconcile net income to net cash (used) provided by operating activities:	'	'
Amortization and depreciation	28.8	27.9
Share-based payments expense	17.8	7.9
Excess tax expense from share-based payment arrangements	-0.2	0
Deferred income tax provision, net	4.4	2.6
Net realized investment gains	-29.6	-18.7
Loss on disposals of fixed assets, net	0.2	0.3
Change in:	'	'
Accrued investment income	5	3.5
Receivables	-3.5	34.6
Deferred acquisition costs	-15.5	-6.2
Ceded unearned premiums	-38.5	-33.9
Reserves for losses and loss adjustment expenses	1.4	-74.6
Unearned premiums	118.8	129.4
Ceded reinsurance payable and funds held	-188.3	-63.4
Income taxes	-2.3	-10.1
Accrued underwriting expenses	27.6	13.2
Other, net	-24.5	-6
Cash (used) provided by operating activities	-3	63.5
Cash flows from investing activities:	'	'
Sales of fixed maturity investments	1,308	886.9
Maturities and mandatory calls of fixed maturity investments	224.2	364.9
Sales of equity securities	65	9.1
Sales of other investments	5.8	2.5
Purchases of fixed maturity investments	-1,264.70	-1,147.30
Purchases of equity securities	-71.4	-91.4
Purchases of other investments	-71.2	-12.7
Change in foreign regulatory deposits and voluntary pools	0.6	-20.2
Change in short-term investments	-76.1	-78.8
Settlements of foreign currency exchange forward contracts	-6.4	-0.6
Purchases of fixed assets	-19.1	-26.9
Acquisition of subsidiaries	0	-1.4
Other, net	4.5	2.7
Cash provided (used) by investing activities	99.2	-113.2
Cash flows from financing activities:	'	'
Proceeds from issuance of senior unsecured fixed rate notes, net	0	139.2
Activity under stock incentive plans	1.4	0.7
Repurchase of Company's common shares	-40.1	-38.6
Excess tax expense from share-based payment arrangements	0.2	0
Payment of cash dividends to common shareholders	-11.8	-9.3
Cash (used) provided by financing activities	-50.3	92
Effect of exchange rate changes on cash	-0.1	-0.5
Change in cash	45.8	41.8
Cash, beginning of period	95.8	100.9
Cash, end of period	$141.60	$142.70

Basis_of_Presentation

Basis of Presentation	9 Months Ended
	Sep. 30, 2013
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 the 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, the 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, the fair value of investments and the assessment of potential impairment, the 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, 2012, filed with the Securities and Exchange Commission on February 28, 2013.
	The interim financial information as of, and for the three and nine months ended, September 30, 2013 and 2012 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.

Recently_Issued_Accounting_Sta

Recently Issued Accounting Standards	9 Months Ended
	Sep. 30, 2013
Accounting Changes And Error Corrections [Abstract]	'
Recently Issued Accounting Standards	'
	2	Recently Issued Accounting Standards
	In December 2011, with further clarification issued in January 2013, the Financial Accounting Standards Board (“FASB”) issued an accounting update requiring disclosures about offsetting and related arrangements to enable users of financial statements to understand the effect of those arrangements on its financial position. Entities are required to disclose both gross information and net information about both instruments and transactions eligible for offset in the statement of financial position and instruments and transactions subject to an agreement similar to a master netting arrangement. This update is applicable to derivatives including bifurcated embedded derivatives, repurchase agreements and reverse repurchase agreements and securities borrowing and securities lending arrangements. The update was effective for annual reporting periods beginning on or after January 1, 2013, and interim periods within those annual periods. The disclosures required by this update should be provided retrospectively for all comparative periods presented. The adoption of this update did not have an impact on our financial results and disclosures.
	In July 2012, the FASB issued an accounting update that allows an entity the option to first assess qualitative factors to determine whether it is more likely than not that an indefinite-lived intangible asset is impaired. The more-likely-than-not threshold is defined as having a likelihood of more than 50 percent. If the more-likely-than-not threshold is met, an entity is required to determine the fair value of the indefinite-lived intangible asset and perform the quantitative impairment test by comparing the fair value with the carrying amount in accordance with accounting guidance. The update was effective for annual and interim impairment tests performed for fiscal years beginning after September 15, 2012. Early adoption was permitted, including for annual and interim impairment tests performed as of a date before July 27, 2012, if an entity’s financial statements for the most recent annual or interim period have not yet been issued or, for nonpublic entities, have not yet been made available for issuance. The adoption of this update did not have an impact on our financial results and disclosures.
	In February 2013, the FASB issued an accounting update to improve the reporting of reclassifications out of accumulated other comprehensive income. An entity is required to report the effect of significant reclassifications, by component, out of accumulated other comprehensive income on the respective line items in net income if the item is required under GAAP to be reclassified in its entirety in the same reporting period. The required disclosures of the update are allowed either in the Statement of Income or in the notes. The amendments in the update were effective for reporting periods beginning after December 15, 2012. See Note 6, “Accumulated Other Comprehensive Income” for the required disclosures.
	In February 2013, the FASB issued amendments to “Liabilities” (Topic 405) in order to resolve diversity in practice related to accounting for the recognition, measurement, and disclosure of obligations resulting from joint and several liability arrangements for which the total amount of the obligation is fixed at the reporting date. The amendments require an entity to measure the liability as the sum of (a) the amount the reporting entity agreed to pay on the basis of its arrangement among its co-obligors, and (b) any additional amount the reporting entity expects to pay on behalf of its co-obligors. The amendments in this update are effective for fiscal years, and interim periods within those years, beginning after December 15, 2013, but early adoption is permitted. The amendments should be applied retrospectively to all prior periods presented for those obligations resulting from joint and several liability arrangements within the update’s scope that exist at the beginning of an entity’s fiscal year of adoption. We do not anticipate this update will have an impact on our financial results and disclosures.
	In March 2013, the FASB issued an amendment to “Consolidation” (Topic 810) to resolve the diversity in practice related to the release of the cumulative translation adjustment into net income when a parent sells a part of all of its investment in a foreign entity or no longer holds a controlling financial interest in a subsidiary or group of assets that is a nonprofit activity or a business within a foreign entity. The amendments in the update are effective prospectively for fiscal years, and interim reporting periods within those years, beginning after December 15, 2013, but early adoption is permitted. The amendments should be applied prospectively to derecognition events occurring after the effective date with no adjustment to prior periods. We do not anticipate this update will have an impact on our financial results and disclosures.

Investments

Investments	9 Months Ended
	Sep. 30, 2013
Investments Debt And Equity Securities [Abstract]	'
Investments	'
	3	Investments
	Composition of Invested Assets
	The amortized cost, gross unrealized gains, gross unrealized losses and fair value of investments as of September 30, 2013 and December 31, 2012 were as follows:
	September 30, 2013					Gross			Gross
			Amortized			Unrealized			Unrealized			Fair
	(in millions)		Cost			Gains			Losses			Value
	Fixed maturities
	USD denominated:
	U.S. Governments		$	275		$	3		$	0.4		$	277.6
	Non-U.S. Governments			76.2			0.4			2.4			74.2
	Obligations of states and political subdivisions			597.5			24.3			13.5			608.3
	Credit-Financial			385.8			15.3			2.3			398.8
	Credit-Industrial			383.4			15.5			3.8			395.1
	Credit-Utility			173.6			5.9			1.4			178.1
	Structured securities:
	CMO/MBS-agency (1)			233.4			10.1			1.5			242
	CMO/MBS-non agency			20.3			0.8			0			21.1
	CMBS (2)			161.5			3.2			1.1			163.6
	ABS-residential (3)			7.6			0.3			0.5			7.4
	ABS-non residential			100.8			0.6			0.1			101.3
	Foreign denominated:
	Governments			227.5			5.2			4			228.7
	Credit			115.3			3.1			2.7			115.7
	Total fixed maturities			2,757.90			87.7			33.7			2,811.90
	Equity securities			386.5			197.4			3.1			580.8
	Other investments			366.9			4.3			0			371.2
	Short-term investments			286.3			0			0			286.3
	Total investments		$	3,797.60		$	289.4		$	36.8		$	4,050.20
	December 31, 2012					Gross			Gross
			Amortized			Unrealized			Unrealized			Fair
	(in millions)		Cost			Gains			Losses			Value
	Fixed maturities
	USD denominated:
	U.S. Governments		$	401.1		$	9.3		$	0		$	410.4
	Non-U.S. Governments			53.1			3.6			0.1			56.6
	Obligations of states and political subdivisions			551.7			44.9			0.5			596.1
	Credit-Financial			382.4			25.7			0.7			407.4
	Credit-Industrial			428.4			31.1			0.7			458.8
	Credit-Utility			189.9			12.6			0.4			202.1
	Structured securities:
	CMO/MBS-agency (1)			375.1			20.2			0.2			395.1
	CMO/MBS-non agency			12.5			1			0.2			13.3
	CMBS (2)			109.5			5.7			0.2			115
	ABS-residential (3)			10.1			0.2			0.8			9.5
	ABS-non residential			81.3			1.2			0			82.5
	Foreign denominated:
	Governments			262.1			9.2			4.3			267
	Credit			135.9			6.1			1.8			140.2
	Total fixed maturities			2,993.10			170.8			9.9			3,154.00
	Equity securities			373.5			153			5.1			521.4
	Other investments			294.8			1.2			5			291
	Short-term investments			234.3			0.1			0.1			234.3
	Total investments		$	3,895.70		$	325.1		$	20.1		$	4,200.70
	-1	Collateralized mortgage obligations/mortgage-backed securities (“CMO/MBS”).
	-2	Commercial mortgage-backed securities (“CMBS”).
	-3	Asset-backed securities (“ABS”).
	Included in total investments at September 30, 2013 and December 31, 2012 is $89.2 million and $143.9 million, respectively, of assets (at fair value) managed on behalf of the trade capital providers, who are third party capital participants that provide underwriting capital to our Syndicate 1200 segment.
	Contractual Maturity
	The amortized cost and fair values of fixed maturity investments as of September 30, 2013, by contractual maturity, were as follows:
	(in millions)		Amortized			Fair
			Cost			Value
	Due in one year or less		$	203.3		$	205.3
	Due after one year through five years			1,170.00			1,196.30
Due after five years through ten years			607.8			629.6
Thereafter			253.2			245.3
Structured securities			523.6			535.4
Total		$	2,757.90		$	2,811.90
The expected maturities may differ from the contractual maturities because debtors may have the right to call or prepay obligations.
Unrealized Losses and Other-Than-Temporary Impairments
An aging of unrealized losses on our investments at September 30, 2013 and December 31, 2012 is presented below:
September 30, 2013		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		$	36.8		$	0.4		$	0		$	0		$	36.8	$	0.4
Non-U.S. Governments (2)			51.3			2.4			0			0			51.3		2.4
Obligations of states and political subdivisions			216.9			13.5			0			0			216.9		13.5
Credit-Financial (2)			82.6			2.3			1.3			0			83.9		2.3
Credit-Industrial			95.9			3.6			2.5			0.2			98.4		3.8
Credit-Utility (2)			34.9			1.4			0			0			34.9		1.4
Structured securities:
CMO/MBS-agency (2)			44.6			1.5			0.2			0			44.8		1.5
CMBS			45.5			1.1			0			0			45.5		1.1
ABS-residential			0			0			3.9			0.5			3.9		0.5
ABS-non residential			60.2			0.1			0			0			60.2		0.1
Foreign denominated:
Governments			174.4			4			0			0			174.4		4
Credit			81.7			2.7			0			0			81.7		2.7
Total fixed maturities			924.8			33			7.9			0.7			932.7		33.7
Equity securities			54.3			3.1			0			0			54.3		3.1
Other investments (1)			0			0			0			0			0		0
Short-term investments (1)			2.2			0			0			0			2.2		0
Total		$	981.3		$	36.1		$	7.9		$	0.7		$	989.2	$	36.8
(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.
December 31, 2012		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 (1)		$	11.9		$	0		$	0		$	0		$	11.9	$	0
Non-U.S. Governments (2)			8.5			0.1			0.5			0			9		0.1
Obligations of states and political subdivisions			28.2			0.3			0.8			0.2			29		0.5
Credit-Financial			25			0.2			8.6			0.5			33.6		0.7
Credit-Industrial			32.2			0.4			3			0.3			35.2		0.7
Credit-Utility			8.4			0.2			0.8			0.2			9.2		0.4
Structured securities:
CMO/MBS-agency (2)			26.4			0.2			0.3			0			26.7		0.2
CMO/MBS-non agency			1.7			0.2			0			0			1.7		0.2
CMBS(1)			5.7			0			3.5			0.2			9.2		0.2
ABS-residential			0			0			3.9			0.8			3.9		0.8
ABS-non residential (1) (2)			13.7			0			0.2			0			13.9		0
Foreign denominated:
Governments (2)			180.5			4.3			0.3			0			180.8		4.3
Credit			76.3			1.8			0			0			76.3		1.8
Total fixed maturities			418.5			7.7			21.9			2.2			440.4		9.9
Equity securities			60.2			4.2			6.3			0.9			66.5		5.1
Other investments			11.6			5			0			0			11.6		5
Short-term investments			0			0.1			0			0			0		0.1
Total		$	490.3		$	17		$	28.2		$	3.1		$	518.5	$	20.1
(1)	Unrealized losses less than one year are less than $0.1 million.
(2)	Unrealized losses one year or greater are less than $0.1 million.
We hold a total of 6,362 securities, of which 1,758 were in an unrealized loss position for less than one year and 25 were in an unrealized loss position for a period one year or greater as of September 30, 2013. 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. We also considered that we do not intend to sell the investments and it is unlikely that we will be required to sell the investments before recovery of their amortized cost bases, which may be maturity. We do not consider these investments to be other-than-temporarily impaired at September 30, 2013.
We regularly evaluate our investments for impairment. For fixed maturity securities, the evaluation for a credit loss is generally based on the present value of expected cash flows of the security as compared to the amortized book value. For structured securities, frequency and severity of loss inputs are used in projecting future cash flows of the securities. Loss frequency is measured as the credit default rate, which includes such factors as loan-to-value ratios and credit scores of borrowers. Loss severity includes such factors as trends in real estate values and proceeds at foreclosure. We recognized other-than-temporary losses on our fixed maturities portfolio of $0.0 million and $0.3 million for the three and nine months ended September 30, 2013, respectively. We recognized other-than-temporary losses on our fixed maturities portfolio of $1.1 million and $1.4 million for the three and nine months ended September 30, 2012, respectively. 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 recognized other-than-temporary losses on our equity portfolio of $0.1 million and $1.6 million for the three and nine months ended September 30, 2013, respectively. We recognized other-than-temporary losses on our equity portfolio of $0.0 million and $0.6 million for the three and nine months ended September 30, 2012, respectively. We recognized other-than-temporary losses on our other investments of $0.0 million and $3.4 million for the three and nine months ended September 30, 2013. We did not recognize other-than-temporary losses on our other investments in 2012. 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 fair value.
Realized Gains and Losses
The following table presents the Company’s gross realized investment gains (losses) for the three and nine months ended September 30:
		For the Three Months						For the Nine Months
(in millions)		Ended September 30,						Ended September 30,
		2013			2012			2013			2012
Realized gains
Fixed maturities		$	5.8		$	5.5		$	27.5		$	21
Equity securities			7.2			0.2			15			0.4
Other investments			11.2			7.7			28.9			20.8
Short-term investments			0.1			0.2			0.1			0.5
Gross realized gains			24.3			13.6			71.5			42.7
Realized losses
Fixed maturities			(6.1	)		(3.0	)		(14.5	)		(8.1	)
Equity securities			(0.1	)		(0.1	)		(0.8	)		(0.4	)
Other investments			(8.6	)		(1.1	)		(20.7	)		(13.2	)
Short-term investments			(0.3	)		0			(0.6	)		(0.3	)
Other-than-temporary impairment losses on fixed maturities			0			(1.1	)		(0.3	)		(1.4	)
Other-than-temporary impairment losses on equity securities			(0.1	)		0			(1.6	)		(0.6	)
Other-than-temporary impairment losses on other investments			0			0			(3.4	)		0
Gross realized losses			(15.2	)		(5.3	)		(41.9	)		(24.0	)
Net realized investment gains		$	9.1		$	8.3		$	29.6		$	18.7
We enter into short-term, currency spot and forward contracts to mitigate foreign exchange rate exposure for certain non-U.S. Dollar denominated fixed maturity investments. The forward contracts used are typically less than sixty days and are renewed, as long as the non-U.S. Dollar denominated fixed maturity investments are held in our portfolio. These forward contracts are designated as fair value hedges for accounting purposes.
As of September 30, 2013 and 2012, we hedged $2.0 million and $2.4 million, respectively, of certain holdings in non-U.S. Dollar denominated fixed maturity investments with $1.8 million and $2.3 million, respectively, of foreign exchange forward contracts. The net realized effect on income was not significant for the reporting periods.
We also enter into foreign currency exchange forward contracts to manage currency exposure on losses related to global catastrophe events. These currency forward contracts are carried at fair value in the Consolidated Balance Sheets in “Other investments”. The realized and unrealized gains and losses are included in realized gains or losses in the Consolidated Statements of Income. The notional amount of the currency forward contracts was $53.2 million and $104.9 million as of September 30, 2013 and 2012, respectively. The fair value of the currency forward contracts was $1.6 million and $2.5 million as of September 30, 2013 and 2012, respectively. For the three and nine months ended September 30, 2013, we recognized $2.4 million in net realized gains and $2.2 million in net realized losses from the currency forward contracts. For the three and nine months ended September 30, 2012, we recognized $2.5 million in net realized gains and $1.1 million in net realized losses from the currency forward contracts.
Regulatory Deposits, Pledged Securities and Letters of Credit
At September 30, 2013, the amortized cost and fair value of investments on deposit for regulatory purposes and reinsurance were $202.4 million and $210.8 million, respectively.
Investments with an amortized cost of $230.4 million and fair value of $232.8 million were pledged as collateral in support of irrevocable letters of credit at September 30, 2013. These assets support irrevocable letters of credit issued under the terms of certain reinsurance agreements in respect of reported loss and loss expense reserves in the amount of $63.3 million and $124.9 million for our Corporate member’s capital as security to support the underwriting business at Lloyd’s.
At September 30, 2013, our Corporate member’s capital supporting our Lloyd’s business consisted of:
(in millions)
Letters of credit		$	124.9
Fixed maturities, at fair value			160.9
Short-term investments, at fair value			0.1
Total securities and letters of credit pledged to Lloyd’s		$	285.9
Fair Value Measurements
Fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. Fair value measurement assumes that the transaction to sell the asset or transfer the liability occurs in the principal market for the asset or liability, or in the absence of a principal market, the most advantageous market. Market participants are buyers and sellers in the principal (or most advantageous) market that are independent, knowledgeable, able to transact for the asset or liability and willing to transfer the asset or liability.
Valuation techniques consistent with the market 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 assumptions about the assumptions market participants would use in pricing the asset or liability based on the best information available in the circumstances.
We receive fair value prices from third-party pricing services and our outside investment managers. These prices are determined using observable market information such as dealer quotes, market spreads, cash flows, yield curves, live trading levels, trade execution data, market consensus prepayment speeds, credit information and the security’s terms and conditions, among other things. We have reviewed the processes used by the third-party providers for pricing the securities, and have determined that these processes result in fair values consistent with GAAP requirements. In addition, we review these prices for reasonableness, and have not adjusted any prices received from the third-party providers as of September 30, 2013. 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.
Fixed Maturities (Available-for-Sale) Level 3:
	•		Corporate securities reported at fair value using Level 3 inputs in 2012 were infrequently traded securities valued by an independent investment manager using unobservable inputs. These securities were sold in 2012.
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 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.
Other Assets Level 3: We have one reinsurance contract deemed a derivative in 2013 and two reinsurance contracts deemed derivatives in 2012. The fair values were estimated by management taking into account changes in the market for catastrophic bond reinsurance contracts with similar economic characteristics and potential recoveries from events preceding the valuation date. See Note 11 “Derivative Instruments” for related disclosures.
Transfers Between Level 1 and Level 2 Securities: There were no transfers between Level 1 and Level 2 securities during the three or nine months ended September 30, 2013.
Based on an analysis of the inputs, our financial assets measured at fair value on a recurring basis at September 30, 2013 and December 31, 2012 have been categorized as follows:
(in millions)					Fair Value Measurements at Reporting Date Using
		September 30, 2013			Level 1 (a)			Level 2 (b)			Level 3  (c)
Fixed maturities
USD denominated:
U.S. Governments		$	277.6		$	142.8		$	134.8		$	0
Non-U.S. Governments			74.2			0			74.2			0
Obligations of states and political subdivisions			608.3			0			608.3			0
Credit-Financial			398.8			0			398.8			0
Credit-Industrial			395.1			0			395.1			0
Credit-Utility			178.1			0			178.1			0
Structured securities:
CMO/MBS-agency			242			0			242			0
CMO/MBS-non agency			21.1			0			21.1			0
CMBS			163.6			0			163.6			0
ABS-residential			7.4			0			7.4			0
ABS-non residential			101.3			0			101.3			0
Foreign denominated:
Governments			228.7			0			228.7			0
Credit			115.7			0			115.7			0
Total fixed maturities			2,811.90			142.8			2,669.10			0
Equity securities			580.8			523.3			56.2			1.3
Other investments			114.3			0			114.3			0
Short-term investments			286.3			284			2.3			0
Other assets			2.8			0			0			2.8
		$	3,796.10		$	950.1		$	2,841.90		$	4.1
(a)	Quoted prices in active markets for identical assets
(b)	Significant other observable inputs
(c)	Significant unobservable inputs
(in millions)					Fair Value Measurements at Reporting Date Using
		December 31, 2012			Level 1 (a)			Level 2 (b)			Level 3  (c)
Fixed maturities
USD denominated:
U.S. Governments		$	410.4		$	219.5		$	190.9		$	0
Non-U.S. Governments			56.6			0			56.6			0
Obligations of states and political subdivisions			596.1			0			596.1			0
Credit-Financial			407.4			0			407.4			0
Credit-Industrial			458.8			0			458.8			0
Credit-Utility			202.1			0			202.1			0
Structured securities:
CMO/MBS-agency			395.1			0			395.1			0
CMO/MBS-non agency			13.3			0			13.3			0
CMBS			115			0			115			0
ABS-residential			9.5			0			9.5			0
ABS-non residential			82.5			0			82.5			0
Foreign denominated:
Governments			267			0			267			0
Credit			140.2			0			140.2			0
Total fixed maturities			3,154.00			219.5			2,934.50			0
Equity securities			521.4			469			50.6			1.8
Other investments			132			0			132			0
Short-term investments			234.3			201.1			33.2			0
Other assets			6.9			0			0			6.9
		$	4,048.60		$	889.6		$	3,150.30		$	8.7
(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 agree to “Total investments” on the Consolidated Balance Sheets as they exclude certain other investments that are accounted for under the equity-method of accounting and include reinsurance contracts that are classified as Other assets.
A reconciliation of the beginning and ending balances for the investments categorized as Level 3 at September 30, 2013 and December 31, 2012 are as follows:
Fair Value Measurements Using Unobservable Inputs (Level 3)
(in millions)		Equity			Other			Total
		Securities			Assets
Beginning balance, January 1, 2013		$	1.8		$	6.9		$	8.7
Transfers into Level 3			0			0			0
Transfers out of Level 3			0			0			0
Total gains or losses (realized/unrealized):
Included in net income			0.1			0			0.1
Included in other comprehensive income			0			0			0
Purchases, issuances, sales, and settlements
Purchases			0			0			0
Issuances			0			0			0
Sales			(0.6	)		0			(0.6	)
Settlements			0			(4.1	)		(4.1	)
Ending balance, September 30, 2013		$	1.3		$	2.8		$	4.1
Amount of total gains or losses for the period included in net income attributable to the change in unrealized gains or losses relating to assets still held at September 30, 2013		$	0		$	0		$	0
Fair Value Measurements Using Unobservable Inputs (Level 3)
(in millions)		Credit			Equity			Other			Total
		Financial			Securities			Assets
Beginning balance, January 1, 2012		$	0.7		$	2.4		$	9		$	12.1
Transfers into Level 3			0			0			0			0
Transfers out of Level 3			0			0			0			0
Total gains or losses (realized/unrealized):
Included in net income			0.1			0			0			0.1
Included in other comprehensive income			0			0			0			0
Purchases, issuances, sales, and settlements
Purchases			0			0			0			0
Issuances			0			0			0			0
Sales			(0.8	)		(0.6	)		0			(1.4	)
Settlements			0			0			(2.1	)		(2.1	)
Ending balance, December 31, 2012		$	0		$	1.8		$	6.9		$	8.7
Amount of total gains or losses for the period included in net loss attributable to the change in unrealized gains or losses relating to assets still held at December 31, 2012		$	0		$	0		$	0		$	0
At September 30, 2013 and December 31, 2012, 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.

Shareholders_Equity

Shareholders Equity	9 Months Ended
	Sep. 30, 2013
Equity [Abstract]	'
Shareholders Equity	'
	4	Shareholders’ Equity
	On May 7, 2013, our Board of Directors declared a 10% stock dividend, payable on June 17, 2013, to shareholders of record at the close of business on June 3, 2013. As a result of the stock dividend, 2,447,839 additional shares were issued. Cash was paid in lieu of fractional shares of the Company’s common shares.
	On August 6, 2013, our Board of Directors declared a quarterly cash dividend in the amount of $0.15 on each share of common stock outstanding. On September 16, 2013, we paid $4.0 million to our shareholders of record on September 2, 2013. On August 7, 2012, our Board of Directors declared a quarterly cash dividend in the amount of $0.12, on each share of common stock outstanding. On September 18, 2012, we paid $3.0 million to our shareholders of record on September 4, 2012.
	For the nine months ended September 30, 2013 and 2012, we have paid cash dividends totaling $11.8 million and $9.3 million to our shareholders.
	On February 18, 2011, our Board of Directors authorized the repurchase of up to $150.0 million of our common shares (“2011 Repurchase Authorization”). The 2011 Repurchase Authorization supersedes the November 13, 2007 repurchase authorization, which also had authorized the repurchase of up to $150.0 million of our common shares. From inception of the repurchase authorizations through September 30, 2013, we have repurchased 7,408,601 shares of our common stock at an average price of $32.96 for a total cost of $244.2 million. These shares are being held as treasury shares in accordance with the provisions of the Bermuda Companies Act 1981. As of September 30, 2013, availability under the 2011 Repurchase Authorization for future repurchases of our common shares was $32.2 million.
	On November 6, 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 the 2011 Repurchase Authorization.
	For the three and nine months ended September 30, 2013, we repurchased a total of 195,412 and 948,988 common shares for $8.6 million and $38.7 million respectively. A summary of activity from January 1, 2013 through September 30, 2013 follows.
	For the three and nine months ended September 30, 2013, we repurchased 94,809 and 376,839 common shares on the open market for $4.2 million and $15.8 million, respectively. In 2013, we repurchased shares under Securities Exchange Act of 1934 Rule 10b5-1 trading plans as follows:
	Date Trading		2013	Number of	Average Price of		Total Cost		Repurchase
	Plan Initiated		Purchase Period	Shares Repurchased	Shares Repurchased		(in millions)		Authorization Year
	12/14/12		01/01/13-01/17/13 (1)	171,480		$        35.41		$        6.0	2011
	3/15/13		03/16/13-05/01/13	205,452		$        40.75		$        8.3	2011
	6/14/13		06/18/13-07/31/13	195,217		$        43.47		$        8.5	2011
	(1)	The above table only reflects the 2013 activity under this Rule 10b5-1 trading plan. In 2012, 117,663 shares were repurchased under this Rule 10b5-1 trading plan for a total cost of $3.9 million. Total shares repurchased in 2012 and 2013 under this Rule 10b5-1 trading plan are 289,143 shares at an average price of $34.58 for a total cost of $9.9 million.

Net_Income_Per_Common_Share

Net Income Per Common Share	9 Months Ended
	Sep. 30, 2013
Earnings Per Share [Abstract]	'
Net Income Per Common Share	'
	5	Net Income Per Common Share
	The following table presents the calculation of net income per common share on a basic and diluted basis for the three and nine months ended September 30, 2013 and 2012:
			For the Three  Months				For the Nine Months
			Ended September 30,				Ended September 30,
	(in millions, except number of shares and per share amounts)		2013		2012		2013		2012
	Net income		$	31	$	13.4	$	95.4	$	57
	Weighted average common shares outstanding - basic			26,682,176		27,751,140		26,961,792		28,265,456
	Effect of dilutive securities
	Equity compensation awards			1,137,374		582,656		1,031,543		447,668
	Weighted average common shares outstanding - diluted			27,819,550		28,333,796		27,993,335		28,713,124
	Net income per common share - basic		$	1.16	$	0.48	$	3.54	$	2.02
	Net income per common share - diluted		$	1.12	$	0.47	$	3.41	$	1.98
	Excluded from the weighted average common shares outstanding calculation at September 30, 2013 and 2012 are 7,408,601 shares and 6,248,209 shares, respectively, which are held as treasury shares. The shares are excluded as of their repurchase date. For the three and nine months ended September 30, 2013, equity compensation awards to purchase 58,784 shares of common stock were excluded from the computation of diluted net income per common share as these instruments were anti-dilutive. These instruments expire at varying times from 2013 through 2020. For the three and nine months ended September 30, 2012, equity compensation awards to purchase 972,175 shares of common stock were excluded from the computation of diluted net income per common share as these instruments were anti-dilutive. These instruments expire at varying times from 2012 through 2019.

Accumulated_Other_Comprehensiv

Accumulated Other Comprehensive Income	9 Months Ended
	Sep. 30, 2013
Text Block [Abstract]	'
Accumulated Other Comprehensive Income	'
	6	Accumulated Other Comprehensive Income
	In February 2013, the FASB issued Accounting Standards Update 2013-02 that amends Accounting Standards Codification (“ASC”) 220, “Comprehensive Income.” See Note 2, “Recently Issued Accounting Standards” for additional information regarding the provisions of this update. Effective January 1, 2013, we adopted the update prospectively.
	A summary of changes in accumulated other comprehensive income (loss), net of taxes (where applicable) by component for the nine months ended September 30, 2013 is presented in the following table:
	(in millions)		Foreign currency			Unrealized holding gains			Defined benefit			Total
			translation adjustments			on securities			pension plans
	Beginning balance, January 1, 2013		$	(8.7	)	$	204.1		$	(5.9	)	$	189.5
	Other comprehensive income before reclassifications			(2.2	)		(23.9	)		0			(26.1	)
	Amounts reclassified from accumulated other comprehensive income			0			(11.6	)		0			(11.6	)
	Net current-period other comprehensive income			(2.2	)		(35.5	)		0			(37.7	)
	Ending balance, September 30, 2013		$	(10.9	)	$	168.6		$	(5.9	)	$	151.8
	The amounts reclassified from accumulated other comprehensive income shown in the above table have been included in the following captions in the Consolidated Statements of Income:
	(in millions)		For the Three Months			For the Nine Months
			Ended September 30, 2013			Ended September 30, 2013
	Unrealized gains and losses on securities
	Net realized investment gains		$	(3.3	)	$	(18.3	)
	Provision for income taxes			2			6.7
	Net of taxes		$	(1.3	)	$	(11.6	)

Commitments_and_Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2013
Commitments And Contingencies Disclosure [Abstract]	'
Commitments and Contingencies	'
	7	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.

Income_Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2013
Income Tax Disclosure [Abstract]	'
Income Taxes	'
	8	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 are therefore 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 the tax laws of that country. 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						For the Nine  Months
			Ended September 30,						Ended September 30,
	(in millions)		2013			2012			2013			2012
	Current tax provision		$	3.5		$	5.7		$	13.4		$	11.2
	Deferred tax provision related to:
	Future tax deductions			(2.4	)		(2.1	)		2			0.8
	Valuation allowance change			0.8			0.7			2.4			1.8
	Income tax provision		$	1.9		$	4.3		$	17.8		$	13.8
	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 and nine months ended September 30, 2013 and 2012, pre-tax income (loss) attributable to our operations and the operations’ effective tax rates were as follows:
			For the Three Months Ended September 30,
	(in millions)		2013						2012
			Pre-tax			Effective Tax			Pre-tax			Effective Tax
			income (loss)			Rate			income (loss)			Rate
	Bermuda		$	14.2			0	%	$	8			0	%
	United States			19.6			22.9	%		12.1			21	%
	United Kingdom			1.6			-164.7	%		0.1			1217.4	%
	Belgium			0.1			91.6	%		0.1			20	%
	Brazil			(1.5	)		0	%		(1.9	)		0	%
	Dubai			(0.4	)		0	%		0			0	%
	Ireland			(0.1	)		0	%		0			0	%
	Malta			(0.6	)		0	%		(0.7	)		0	%
	Switzerland			0	(1)		19.7	%		0	(1)		25	%
	Pre-tax income (loss)		$	32.9					$	17.7
	-1	Pre-tax income for the respective period was less than $0.1 million.
			For the Nine Months Ended September 30,
	(in millions)		2013						2012
			Pre-tax			Effective Tax			Pre-tax			Effective Tax
			income (loss)			Rate			income (loss)			Rate
	Bermuda		$	34.2			0	%	$	18.2			0	%
	United States			68.6			23.5	%		45.8			22.8	%
	United Kingdom			18.3			9.2	%		13.8			24	%
	Belgium			0.1			26.9	%		0.2			33.6	%
	Brazil			(6.2	)		0	%		(5.4	)		0	%
	Dubai			(0.4	)		0	%		0.1			0	%
	Ireland			(0.1	)		0	%		(0.1	)		0	%
	Malta			(1.3	)		0	%		(1.8	)		0	%
	Switzerland			0	(1)		21	%		0	(1)		24	%
	Pre-tax income (loss)		$	113.2					$	70.8
	-1	Pre-tax income for the respective period 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 for the three and nine months ended September 30, 2013 and 2012 is as follows:
			For the Three Months						For the Nine Months
			Ended September 30,						Ended September 30,
	(in millions)		2013			2012			2013			2012
	Income tax provision at expected rate		$	6.2		$	3.3		$	25.2		$	17
	Tax effect of:
	Tax-exempt interest			(1.3	)		(1.2	)		(4.1	)		(4.2	)
	Dividends received deduction			(0.7	)		(0.4	)		(1.8	)		(1.3	)
	Valuation allowance change			0.8			0.7			2.4			1.8
	Other permanent adjustments, net			(0.5	)		0.1			(1.4	)		0.8
	Adjustment for annualized rate			0.3			0			(0.1	)		0
	United States state tax benefit			(0.1	)		0			(0.4	)		(0.2	)
	Other foreign adjustments			0			0			0			0.1
	Deferred tax rate reduction			(1.3	)		(0.6	)		(1.3	)		(0.6	)
	Foreign exchange adjustments			(1.7	)		2.4			(1.3	)		0.4
	Foreign withholding taxes			0.2			0			0.6			0
	Income tax provision		$	1.9		$	4.3		$	17.8		$	13.8
Income tax provision (benefit) - Foreign		$	(2.6	)	$	1.8		$	1.7		$	3.4
Income tax provision - United States Federal			4.4			2.3			16.1			10.6
Income tax provision (benefit) - United States State			(0.1	)		0.2			(0.6	)		(0.2	)
Foreign withholding tax - United States			0.2			0			0.6			0
Income tax provision		$	1.9		$	4.3		$	17.8		$	13.8
Our net deferred tax assets (liabilities) are supported by taxes paid in previous periods, the reversal of the taxable temporary differences and the 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
operations in the United States. At September 30, 2013, we had a total net deferred tax asset of $24.2 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 $30.0 million generated from the sale of PXRE Reinsurance Company, and a full valuation allowance is required for the tax effected net operating loss carryforward of $18.7 million from PXRE Corporation and for the tax effected net operating loss carryforward of $1.0 million from ARIS Title Insurance Corporation (“ARIS”). The capital loss carryforward generated from the sale of PXRE Reinsurance Company will expire if not used by December 31, 2013. Of the PXRE Corporation loss carryforwards, $17.2 million will expire if not used by December 31, 2025 and $1.5 million will expire if not utilized by December 31, 2027. 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. Accordingly, a valuation allowance of $54.9 million is required as of September 30, 2013. The loss carryforwards available per year are $2.8 million as required by Internal Revenue Code Section 382. For the nine months ended September 30, 2013, the valuation allowance was reduced by $0.7 million pertaining to the use of the PXRE and ARIS loss carryforwards, was increased by $0.4 million pertaining to the Malta operations and was increased by $2.7 million pertaining to the Brazil operations.
We had no material unrecognized tax benefits as of September 30, 2013. Our United States subsidiaries are no longer subject to U.S. federal and state income tax examinations by tax authorities for years before 2009. Our United Kingdom subsidiaries are no longer subject to United Kingdom income tax examinations by Her Majesty’s Revenue and Customs for years before 2011.

Sharebased_Compensation

Share-based Compensation	9 Months Ended
	Sep. 30, 2013
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]	'
Share-based Compensation	'
	9	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 Argo Group’s 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 following table summarizes the assumptions we used for the nine months ended September 30, 2013 and 2012:
			2013			2012
	Risk-free rate of return			1.44	%		1.09	%
	Expected dividend yields			1.55	%		1.67	%
	Expected award life (years)			4.92			5.03
	Expected volatility			31.1	%		32.6	%
	Argo Group’s 2007 Long-Term Incentive Plan
	In November 2007, the shareholders of Argo Group 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. The share awards may be in the form of share options, SARs, restricted shares, restricted share units, performance units, performance shares or other share-based incentive 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 2007 Plan; 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 2007 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. The term of the share options cannot exceed seven years from the grant date.
	A summary of option activity under the 2007 Plan as of September 30, 2013, and changes during the nine months then ended is as follows:
			Shares			Weighted-Average
						Exercise Price
	Outstanding at January 1, 2013			418,325		$	33.56
	Granted			0		$	0
	Exercised			(151,286	)	$	31.62
	Expired or forfeited			(45,051	)	$	34.21
	Outstanding at September 30, 2013			221,988		$	34.74
	Options outstanding under this plan vested over a one to five year period, subject to continued employment. As of December 31, 2012, all options under this plan were fully vested. Expense recognized under this plan for share options was $0.5 million for the nine months ended September 30, 2012. Compensation expense from all equity-based compensation awards is included in “Underwriting, acquisition and insurance expense” in the accompanying Consolidated Statements of Income.
	A summary of restricted share activity under the 2007 Plan as of September 30, 2013, and changes during the nine months then ended is as follows:
			Shares			Weighted-Average
						Grant Date
						Fair Value
	Outstanding at January 1, 2013			236,139		$	29.76
	Granted			117,788		$	37.05
	Vested and issued			(54,034	)	$	29.8
	Expired or forfeited			(41,077	)	$	35.62
	Outstanding at September 30, 2013			258,816		$	32.71
	The restricted shares will vest over two to four years. Expense recognized under this plan for the restricted shares was $2.2 million and $1.4 million for the nine months ended September 30, 2013 and 2012, respectively. As of September 30, 2013, there was $6.2 million of total unrecognized compensation cost related to restricted shares.
	A summary of stock-settled SARs activity under the 2007 Plan as of September 30, 2013, and changes during the nine months then ended is as follows:
			Shares			Weighted-Average
						Exercise Price
	Outstanding at January 1, 2013			1,261,448		$	27.04
	Granted			332,004		$	37.21
	Exercised			(167,693	)	$	26.69
	Expired or forfeited			(124,711	)	$	29.27
	Outstanding at September 30, 2013			1,301,048		$	29.46
	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 $2.1 million and $1.7 million for each of the nine months ended September 30, 2013 and 2012. As of September 30, 2013, there was $4.9 million of total unrecognized compensation cost related to stock-settled SARs.
	A summary of cash-settled SARs activity under the 2007 Plan as of September 30, 2013, and changes during the nine months then ended is as follows:
			Shares			Weighted-Average
						Exercise Price
	Outstanding at January 1, 2013			1,886,124		$	29.75
	Granted			556,598		$	39.67
	Exercised			(451,792	)	$	29.46
	Expired or forfeited			(175,284	)	$	29.85
	Outstanding at September 30, 2013			1,815,646		$	32.85
	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 are accounting 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 $12.7 million and $3.6 million for the nine months ended September 30, 2013 and 2012, respectively. As of September 30, 2013, there was $13.5 million of total unrecognized compensation cost related to cash-settled SARs.
	Argo Group International Holdings Ltd. Deferred Compensation Plan for Non-Employee Directors
	In February 2008, the Board of Directors approved the Argo Group International Holdings, Ltd. Deferred Compensation Plan for Non-Employee Directors (“Directors Plan”), a non-funded and non-qualified deferred compensation plan. Under the Directors Plan, non-employee directors can elect each year to defer payment of 50% or 100% of their cash compensation payable during the next calendar year. During the time that the cash compensation amounts are deferred, such amounts are credited with interest earned at a rate two percent above the prime rate, to be re-set each May 1. In addition, the Directors Plan calls for us to grant a match equal to 75% of the cash compensation amounts deferred in the form of “Stock Units,” which provide directors with the economic equivalent of stock ownership and are credited as a bookkeeping entry to each director’s “Stock Unit Account.” Each Stock Unit is valued at the closing price of our common stock on the national exchange on which it is listed as of the date credited for all purposes under the Directors Plan and fluctuates daily thereafter on that same basis. Distributions from the Directors Plan will occur six months after the non-employee director ceases to be a member of the Board due to retirement or termination without cause or change in control, or immediately upon disability or death. The non-employee directors are responsible for all tax requirements on the deferred compensation and any related earnings. The Directors Plan provides for a Stock Unit Account to be established for each non-employee director upon their election to the Board and credits their account with an initial bookkeeping entry for 1,650 Stock Units. Under the Directors Plan, we recorded compensation expense of $1.5 million and $0.9 million for the nine months ended September 30, 2013 and 2012, respectively.
	Argonaut Group’s Amended and Restated Stock Incentive Plan
	Argonaut Group, Inc.’s Amended and Restated Stock Incentive Plan, as approved by the shareholders (the “Amended Plan”), provided for an aggregate of up to 6,250,000 shares of our common stock that may be issued to certain executives and other key employees. The stock awards were issued in the form of non-qualified stock options and non-vested stock.
	A summary of option activity under the Amended Plan as of September 30, 2013, and changes during the nine months then ended is as follows:
			Shares			Weighted-Average
						Exercise Price
	Outstanding at January 1, 2013			210,097		$	33.13
	Granted			0		$	0
	Exercised			(115,555	)	$	27.75
	Expired or forfeited			(41,814	)	$	47.71
	Outstanding at September 30, 2013			52,728		$	33.37
	All options granted under the Amended Plan were fully vested in August 2011.

Reserves_for_Losses_and_Loss_A

Reserves for Losses and Loss Adjustment Expenses	9 Months Ended
	Sep. 30, 2013
Insurance [Abstract]	'
Reserves for Losses and Loss Adjustment Expenses	'
	10	Reserves for Losses and Loss Adjustment Expenses
	The following table provides a reconciliation of reserves for losses and loss adjustment expenses (“LAE”), net of reinsurance, to the gross amounts reported in the Consolidated Balance Sheets. Reinsurance recoverables in this note exclude paid loss recoverables of $142.7 million and $106.5 million as of September 30, 2013 and 2012, respectively:
			For the Nine Months
			Ended September 30,
	(in millions)		2013			2012
	Net reserves beginning of the year		$	2,110.90		$	2,336.70
	Add:
	Net reserves from assumed retroactive insurance contract (1)			0			13
	Losses and LAE incurred during current calendar year, net of reinsurance:
	Current accident year			575.6			545.7
	Prior accident years			(21.5	)		(17.8	)
	Losses and LAE incurred during calendar year, net of reinsurance			554.1			527.9
	Deduct:
	Losses and LAE payments made during current calendar year, net of reinsurance:
	Current accident year			131.7			111.9
	Prior accident years			429			450.1
	Losses and LAE payments made during current calendar year, net of reinsurance:			560.7			562
	Change in participation interest (2)			10.4			(3.1	)
	Foreign exchange adjustments			(6.3	)		5.4
	Net reserves - end of period			2,108.40			2,317.90
	Add:
	Reinsurance recoverables on unpaid losses and LAE, end of period			1,114.50			910
	Gross reserves - end of period		$	3,222.90		$	3,227.90
	-1	Amount represents reserves assumed resulting from participation in Brazilian Motor Third-Party Liability Insurance Pool effective January 1, 2012.
	-2	Amount represents (decrease) increase in reserves due to change in syndicate participation.
	Included in losses and LAE for the nine months ended September 30, 2013 was $21.5 million in favorable prior years’ loss reserve development comprised of the following: $26.8 million net favorable development in the Excess and Surplus Lines segment primarily caused by favorable development of $35.9 million in the general and products liability lines, partially offset with $9.1 million of unfavorable development in commercial automobile; $0.4 million of net unfavorable development in the Commercial Specialty segment, primarily driven by $11.4 million unfavorable development in general liability due to increases in claim severity and $2.4 million unfavorable development in automobile liability, offset by $6.7 million favorable development in short tail lines and $5.8 million favorable development in workers compensation; $0.6 million net favorable development in the International Specialty segment primarily driven by $3.0 million of favorable development on professional lines due to favorable loss activity and $1.2 million on assumed contract from older years, partially offset by $2.6 million of unfavorable development from a marine industry loss warranty and crop losses and $0.8 million in unfavorable development in our Brazil unit; $4.1 million net favorable development in the Syndicate 1200 segment driven by favorable development in various property classes, partially offset by unfavorable development in the onshore energy and liability classes of business; $9.6 million unfavorable development in the Runoff Lines segment primarily caused by $6.4 million of unfavorable development in asbestos driven by an increase in defense costs for claims from general liability policies written on a direct basis and commutation and settlement activity and $2.0 million of unfavorable development in the medical malpractice liability line due to the loss of the New York Liquidation Bureau funding for structured settlement annuity payments.
	Included in losses and LAE for the nine months ended September 30, 2012 was $17.8 million in favorable prior years’ loss reserve development comprised of the following: $33.5 million of favorable development in the Excess and Surplus Lines segment primarily driven by $23.6 million of favorable development in the general and products liability lines of business in accident years 2009 and prior, $3.1 million of favorable development in the property lines, primarily in accident year 2010 and $2.4 million of favorable development in the automobile liability lines of business, primarily in accident year 2009; $18.6 million of net unfavorable development in the Commercial Specialty segment driven by $24.9 million of unfavorable development in general liability due to increases in claim severity, $3.8 million of unfavorable development in the automobile liability lines of business, partially offset by $7.7 million of favorable development in workers compensation and $2.4 million of favorable development in short-tail lines; $4.5 million of net favorable development in the International Specialty segment primarily driven by favorable development attributable to short-tail non-catastrophe losses; $4.4 million of net favorable development in the Syndicate 1200 segment primarily driven by favorable development in the property facultative class of business, partially offset by unfavorable development in the general liability class of business; and $6.0 million of net unfavorable development in the Run-off Lines segment primarily due to $5.5 million of unfavorable development in asbestos driven by increasing defense costs, unfavorable development in other run-off lines, partially offset by favorable development in workers compensation and legacy PXRE claims.
	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.

Derivative_Instruments

Derivative Instruments	9 Months Ended
	Sep. 30, 2013
Derivative Instruments And Hedging Activities Disclosure [Abstract]	'
Derivative Instruments	'
	11	Derivative Instruments
	Through our subsidiary Argo Re, Ltd., in 2011 we entered into two reinsurance contracts with a special purpose reinsurance company that provided us with protection against certain severe catastrophe events and the occurrence of multiple significant catastrophe events during the same year. The first contract was effective June 18, 2011 and provided coverage of $100 million for hurricanes and earthquakes in the U.S., windstorms in Europe, and earthquakes in Japan based on the occurrence of second and subsequent events on a per-occurrence basis over an 18-month coverage period. Each event had an activation level, which, if attained, put the notes on risk for a subsequent event. Once the coverage had been activated, a second loss during the Coverage Period in excess of the loss trigger level results in a loss to the note holders. The first contract expired in December 2012. The second contract entered into on December 28, 2011 and effective January 1, 2012, provides coverage of $100 million for hurricanes and earthquakes (including fire) in the U.S. and covers losses for the first and subsequent events on a per-occurrence basis over a 24-month coverage period. Both of these transactions ignore the effects of inuring reinsurance, creating the remote possibility of a double recovery on covered events, and are therefore deemed to be derivatives.
	We recorded these contracts at fair value, and such fair value is included in “Other assets” in our Consolidated Balance Sheets with any changes in the value reflected in “Other reinsurance-related expenses” in the Consolidated Statements of Income. As there is no quoted fair value available for these derivatives, the fair value was estimated by management taking into account changes in the market for catastrophe bond reinsurance contracts with similar economic characteristics and potential recoveries from events preceding the valuation date. Included in “Other reinsurance-related expenses” for the three and nine months ended September 30, 2013 was $4.6 million and $14.4 million, respectively, and $7.0 million and $20.8 million for the three and nine months ended September 30, 2012. The expense in each respective period was due to the change in the fair value of the derivatives, principally due to the passage of the transaction’s risk coverage term. Included in “Other assets” in our Consolidated Balance Sheets were $2.9 million and $6.9 million, which represent the fair value of these contracts at September 30, 2013 and December 31, 2012, respectively.
	The special purpose reinsurance company that is the counterparty to these transactions is a variable interest entity under the provisions of ASC Topic 810-10. Argo Group is not the primary beneficiary of this entity and is therefore not required to consolidate it in its consolidated financial statements.

Underwriting_Acquisition_and_I

Underwriting, Acquisition and Insurance Expenses	9 Months Ended
	Sep. 30, 2013
Text Block [Abstract]	'
Underwriting, Acquisition and Insurance Expenses	'
	12	Underwriting, Acquisition and Insurance Expenses
	Underwriting, acquisition and insurance expenses for the three and nine months ended September 30, 2013 and 2012 were as follows:
			For the Three Months						For the Nine Months
			Ended September 30,						Ended September 30,
	(in millions)		2013			2012			2013			2012
	Commissions		$	57.2		$	55.3		$	174.7		$	156
	General expenses			65.4			60.2			200.8			182.2
	Premium taxes, boards and bureaus			6.8			7.1			19.7			19.2
				129.4			122.6			395.2			357.4
	Net deferral of policy acquisition costs			(2.7	)		(4.6	)		(17.2	)		(11.1	)
	Total underwriting, acquisition and insurance expenses		$	126.7		$	118		$	378		$	346.3
	Included in general expenses for the three and nine months ended September 30, 2013, was $1.7 million and $12.7 million in additional expense for our cash settled stock appreciation rights, due to the increase in our stock price.

Segment_Information

Segment Information	9 Months Ended
	Sep. 30, 2013
Segment Reporting [Abstract]	'
Segment Information	'
	13	Segment Information
	We are primarily engaged in writing 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 write.
	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. Intersegment transactions are allocated to the segment that initiated the transaction. 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 for the three and nine months ended September 30, 2013 and 2012 were as follows:
			For the Three Months						For the Nine Months
			Ended September 30,						Ended September 30,
	(in millions)		2013			2012			2013			2012
	Revenue:
	Earned premiums
	Excess and Surplus Lines		$	120.6		$	100.8		$	334.4		$	295.5
	Commercial Specialty			74.6			78.6			225.1			241.8
	International Specialty			37.5			33.4			106.6			90.8
	Syndicate 1200			97.3			90.8			295.6			242.3
	Run-off Lines			0.5			0.7			0.5			1.4
	Total earned premiums			330.5			304.3			962.2			871.8
	Net investment income
	Excess and Surplus Lines			10.2			12.5			32.3			38.5
	Commercial Specialty			5.6			6.7			17.6			20.6
	International Specialty			2.4			3.1			6.4			10
	Syndicate 1200			2.5			3.8			8.1			11.4
	Run-off Lines			2.6			2.8			8.2			9.8
	Corporate and Other			0.8			0			4.7			0
	Total net investment income			24.1			28.9			77.3			90.3
	Fee income, net			0.3			3			0.5			4.8
	Net realized investment gains			9.1			8.3			29.6			18.7
	Total revenue		$	364		$	344.5		$	1,069.60		$	985.6
			For the Three Months						For the Nine Months
			Ended September 30,						Ended September 30,
	(in millions)		2013			2012			2013			2012
	Income (loss) before income taxes
	Excess and Surplus Lines		$	29.6		$	15.1		$	60.2		$	56.3
	Commercial Specialty			10.1			(2.7	)		17.3			(11.8	)
	International Specialty			0.5			6.6			8.1			22
	Syndicate 1200			8.5			11.8			26.7			18
	Run-off Lines			(9.1	)		(2.5	)		(5.8	)		(1.5	)
	Total segment income before taxes			39.6			28.3			106.5			83
	Corporate and Other			(15.8	)		(18.9	)		(22.9	)		(30.9	)
	Net realized investment gains			9.1			8.3			29.6			18.7
	Total income before income taxes		$	32.9		$	17.7		$	113.2		$	70.8
	The table below presents earned premiums by geographic location for the three and nine months ended September 30, 2013 and 2012. For this disclosure, we determine geographic location by the country of domicile of our subsidiaries that write the business and not by the location of insureds or reinsureds from whom the business was generated.
			For the Three Months						For the Nine Months
			Ended September 30,						Ended September 30,
	(in millions)		2013			2012			2013			2012
	Bermuda		$	27.4		$	32.6		$	75.3		$	83.5
	Brazil			9.9			6.7			29.9			18.4
	Malta			0.5			0			1.3			0.1
	United Kingdom			97.7			84.9			296			231.1
	United States			195			180.1			559.7			538.7
	Total earned premiums		$	330.5		$	304.3		$	962.2		$	871.8
	The following table represents identifiable assets as of September 30, 2013 and December 31, 2012:
	(in millions)		September 30,			December 31,
			2013			2012
	Excess and Surplus Lines		$	2,248.60		$	2,206.70
	Commercial Specialty			1,316.50			1,337.70
	International Specialty			785.9			731.8
	Syndicate 1200			1,670.50			1,811.40
	Run-off Lines			558			516.5
	Corporate and Other			71.1			84.8
	Total		$	6,650.60		$	6,688.90
	Included in total assets at September 30, 2013 and December 31, 2012 are $378.9 million and $505.7 million, respectively, in assets associated with trade capital providers.

Supplemental_Cash_Flow_Informa

Supplemental Cash Flow Information	9 Months Ended
	Sep. 30, 2013
Supplemental Cash Flow Elements [Abstract]	'
Supplemental Cash Flow Information	'
	14	Supplemental Cash Flow Information
	Income taxes paid. We paid income taxes of $14.1 million and $18.1 million during the nine months ended September 30, 2013 and 2012, respectively.
	Interest paid. Interest paid for the nine months ended September 30, was as follows:
			For the Nine Months
			Ended September 30,
	(in millions)		2013		2012
	Senior unsecured fixed rate notes		$	7	$	0
	Junior subordinated debentures			6		16.3
	Other indebtedness			0.7		2.5
	Revolving credit facility			0		0
	Total interest paid		$	13.7	$	18.8

Disclosures_about_Fair_Value_o

Disclosures about Fair Value of Financial Instruments	9 Months Ended
	Sep. 30, 2013
Fair Value Disclosures [Abstract]	'
Disclosures about Fair Value of Financial Instruments	'
	15	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 September 30, 2013 and December 31, 2012, the carrying values of premiums receivable over 90 days were $17.0 million and $24.3 million, respectively. Included in “Reinsurance recoverables” in the Consolidated Balance Sheets at September 30, 2013 and December 31, 2012, are amounts that are due from third party trade capital providers associated with the operations of Argo Underwriting Agency Limited (“Argo International”). Upon settlement, the receivable is offset against the liability also reflected in the accompanying Consolidated Balance Sheets. At September 30, 2013 and December 31, 2012, the payable was in excess of the receivable. Of our paid losses on reinsurance recoverable balances, excluding amounts attributable to Argo International’s third party trade capital providers, at September 30, 2013 and December 31, 2012, the carrying values over 90 days were $17.9 million and $18.0 million, respectively. Our methodology for establishing our allowances for doubtful accounts includes specifically identifying all potential uncollectible balances regardless of aging. At September 30, 2013 and December 31, 2012, the allowance for doubtful accounts for premiums receivable was $4.3 million and $2.5 million, respectively. The allowance for doubtful accounts for reinsurance recoverables on paid losses was $2.3 million and $2.2 million at September 30, 2013 and December 31, 2012, respectively. Premiums receivable over 90 days were secured by collateral in the amount of $0.6 million and $0.4 million at September 30, 2013 and December 31, 2012, respectively. Reinsurance recoverables on paid losses over 90 days were secured by collateral in the amount of $1.6 million and $0.3 million at September 30, 2013 and December 31, 2012, respectively.
	At September 30, 2013 and December 31, 2012, 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 at September 30, 2013 and December 31, 2012 is shown below:
			September 30, 2013				December 31, 2012
	(in millions)		Carrying		Fair Value		Carrying		Fair
			Amount				Amount		Value
	Junior subordinated debentures		$	193.3	$	155.2	$	193.3	$	151.8
	Senior unsecured fixed rate notes			143.8		115.2		143.8		143.2
	Other indebtedness:
	Floating rate loan stock			64.1		51.4		63		49.5
	Note payable			0.8		0.6		0.8		0.6
			$	402	$	322.4	$	400.9	$	345.1

Information_Provided_in_Connec

Information Provided in Connection with Outstanding Debt of Subsidiaries	9 Months Ended
	Sep. 30, 2013
Text Block [Abstract]	'
Information Provided in Connection with Outstanding Debt of Subsidiaries	'
	16	Information Provided in Connection with Outstanding Debt of Subsidiaries
	In September 2012, through our wholly owned subsidiary Argo Group US, Inc. (“Argo Group US”), we issued $143,750,000 aggregate principal amount of Argo Group US’s 6.5% Senior Notes due September 15, 2042 (the “Notes”). The Notes bear interest at 6.5%, payable quarterly in cash in arrears on the 15th day of March, June, September and December of each year, beginning on December 15, 2012. The Notes are unsecured and unsubordinated obligations of Argo Group US and rank equally in right of payment with all of Argo Group US’s other unsecured and unsubordinated debt. The Notes are guaranteed on a full and unconditional senior unsecured basis by Argo Group. The Notes may be redeemed, for cash, in whole or in part, on or after September 15, 2017, at Argo Group US’s option, at any time and from time to time, prior to maturity at a redemption price equal to 100% of the principal amount of the Notes to be redeemed, plus accrued but unpaid interest on the principal amount being redeemed to, but not including, the redemption date.
	The following tables present condensed consolidating financial information at September 30, 2013 and December 31, 2012 and for the three and nine months ended September 30, 2013 and 2012, 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
	September 30, 2013
	(in millions)
			Argo Group			Argo Group US, Inc.			Other Subsidiaries			Consolidating			Total
			International			and Subsidiaries			and Eliminations (1)			Adjustments (2)
			Holdings, Ltd (Parent			(Subsidiary Issuer)
			Guarantor)
	Assets
	Investments		$	3.4		$	2,750.20		$	1,296.60		$	0		$	4,050.20
	Cash			0			117.5			24.1			0			141.6
	Accrued investment income			0			19.5			5.8			0			25.3
	Premiums receivable			0			150.3			276.1			0			426.4
	Reinsurance recoverables			0			1,165.40			91.8			0			1,257.20
	Goodwill and other intangible assets			0			137.9			103.2			0			241.1
	Current income taxes receivable, net			0			10.1			5.2			0			15.3
	Deferred acquisition costs, net			0			55.5			59.8			0			115.3
	Ceded unearned premiums			0			99.4			131.6			0			231
	Other assets			4.2			93.5			49.5			0			147.2
	Due from affiliates			0			0.2			(0.2	)		0			0
	Intercompany note receivable			0			102.7			(102.7	)		0			0
	Investments in subsidiaries			1,662.10			0			0			(1,662.1	)		0
	Total assets		$	1,669.70		$	4,702.20		$	1,940.80		$	(1,662.1	)	$	6,650.60
	Liabilities and Shareholders’ Equity
	Reserves for losses and loss adjustment expenses		$	0		$	2,178.70			1,044.20		$	0		$	3,222.90
	Unearned premiums			0			448.8			398.2			0			847
	Funds held and ceded reinsurance payable, net			0			638			(181.4	)		0			456.6
	Long-term debt			49			288.9			64.1			0			402
	Current income taxes payable, net			0			0			0			0			0
	Deferred tax liabilities, net			0			23.1			7.6			0			30.7
	Accrued underwriting expenses and other liabilities			14.2			98			51.8			0			164
	Due to affiliates			(1.5	)		0			0			1.5			0
	Intercompany note payable			80.6			0			0			(80.6	)		0
	Total liabilities			142.3			3,675.50			1,384.50			(79.1	)		5,123.20
	Total shareholders’ equity			1,527.40			1,026.80			556.2			(1,583.0	)		1,527.40
	Total liabilities and shareholders’ equity		$	1,669.70		$	4,702.30		$	1,940.70		$	(1,662.1	)	$	6,650.60
	(1)	Includes all other subsidiaries of Argo Group International Holdings, Ltd and all intercompany eliminations
	(2)	Includes all Argo Group parent company eliminations
	CONDENSED CONSOLIDATING BALANCE SHEET
	December 31, 2012
	(in millions)
			Argo Group			Argo Group US, Inc.			Other Subsidiaries			Consolidating			Total
			International			and Subsidiaries			and Eliminations (1)			Adjustments (2)
			Holdings, Ltd (Parent			(Subsidiary Issuer)
			Guarantor)
	Assets
	Investments		$	(0.9	)	$	2,794.70		$	1,406.90		$	0		$	4,200.70
	Cash			0			73.9			21.9			0			95.8
	Accrued investment income			0			21.3			9			0			30.3
	Premiums receivable			0			155.1			205.9			0			361
	Reinsurance recoverables			0			1,155.10			165.8			0			1,320.90
	Goodwill and other intangible assets			0			139.5			105.8			0			245.3
	Current income taxes receivable, net			0			8.7			4.2			0			12.9
	Deferred acquisition costs, net			0			54.6			44.8			0			99.4
	Ceded unearned premiums			0			85.9			107.7			0			193.6
	Other assets			4.3			82.7			42			0			129
	Due from affiliates			5			1.2			(1.2	)		(5.0	)		0
	Intercompany note receivable			0			119.6			(119.6	)		0			0
	Investments in subsidiaries			1,621.10			0			0			(1,621.1	)		0
	Total assets		$	1,629.50		$	4,692.30		$	1,993.20		$	(1,626.1	)	$	6,688.90
	Liabilities and Shareholders’ Equity
Reserves for losses and loss adjustment expenses		$	0		$	2,226.30		$	997.2		$	0		$	3,223.50
Unearned premiums			0			415.6			314.6			0			730.2
Funds held and ceded reinsurance payable, net			0			598.4			47.4			0			645.8
Long-term debt			49			288.9			63			0			400.9
Current income taxes payable, net			0			0			0			0			0
Deferred tax liabilities, net			0			36.8			7			0			43.8
Accrued underwriting expenses and other liabilities			11.3			84.1			35.2			0			130.6
Due to affiliates			0			0			0			0			0
Intercompany note payable			55.1			0			0			(55.1	)		0
Total liabilities			115.4			3,650.10			1,464.40			(55.1	)		5,174.80
Total shareholders’ equity			1,514.10			1,042.20			528.8			(1,571.0	)		1,514.10
Total liabilities and shareholders’ equity		$	1,629.50		$	4,692.30		$	1,993.20		$	(1,626.1	)	$	6,688.90
(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 SEPTEMBER 30, 2013
(in millions)
		Argo Group International			Argo Group US, Inc. and			Other Subsidiaries			Consolidating			Total
		Holdings, Ltd (Parent			Subsidiaries			and Eliminations (1)			Adjustments (2)
		Guarantor)			(Subsidiary Issuer)
Premiums and other revenue:
Earned premiums		$	0		$	105.2		$	225.3		$	0		$	330.5
Net investment income			0			16.7			7.7			(0.3	)		24.1
Fee income, net			0			(0.6	)		0.9			0			0.3
Net realized investment gains			0			7.2			1.9			0			9.1
Total revenue			0			128.5			235.8			(0.3	)		364
Expenses:
Losses and loss adjustment expenses			0			65.9			125			0			190.9
Other reinsurance-related expenses			0			(2.9	)		7.5			0			4.6
Underwriting, acquisition and insurance expenses			5.8			42.1			78.8			0			126.7
Interest expense			0.7			3.8			0.9			(0.3	)		5.1
Foreign currency exchange (gain) loss			0			(0.1	)		3.9			0			3.8
Total expenses			6.5			108.8			216.1			(0.3	)		331.1
Income (loss) before income taxes			(6.5	)		19.7			19.7			0			32.9
Provision for income taxes			0			4.4			(2.5	)		0			1.9
Net (loss) income before equity in earnings of subsidiaries			(6.5	)		15.3			22.2			0			31
Equity in undistributed earnings of subsidiaries			37.5			0			0			(37.5	)		0
Net income		$	31		$	15.3		$	22.2		$	(37.5	)	$	31
(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 (LOSS) INCOME
FOR THE THREE MONTHS ENDED SEPTEMBER 30, 2012
(in millions)
		Argo Group International			Argo Group US, Inc. and			Other Subsidiaries			Consolidating			Total
		Holdings, Ltd (Parent			Subsidiaries			and Eliminations (1)			Adjustments (2)
		Guarantor)			(Subsidiary Issuer)
Premiums and other revenue:
Earned premiums		$	0		$	108.1		$	196.2		$	0		$	304.3
Net investment income			0			21.2			7.8			(0.1	)		28.9
Fee income, net			0			2.4			0.6			0			3
Net realized investment gains (losses)			(0.5	)		4.2			4.6			0			8.3
Total revenue			(0.5	)		135.9			209.2			(0.1	)		344.5
Expenses:
Losses and loss adjustment expenses			0			78			108.3			0			186.3
Other reinsurance-related expenses			0			0			7			0			7
Underwriting, acquisition and insurance expenses			10.1			41.8			66.1			0			118
Interest expense			0.1			4.1			1.7			(0.1	)		5.8
Foreign currency exchange (gain) loss			0			(0.1	)		9.8			0			9.7
Total expenses			10.2			123.8			192.9			(0.1	)		326.8
Income (loss) before income taxes			(10.7	)		12.1			16.3			0			17.7
Provision for income taxes			0			2.5			1.8			0			4.3
Net income (loss) before equity in earnings of subsidiaries			(10.7	)		9.6			14.5			0			13.4
Equity in undistributed earnings of subsidiaries			24.1			0			0			(24.1	)		0
Net income		$	13.4		$	9.6		$	14.5		$	(24.1	)	$	13.4
(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 NINE MONTHS ENDED SEPTEMBER 30, 2013
(in millions)
		Argo Group International			Argo Group US, Inc. and			Other Subsidiaries			Consolidating			Total
		Holdings, Ltd (Parent			Subsidiaries			and Eliminations (1)			Adjustments (2)
		Guarantor)			(Subsidiary Issuer)
Premiums and other revenue:
Earned premiums		$	0		$	321.2		$	641		$	0		$	962.2
Net investment income			0			57.2			21			(0.9	)		77.3
Fee income, net			0			(1.9	)		2.4			0			0.5
Net realized investment gains (losses)			0			32.8			(3.2	)		0			29.6
Total revenue			0			409.3			661.2			(0.9	)		1,069.60
Expenses:
Losses and loss adjustment expenses			0			194.9			359.2			0			554.1
Other reinsurance-related expenses			0			0			14.4			0			14.4
Underwriting, acquisition and insurance expenses			18.3			134.3			225.4			0			378
Interest expense			2.3			11.4			2.3			(0.9	)		15.1
Foreign currency exchange (gain) loss			0			0.1			(5.3	)		0			(5.2	)
Total expenses			20.6			340.7			596			(0.9	)		956.4
(Loss) income before income taxes			(20.6	)		68.6			65.2			0			113.2
Provision for income taxes			0			16			1.8			0			17.8
Net (loss) income before equity in earnings of subsidiaries			(20.6	)		52.6			63.4			0			95.4
Equity in undistributed earnings of subsidiaries			116			0			0			(116.0	)		0
Net income		$	95.4		$	52.6		$	63.4		$	(116.0	)	$	95.4
(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 (LOSS) INCOME
FOR THE NINE MONTHS ENDED SEPTEMBER 30, 2012
(in millions)
		Argo Group International			Argo Group US, Inc. and			Other Subsidiaries			Consolidating			Total
		Holdings, Ltd (Parent			Subsidiaries			and Eliminations (1)			Adjustments (2)
		Guarantor)			(Subsidiary Issuer)
Premiums and other revenue:
Earned premiums		$	0		$	327		$	544.8		$	0		$	871.8
Net investment income			0.1			65.3			25.1			(0.2	)		90.3
Fee income, net			0			2.4			2.4			0			4.8
Net realized investment gains (losses)			(0.5	)		11.8			7.4			0			18.7
Total revenue			(0.4	)		406.5			579.7			(0.2	)		985.6
Expenses:
Losses and loss adjustment expenses			0			220.9			307			0			527.9
Other reinsurance-related expenses			0			0			20.8			0			20.8
Underwriting, acquisition and insurance expenses			21.4			127.9			197			0			346.3
Interest expense			2.9			11.8			2.5			(0.2	)		17
Foreign currency exchange loss			0			0.1			2.7			0			2.8
Total expenses			24.3			360.7			530			(0.2	)		914.8
(Loss) income before income taxes			(24.7	)		45.8			49.7			0			70.8
Provision for income taxes			0			10.4			3.4			0			13.8
Net (loss) income before equity in earnings of subsidiaries			(24.7	)		35.4			46.3			0			57
Equity in undistributed earnings of subsidiaries			81.7			0			0			(81.7	)		0
Net income		$	57		$	35.4		$	46.3		$	(81.7	)	$	57
(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 NINE MONTHS ENDED SEPTEMBER 30, 2013
(in millions)
		Argo Group			Argo Group US, Inc. and			Other Subsidiaries			Consolidating			Total
		International Holdings,			Subsidiaries			and Eliminations (1)			Adjustments (2)
		Ltd (Parent Guarantor)			(Subsidiary Issuer)
Net cash flows from operating activities		$	(15.5	)	$	75.9			(63.4	)	$	0		$	(3.0	)
Cash flows from investing activities:
Proceeds from sales of investments			0			863.2			515.6			0			1,378.80
Proceeds from maturities and mandatory calls of investments			0			139			85.2			0			224.2
Purchases of investments			0			(942.4	)		(464.9	)		0			(1,407.3	)
Change in short-term investments and foreign regulatory deposits			(0.8	)		(38.5	)		(36.2	)		0			(75.5	)
Settlements of foreign currency exchange forward contracts			1.9			0			(8.3	)		0			(6.4	)
Issuance of intercompany note, net			0			0			(24.9	)		24.9			0
Purchase of fixed assets			0			(17.7	)		(1.4	)		0			(19.1	)
Other, net			(0.1	)		4.4			0.2			0			4.5
Cash provided by investing activities			1			8			65.3			24.9			99.2
Cash flows from financing activities:
Borrowings under intercompany note, net			24.9			0			0			(24.9	)		0
Proceeds from issuance of senior unsecured fixed rate notes			0			0			0			0			0
Activity under stock incentive plans			1.4			0			0			0			1.4
Repurchase of Company’s common shares			0			(40.1	)		0			0			(40.1	)
Excess tax expense from share-based payment arrangements			0			(0.2	)		0.4			0			0.2
Payment of cash dividends to common shareholders			(11.8	)		0			0			0			(11.8	)
Cash provided (used) by financing activities			14.5			(40.3	)		0.4			(24.9	)		(50.3	)
Effect of exchange rate changes on cash			0			0			(0.1	)		0			(0.1	)
Change in cash			(0.0	)		43.6			2.2			0			45.8
Cash, beginning of period			0			73.9			21.9			0			95.8
Cash, end of period		$	(0.0	)	$	117.5		$	24.1		$	0		$	141.6
(1)	Includes all other subsidiaries of Argo Group International Holdings, Ltd and all intercompany eliminations
(2)	Includes all Argo Group parent company eliminations
CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS
FOR THE NINE MONTHS ENDED SEPTEMBER 30, 2012
(in millions)
		Argo Group			Argo Group US,			Other Subsidiaries			Consolidating			Total
		International Holdings,			Inc. and			and Eliminations (1)			Adjustments (2)
		Ltd (Parent Guarantor)			Subsidiaries
					(Subsidiary Issuer)
Net cash flows from operating activities		$	(17.9	)	$	13.7		$	67.7		$	0		$	63.5
Cash flows from investing activities:
Proceeds from sales of investments			0			458.9			439.6			0			898.5
Proceeds from maturities and mandatory calls of investments			0			253.9			111			0			364.9
Purchases of investments			0			(712.1	)		(539.3	)		0			(1,251.4	)
Change in short-term investments and foreign regulatory deposits			(0.1	)		(64.3	)		(34.6	)		0			(99.0	)
Settlements of foreign currency exchange forward contracts			(0.6	)		0			0			0			(0.6	)
Acquisitions			0			(1.4	)		0			0			(1.4	)
Issuance of intercompany note, net			0			(10.0	)		(17.3	)		27.3			0
Purchase of fixed assets			0			(23.6	)		(3.3	)		0			(26.9	)
Other, net			0			2			0.7			0			2.7
Cash provided (used) by investing activities			(0.7	)		(96.6	)		(43.2	)		27.3			(113.2	)
Cash flows from financing activities:
Borrowings under intercompany note, net			27.3			25			(25.0	)		(27.3	)		0
Proceeds from issuance of senior unsecured fixed rate notes			0			139.2			0			0			139.2
Activity under stock incentive plans			0.7			0			0			0			0.7
Repurchase of Company’s common shares			0			(38.6	)		0			0			(38.6	)
Excess tax expense from share-based payment arrangements			0			0			0			0			0
Payment of cash dividends to common shareholders			(9.3	)		0			0			0			(9.3	)
Cash (used) provided by financing activities			18.7			125.6			(25.0	)		(27.3	)		92
Effect of exchange rate changes on cash			0			0			(0.5	)		0			(0.5	)
Change in cash			0.1			42.7			(1.0	)		0			41.8
Cash, beginning of period			0			85.1			15.8			0			100.9
Cash, end of period		$	0.1		$	127.8		$	14.8		$	0		$	142.7
(1)	Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations
(2)	Includes all Argo Group parent company eliminations

Recently_Issued_Accounting_Sta1

Recently Issued Accounting Standards (Policies)	9 Months Ended
	Sep. 30, 2013
Accounting Changes And Error Corrections [Abstract]	'
Recently Issued Accounting Pronouncements	'
	In December 2011, with further clarification issued in January 2013, the Financial Accounting Standards Board (“FASB”) issued an accounting update requiring disclosures about offsetting and related arrangements to enable users of financial statements to understand the effect of those arrangements on its financial position. Entities are required to disclose both gross information and net information about both instruments and transactions eligible for offset in the statement of financial position and instruments and transactions subject to an agreement similar to a master netting arrangement. This update is applicable to derivatives including bifurcated embedded derivatives, repurchase agreements and reverse repurchase agreements and securities borrowing and securities lending arrangements. The update was effective for annual reporting periods beginning on or after January 1, 2013, and interim periods within those annual periods. The disclosures required by this update should be provided retrospectively for all comparative periods presented. The adoption of this update did not have an impact on our financial results and disclosures.
	In July 2012, the FASB issued an accounting update that allows an entity the option to first assess qualitative factors to determine whether it is more likely than not that an indefinite-lived intangible asset is impaired. The more-likely-than-not threshold is defined as having a likelihood of more than 50 percent. If the more-likely-than-not threshold is met, an entity is required to determine the fair value of the indefinite-lived intangible asset and perform the quantitative impairment test by comparing the fair value with the carrying amount in accordance with accounting guidance. The update was effective for annual and interim impairment tests performed for fiscal years beginning after September 15, 2012. Early adoption was permitted, including for annual and interim impairment tests performed as of a date before July 27, 2012, if an entity’s financial statements for the most recent annual or interim period have not yet been issued or, for nonpublic entities, have not yet been made available for issuance. The adoption of this update did not have an impact on our financial results and disclosures.
	In February 2013, the FASB issued an accounting update to improve the reporting of reclassifications out of accumulated other comprehensive income. An entity is required to report the effect of significant reclassifications, by component, out of accumulated other comprehensive income on the respective line items in net income if the item is required under GAAP to be reclassified in its entirety in the same reporting period. The required disclosures of the update are allowed either in the Statement of Income or in the notes. The amendments in the update were effective for reporting periods beginning after December 15, 2012. See Note 6, “Accumulated Other Comprehensive Income” for the required disclosures.
	In February 2013, the FASB issued amendments to “Liabilities” (Topic 405) in order to resolve diversity in practice related to accounting for the recognition, measurement, and disclosure of obligations resulting from joint and several liability arrangements for which the total amount of the obligation is fixed at the reporting date. The amendments require an entity to measure the liability as the sum of (a) the amount the reporting entity agreed to pay on the basis of its arrangement among its co-obligors, and (b) any additional amount the reporting entity expects to pay on behalf of its co-obligors. The amendments in this update are effective for fiscal years, and interim periods within those years, beginning after December 15, 2013, but early adoption is permitted. The amendments should be applied retrospectively to all prior periods presented for those obligations resulting from joint and several liability arrangements within the update’s scope that exist at the beginning of an entity’s fiscal year of adoption. We do not anticipate this update will have an impact on our financial results and disclosures.
	In March 2013, the FASB issued an amendment to “Consolidation” (Topic 810) to resolve the diversity in practice related to the release of the cumulative translation adjustment into net income when a parent sells a part of all of its investment in a foreign entity or no longer holds a controlling financial interest in a subsidiary or group of assets that is a nonprofit activity or a business within a foreign entity. The amendments in the update are effective prospectively for fiscal years, and interim reporting periods within those years, beginning after December 15, 2013, but early adoption is permitted. The amendments should be applied prospectively to derecognition events occurring after the effective date with no adjustment to prior periods. We do not anticipate this update will have an impact on our financial results and disclosures.

Investments_Tables

Investments (Tables)	9 Months Ended
	Sep. 30, 2013
Investments Debt And Equity Securities [Abstract]	'
Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments	'
	The amortized cost, gross unrealized gains, gross unrealized losses and fair value of investments as of September 30, 2013 and December 31, 2012 were as follows:
	September 30, 2013					Gross			Gross
			Amortized			Unrealized			Unrealized			Fair
	(in millions)		Cost			Gains			Losses			Value
	Fixed maturities
	USD denominated:
	U.S. Governments		$	275		$	3		$	0.4		$	277.6
	Non-U.S. Governments			76.2			0.4			2.4			74.2
	Obligations of states and political subdivisions			597.5			24.3			13.5			608.3
	Credit-Financial			385.8			15.3			2.3			398.8
	Credit-Industrial			383.4			15.5			3.8			395.1
	Credit-Utility			173.6			5.9			1.4			178.1
	Structured securities:
	CMO/MBS-agency (1)			233.4			10.1			1.5			242
	CMO/MBS-non agency			20.3			0.8			0			21.1
	CMBS (2)			161.5			3.2			1.1			163.6
	ABS-residential (3)			7.6			0.3			0.5			7.4
	ABS-non residential			100.8			0.6			0.1			101.3
	Foreign denominated:
	Governments			227.5			5.2			4			228.7
	Credit			115.3			3.1			2.7			115.7
	Total fixed maturities			2,757.90			87.7			33.7			2,811.90
	Equity securities			386.5			197.4			3.1			580.8
	Other investments			366.9			4.3			0			371.2
	Short-term investments			286.3			0			0			286.3
	Total investments		$	3,797.60		$	289.4		$	36.8		$	4,050.20
	December 31, 2012					Gross			Gross
			Amortized			Unrealized			Unrealized			Fair
	(in millions)		Cost			Gains			Losses			Value
	Fixed maturities
	USD denominated:
	U.S. Governments		$	401.1		$	9.3		$	0		$	410.4
	Non-U.S. Governments			53.1			3.6			0.1			56.6
	Obligations of states and political subdivisions			551.7			44.9			0.5			596.1
	Credit-Financial			382.4			25.7			0.7			407.4
	Credit-Industrial			428.4			31.1			0.7			458.8
	Credit-Utility			189.9			12.6			0.4			202.1
	Structured securities:
	CMO/MBS-agency (1)			375.1			20.2			0.2			395.1
	CMO/MBS-non agency			12.5			1			0.2			13.3
	CMBS (2)			109.5			5.7			0.2			115
	ABS-residential (3)			10.1			0.2			0.8			9.5
	ABS-non residential			81.3			1.2			0			82.5
	Foreign denominated:
	Governments			262.1			9.2			4.3			267
	Credit			135.9			6.1			1.8			140.2
	Total fixed maturities			2,993.10			170.8			9.9			3,154.00
	Equity securities			373.5			153			5.1			521.4
	Other investments			294.8			1.2			5			291
	Short-term investments			234.3			0.1			0.1			234.3
	Total investments		$	3,895.70		$	325.1		$	20.1		$	4,200.70
	-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 September 30, 2013, by contractual maturity, were as follows:
	(in millions)		Amortized			Fair
			Cost			Value
	Due in one year or less		$	203.3		$	205.3
	Due after one year through five years			1,170.00			1,196.30
	Due after five years through ten years			607.8			629.6
	Thereafter			253.2			245.3
	Structured securities			523.6			535.4
	Total		$	2,757.90		$	2,811.90
Schedule of Aging of Unrealized Losses on Investments	'
	An aging of unrealized losses on our investments at September 30, 2013 and December 31, 2012 is presented below:
	September 30, 2013		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		$	36.8		$	0.4		$	0		$	0		$	36.8	$	0.4
	Non-U.S. Governments (2)			51.3			2.4			0			0			51.3		2.4
	Obligations of states and political subdivisions			216.9			13.5			0			0			216.9		13.5
	Credit-Financial (2)			82.6			2.3			1.3			0			83.9		2.3
	Credit-Industrial			95.9			3.6			2.5			0.2			98.4		3.8
	Credit-Utility (2)			34.9			1.4			0			0			34.9		1.4
	Structured securities:
	CMO/MBS-agency (2)			44.6			1.5			0.2			0			44.8		1.5
	CMBS			45.5			1.1			0			0			45.5		1.1
	ABS-residential			0			0			3.9			0.5			3.9		0.5
	ABS-non residential			60.2			0.1			0			0			60.2		0.1
	Foreign denominated:
	Governments			174.4			4			0			0			174.4		4
	Credit			81.7			2.7			0			0			81.7		2.7
	Total fixed maturities			924.8			33			7.9			0.7			932.7		33.7
	Equity securities			54.3			3.1			0			0			54.3		3.1
	Other investments (1)			0			0			0			0			0		0
	Short-term investments (1)			2.2			0			0			0			2.2		0
	Total		$	981.3		$	36.1		$	7.9		$	0.7		$	989.2	$	36.8
	(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.
	December 31, 2012		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 (1)		$	11.9		$	0		$	0		$	0		$	11.9	$	0
	Non-U.S. Governments (2)			8.5			0.1			0.5			0			9		0.1
	Obligations of states and political subdivisions			28.2			0.3			0.8			0.2			29		0.5
	Credit-Financial			25			0.2			8.6			0.5			33.6		0.7
	Credit-Industrial			32.2			0.4			3			0.3			35.2		0.7
	Credit-Utility			8.4			0.2			0.8			0.2			9.2		0.4
	Structured securities:
	CMO/MBS-agency (2)			26.4			0.2			0.3			0			26.7		0.2
	CMO/MBS-non agency			1.7			0.2			0			0			1.7		0.2
	CMBS(1)			5.7			0			3.5			0.2			9.2		0.2
	ABS-residential			0			0			3.9			0.8			3.9		0.8
	ABS-non residential (1) (2)			13.7			0			0.2			0			13.9		0
	Foreign denominated:
	Governments (2)			180.5			4.3			0.3			0			180.8		4.3
	Credit			76.3			1.8			0			0			76.3		1.8
	Total fixed maturities			418.5			7.7			21.9			2.2			440.4		9.9
	Equity securities			60.2			4.2			6.3			0.9			66.5		5.1
	Other investments			11.6			5			0			0			11.6		5
	Short-term investments			0			0.1			0			0			0		0.1
	Total		$	490.3		$	17		$	28.2		$	3.1		$	518.5	$	20.1
	(1)	Unrealized losses less than one year are less than $0.1 million.
	(2)	Unrealized losses one year or greater are less than $0.1 million.
Schedule of Company's Gross Realized Investment Gains (Losses)	'
	The following table presents the Company’s gross realized investment gains (losses) for the three and nine months ended September 30:
			For the Three Months						For the Nine Months
	(in millions)		Ended September 30,						Ended September 30,
			2013			2012			2013			2012
	Realized gains
	Fixed maturities		$	5.8		$	5.5		$	27.5		$	21
	Equity securities			7.2			0.2			15			0.4
	Other investments			11.2			7.7			28.9			20.8
	Short-term investments			0.1			0.2			0.1			0.5
	Gross realized gains			24.3			13.6			71.5			42.7
	Realized losses
	Fixed maturities			(6.1	)		(3.0	)		(14.5	)		(8.1	)
	Equity securities			(0.1	)		(0.1	)		(0.8	)		(0.4	)
	Other investments			(8.6	)		(1.1	)		(20.7	)		(13.2	)
	Short-term investments			(0.3	)		0			(0.6	)		(0.3	)
	Other-than-temporary impairment losses on fixed maturities			0			(1.1	)		(0.3	)		(1.4	)
	Other-than-temporary impairment losses on equity securities			(0.1	)		0			(1.6	)		(0.6	)
	Other-than-temporary impairment losses on other investments			0			0			(3.4	)		0
	Gross realized losses			(15.2	)		(5.3	)		(41.9	)		(24.0	)
	Net realized investment gains		$	9.1		$	8.3		$	29.6		$	18.7
Securities and Letters of Credit Pledged to Lloyd's as Company's Corporate Member's Capital	'
	At September 30, 2013, our Corporate member’s capital supporting our Lloyd’s business consisted of:
	(in millions)
	Letters of credit		$	124.9
	Fixed maturities, at fair value			160.9
	Short-term investments, at fair value			0.1
	Total securities and letters of credit pledged to Lloyd’s		$	285.9
Financial Assets Measured at Fair Value on Recurring Basis	'
	Based on an analysis of the inputs, our financial assets measured at fair value on a recurring basis at September 30, 2013 and December 31, 2012 have been categorized as follows:
	(in millions)					Fair Value Measurements at Reporting Date Using
			September 30, 2013			Level 1 (a)			Level 2 (b)			Level 3  (c)
	Fixed maturities
	USD denominated:
	U.S. Governments		$	277.6		$	142.8		$	134.8		$	0
	Non-U.S. Governments			74.2			0			74.2			0
	Obligations of states and political subdivisions			608.3			0			608.3			0
	Credit-Financial			398.8			0			398.8			0
	Credit-Industrial			395.1			0			395.1			0
	Credit-Utility			178.1			0			178.1			0
	Structured securities:
	CMO/MBS-agency			242			0			242			0
	CMO/MBS-non agency			21.1			0			21.1			0
	CMBS			163.6			0			163.6			0
	ABS-residential			7.4			0			7.4			0
	ABS-non residential			101.3			0			101.3			0
	Foreign denominated:
	Governments			228.7			0			228.7			0
	Credit			115.7			0			115.7			0
	Total fixed maturities			2,811.90			142.8			2,669.10			0
	Equity securities			580.8			523.3			56.2			1.3
	Other investments			114.3			0			114.3			0
	Short-term investments			286.3			284			2.3			0
	Other assets			2.8			0			0			2.8
			$	3,796.10		$	950.1		$	2,841.90		$	4.1
	(a)	Quoted prices in active markets for identical assets
	(b)	Significant other observable inputs
	(c)	Significant unobservable inputs
	(in millions)					Fair Value Measurements at Reporting Date Using
			December 31, 2012			Level 1 (a)			Level 2 (b)			Level 3  (c)
	Fixed maturities
	USD denominated:
	U.S. Governments		$	410.4		$	219.5		$	190.9		$	0
	Non-U.S. Governments			56.6			0			56.6			0
	Obligations of states and political subdivisions			596.1			0			596.1			0
	Credit-Financial			407.4			0			407.4			0
	Credit-Industrial			458.8			0			458.8			0
	Credit-Utility			202.1			0			202.1			0
	Structured securities:
	CMO/MBS-agency			395.1			0			395.1			0
	CMO/MBS-non agency			13.3			0			13.3			0
	CMBS			115			0			115			0
	ABS-residential			9.5			0			9.5			0
	ABS-non residential			82.5			0			82.5			0
	Foreign denominated:
	Governments			267			0			267			0
	Credit			140.2			0			140.2			0
	Total fixed maturities			3,154.00			219.5			2,934.50			0
	Equity securities			521.4			469			50.6			1.8
	Other investments			132			0			132			0
	Short-term investments			234.3			201.1			33.2			0
	Other assets			6.9			0			0			6.9
			$	4,048.60		$	889.6		$	3,150.30		$	8.7
	(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 at September 30, 2013 and December 31, 2012 are as follows:
	Fair Value Measurements Using Unobservable Inputs (Level 3)
	(in millions)		Equity			Other			Total
			Securities			Assets
	Beginning balance, January 1, 2013		$	1.8		$	6.9		$	8.7
	Transfers into Level 3			0			0			0
	Transfers out of Level 3			0			0			0
	Total gains or losses (realized/unrealized):
	Included in net income			0.1			0			0.1
	Included in other comprehensive income			0			0			0
	Purchases, issuances, sales, and settlements
	Purchases			0			0			0
	Issuances			0			0			0
	Sales			(0.6	)		0			(0.6	)
	Settlements			0			(4.1	)		(4.1	)
	Ending balance, September 30, 2013		$	1.3		$	2.8		$	4.1
	Amount of total gains or losses for the period included in net income attributable to the change in unrealized gains or losses relating to assets still held at September 30, 2013		$	0		$	0		$	0
	Fair Value Measurements Using Unobservable Inputs (Level 3)
	(in millions)		Credit			Equity			Other			Total
			Financial			Securities			Assets
	Beginning balance, January 1, 2012		$	0.7		$	2.4		$	9		$	12.1
	Transfers into Level 3			0			0			0			0
	Transfers out of Level 3			0			0			0			0
	Total gains or losses (realized/unrealized):
	Included in net income			0.1			0			0			0.1
	Included in other comprehensive income			0			0			0			0
	Purchases, issuances, sales, and settlements
	Purchases			0			0			0			0
	Issuances			0			0			0			0
	Sales			(0.8	)		(0.6	)		0			(1.4	)
	Settlements			0			0			(2.1	)		(2.1	)
	Ending balance, December 31, 2012		$	0		$	1.8		$	6.9		$	8.7
	Amount of total gains or losses for the period included in net loss attributable to the change in unrealized gains or losses relating to assets still held at December 31, 2012		$	0		$	0		$	0		$	0

Shareholders_Equity_Tables

Shareholders Equity (Tables)	9 Months Ended
	Sep. 30, 2013
Equity [Abstract]	'
Schedule of Repurchase of Shares	'
	In 2013, we repurchased shares under Securities Exchange Act of 1934 Rule 10b5-1 trading plans as follows:
	Date Trading		2013	Number of	Average Price of		Total Cost		Repurchase
	Plan Initiated		Purchase Period	Shares Repurchased	Shares Repurchased		(in millions)		Authorization Year
	12/14/12		01/01/13-01/17/13 (1)	171,480		$        35.41		$        6.0	2011
	3/15/13		03/16/13-05/01/13	205,452		$        40.75		$        8.3	2011
	6/14/13		06/18/13-07/31/13	195,217		$        43.47		$        8.5	2011
	(1)	The above table only reflects the 2013 activity under this Rule 10b5-1 trading plan. In 2012, 117,663 shares were repurchased under this Rule 10b5-1 trading plan for a total cost of $3.9 million. Total shares repurchased in 2012 and 2013 under this Rule 10b5-1 trading plan are 289,143 shares at an average price of $34.58 for a total cost of $9.9 million.

Net_Income_Per_Common_Share_Ta

Net Income Per Common Share (Tables)	9 Months Ended
	Sep. 30, 2013
Earnings Per Share [Abstract]	'
Net Income Per Common Share on Basic and Diluted Basis	'
	The following table presents the calculation of net income per common share on a basic and diluted basis for the three and nine months ended September 30, 2013 and 2012:
		For the Three  Months				For the Nine Months
		Ended September 30,				Ended September 30,
	(in millions, except number of shares and per share amounts)	2013		2012		2013		2012
	Net income	$	31	$	13.4	$	95.4	$	57
	Weighted average common shares outstanding - basic		26,682,176		27,751,140		26,961,792		28,265,456
	Effect of dilutive securities
	Equity compensation awards		1,137,374		582,656		1,031,543		447,668
	Weighted average common shares outstanding - diluted		27,819,550		28,333,796		27,993,335		28,713,124
	Net income per common share - basic	$	1.16	$	0.48	$	3.54	$	2.02
	Net income per common share - diluted	$	1.12	$	0.47	$	3.41	$	1.98

Accumulated_Other_Comprehensiv1

Accumulated Other Comprehensive Income (Tables)	9 Months Ended
	Sep. 30, 2013
Text Block [Abstract]	'
Changes in Accumulated Other Comprehensive Income	'
	A summary of changes in accumulated other comprehensive income (loss), net of taxes (where applicable) by component for the nine months ended September 30, 2013 is presented in the following table:
	(in millions)	Foreign currency			Unrealized holding gains			Defined benefit			Total
		translation adjustments			on securities			pension plans
	Beginning balance, January 1, 2013	$	(8.7	)	$	204.1		$	(5.9	)	$	189.5
	Other comprehensive income before reclassifications		(2.2	)		(23.9	)		0			(26.1	)
	Amounts reclassified from accumulated other comprehensive income		0			(11.6	)		0			(11.6	)
	Net current-period other comprehensive income		(2.2	)		(35.5	)		0			(37.7	)
	Ending balance, September 30, 2013	$	(10.9	)	$	168.6		$	(5.9	)	$	151.8
Amounts Reclassified from Accumulated Other Comprehensive Income	'
	The amounts reclassified from accumulated other comprehensive income shown in the above table have been included in the following captions in the Consolidated Statements of Income:
	(in millions)	For the Three Months			For the Nine Months
		Ended September 30, 2013			Ended September 30, 2013
	Unrealized gains and losses on securities
	Net realized investment gains	$	(3.3	)	$	(18.3	)
	Provision for income taxes		2			6.7
	Net of taxes	$	(1.3	)	$	(11.6	)

Income_Taxes_Tables

Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2013
Income Tax Disclosure [Abstract]	'
Income Tax Provision (Benefit)	'
	Our income tax provision includes the following components:
			For the Three Months						For the Nine  Months
			Ended September 30,						Ended September 30,
	(in millions)		2013			2012			2013			2012
	Current tax provision		$	3.5		$	5.7		$	13.4		$	11.2
	Deferred tax provision related to:
	Future tax deductions			(2.4	)		(2.1	)		2			0.8
	Valuation allowance change			0.8			0.7			2.4			1.8
	Income tax provision		$	1.9		$	4.3		$	17.8		$	13.8
Schedule of Pre-Tax Income (Loss) and Effective Income Tax Rates	'
	For the three and nine months ended September 30, 2013 and 2012, pre-tax income (loss) attributable to our operations and the operations’ effective tax rates were as follows:
			For the Three Months Ended September 30,
	(in millions)		2013						2012
			Pre-tax			Effective Tax			Pre-tax			Effective Tax
			income (loss)			Rate			income (loss)			Rate
	Bermuda		$	14.2			0	%	$	8			0	%
	United States			19.6			22.9	%		12.1			21	%
	United Kingdom			1.6			-164.7	%		0.1			1217.4	%
	Belgium			0.1			91.6	%		0.1			20	%
	Brazil			(1.5	)		0	%		(1.9	)		0	%
	Dubai			(0.4	)		0	%		0			0	%
	Ireland			(0.1	)		0	%		0			0	%
	Malta			(0.6	)		0	%		(0.7	)		0	%
	Switzerland			0	(1)		19.7	%		0	(1)		25	%
	Pre-tax income (loss)		$	32.9					$	17.7
	-1	Pre-tax income for the respective period was less than $0.1 million.
			For the Nine Months Ended September 30,
	(in millions)		2013						2012
			Pre-tax			Effective Tax			Pre-tax			Effective Tax
			income (loss)			Rate			income (loss)			Rate
	Bermuda		$	34.2			0	%	$	18.2			0	%
	United States			68.6			23.5	%		45.8			22.8	%
	United Kingdom			18.3			9.2	%		13.8			24	%
	Belgium			0.1			26.9	%		0.2			33.6	%
	Brazil			(6.2	)		0	%		(5.4	)		0	%
	Dubai			(0.4	)		0	%		0.1			0	%
	Ireland			(0.1	)		0	%		(0.1	)		0	%
	Malta			(1.3	)		0	%		(1.8	)		0	%
	Switzerland			0	(1)		21	%		0	(1)		24	%
	Pre-tax income (loss)		$	113.2					$	70.8
	-1	Pre-tax income for the respective period was less than $0.1 million.
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 for the three and nine months ended September 30, 2013 and 2012 is as follows:
			For the Three Months						For the Nine Months
			Ended September 30,						Ended September 30,
	(in millions)		2013			2012			2013			2012
	Income tax provision at expected rate		$	6.2		$	3.3		$	25.2		$	17
	Tax effect of:
	Tax-exempt interest			(1.3	)		(1.2	)		(4.1	)		(4.2	)
	Dividends received deduction			(0.7	)		(0.4	)		(1.8	)		(1.3	)
	Valuation allowance change			0.8			0.7			2.4			1.8
	Other permanent adjustments, net			(0.5	)		0.1			(1.4	)		0.8
	Adjustment for annualized rate			0.3			0			(0.1	)		0
	United States state tax benefit			(0.1	)		0			(0.4	)		(0.2	)
	Other foreign adjustments			0			0			0			0.1
	Deferred tax rate reduction			(1.3	)		(0.6	)		(1.3	)		(0.6	)
	Foreign exchange adjustments			(1.7	)		2.4			(1.3	)		0.4
	Foreign withholding taxes			0.2			0			0.6			0
	Income tax provision		$	1.9		$	4.3		$	17.8		$	13.8
	Income tax provision (benefit) - Foreign		$	(2.6	)	$	1.8		$	1.7		$	3.4
	Income tax provision - United States Federal			4.4			2.3			16.1			10.6
	Income tax provision (benefit) - United States State			(0.1	)		0.2			(0.6	)		(0.2	)
	Foreign withholding tax - United States			0.2			0			0.6			0
	Income tax provision		$	1.9		$	4.3		$	17.8		$	13.8

Sharebased_Compensation_Tables

Share-based Compensation (Tables)	9 Months Ended
	Sep. 30, 2013
Fair Value Assumptions	'
	The expected award life is based upon the average holding period over the history of the incentive plan. The following table summarizes the assumptions we used for the nine months ended September 30, 2013 and 2012:
		2013			2012
	Risk-free rate of return		1.44	%		1.09	%
	Expected dividend yields		1.55	%		1.67	%
	Expected award life (years)		4.92			5.03
	Expected volatility		31.1	%		32.6	%
2007 Plan [Member]	'
Summary of Stock Option Activity	'
	A summary of option activity under the 2007 Plan as of September 30, 2013, and changes during the nine months then ended is as follows:
		Shares			Weighted-Average
					Exercise Price
	Outstanding at January 1, 2013		418,325		$	33.56
	Granted		0		$	0
	Exercised		(151,286	)	$	31.62
	Expired or forfeited		(45,051	)	$	34.21
	Outstanding at September 30, 2013		221,988		$	34.74
Summary of Restricted Share Units Activity Under 2007 Plan	'
	A summary of restricted share activity under the 2007 Plan as of September 30, 2013, and changes during the nine months then ended is as follows:
		Shares			Weighted-Average
					Grant Date
					Fair Value
	Outstanding at January 1, 2013		236,139		$	29.76
	Granted		117,788		$	37.05
	Vested and issued		(54,034	)	$	29.8
	Expired or forfeited		(41,077	)	$	35.62
	Outstanding at September 30, 2013		258,816		$	32.71
Summary of Stock-Settled SARs Activity Under 2007 Plan	'
	A summary of stock-settled SARs activity under the 2007 Plan as of September 30, 2013, and changes during the nine months then ended is as follows:
		Shares			Weighted-Average
					Exercise Price
	Outstanding at January 1, 2013		1,261,448		$	27.04
	Granted		332,004		$	37.21
	Exercised		(167,693	)	$	26.69
	Expired or forfeited		(124,711	)	$	29.27
	Outstanding at September 30, 2013		1,301,048		$	29.46
Summary of Cash-Settled SARs Activity Under 2007 Plan	'
	A summary of cash-settled SARs activity under the 2007 Plan as of September 30, 2013, and changes during the nine months then ended is as follows:
		Shares			Weighted-Average
					Exercise Price
	Outstanding at January 1, 2013		1,886,124		$	29.75
	Granted		556,598		$	39.67
	Exercised		(451,792	)	$	29.46
	Expired or forfeited		(175,284	)	$	29.85
	Outstanding at September 30, 2013		1,815,646		$	32.85
Amended Plan [Member]	'
Summary of Stock Option Activity	'
	A summary of option activity under the Amended Plan as of September 30, 2013, and changes during the nine months then ended is as follows:
		Shares			Weighted-Average
					Exercise Price
	Outstanding at January 1, 2013		210,097		$	33.13
	Granted		0		$	0
	Exercised		(115,555	)	$	27.75
	Expired or forfeited		(41,814	)	$	47.71
	Outstanding at September 30, 2013		52,728		$	33.37

Reserves_for_Losses_and_Loss_A1

Reserves for Losses and Loss Adjustment Expenses (Tables)	9 Months Ended
	Sep. 30, 2013
Insurance [Abstract]	'
Reinsurance Recoverables	'
	The following table provides a reconciliation of reserves for losses and loss adjustment expenses (“LAE”), net of reinsurance, to the gross amounts reported in the Consolidated Balance Sheets. Reinsurance recoverables in this note exclude paid loss recoverables of $142.7 million and $106.5 million as of September 30, 2013 and 2012, respectively:
			For the Nine Months
			Ended September 30,
	(in millions)		2013			2012
	Net reserves beginning of the year		$	2,110.90		$	2,336.70
	Add:
	Net reserves from assumed retroactive insurance contract (1)			0			13
	Losses and LAE incurred during current calendar year, net of reinsurance:
	Current accident year			575.6			545.7
	Prior accident years			(21.5	)		(17.8	)
	Losses and LAE incurred during calendar year, net of reinsurance			554.1			527.9
	Deduct:
	Losses and LAE payments made during current calendar year, net of reinsurance:
	Current accident year			131.7			111.9
	Prior accident years			429			450.1
	Losses and LAE payments made during current calendar year, net of reinsurance:			560.7			562
	Change in participation interest (2)			10.4			(3.1	)
	Foreign exchange adjustments			(6.3	)		5.4
	Net reserves - end of period			2,108.40			2,317.90
	Add:
	Reinsurance recoverables on unpaid losses and LAE, end of period			1,114.50			910
	Gross reserves - end of period		$	3,222.90		$	3,227.90
	-1	Amount represents reserves assumed resulting from participation in Brazilian Motor Third-Party Liability Insurance Pool effective January 1, 2012.
	-2	Amount represents (decrease) increase in reserves due to change in syndicate participation.

Underwriting_Acquisition_and_I1

Underwriting, Acquisition and Insurance Expenses (Tables)	9 Months Ended
	Sep. 30, 2013
Text Block [Abstract]	'
Underwriting, Acquisition and Insurance Expenses	'
	Underwriting, acquisition and insurance expenses for the three and nine months ended September 30, 2013 and 2012 were as follows:
		For the Three Months						For the Nine Months
		Ended September 30,						Ended September 30,
	(in millions)	2013			2012			2013			2012
	Commissions	$	57.2		$	55.3		$	174.7		$	156
	General expenses		65.4			60.2			200.8			182.2
	Premium taxes, boards and bureaus		6.8			7.1			19.7			19.2
			129.4			122.6			395.2			357.4
	Net deferral of policy acquisition costs		(2.7	)		(4.6	)		(17.2	)		(11.1	)
	Total underwriting, acquisition and insurance expenses	$	126.7		$	118		$	378		$	346.3

Segment_Information_Tables

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2013
Segment Reporting [Abstract]	'
Revenue and (Loss) Income Before Income Taxes for Each Segment	'
	Revenue and income (loss) before income taxes for each segment for the three and nine months ended September 30, 2013 and 2012 were as follows:
		For the Three Months						For the Nine Months
		Ended September 30,						Ended September 30,
	(in millions)	2013			2012			2013			2012
	Revenue:
	Earned premiums
	Excess and Surplus Lines	$	120.6		$	100.8		$	334.4		$	295.5
	Commercial Specialty		74.6			78.6			225.1			241.8
	International Specialty		37.5			33.4			106.6			90.8
	Syndicate 1200		97.3			90.8			295.6			242.3
	Run-off Lines		0.5			0.7			0.5			1.4
	Total earned premiums		330.5			304.3			962.2			871.8
	Net investment income
	Excess and Surplus Lines		10.2			12.5			32.3			38.5
	Commercial Specialty		5.6			6.7			17.6			20.6
	International Specialty		2.4			3.1			6.4			10
	Syndicate 1200		2.5			3.8			8.1			11.4
	Run-off Lines		2.6			2.8			8.2			9.8
	Corporate and Other		0.8			0			4.7			0
	Total net investment income		24.1			28.9			77.3			90.3
	Fee income, net		0.3			3			0.5			4.8
	Net realized investment gains		9.1			8.3			29.6			18.7
	Total revenue	$	364		$	344.5		$	1,069.60		$	985.6
		For the Three Months						For the Nine Months
		Ended September 30,						Ended September 30,
	(in millions)	2013			2012			2013			2012
	Income (loss) before income taxes
	Excess and Surplus Lines	$	29.6		$	15.1		$	60.2		$	56.3
	Commercial Specialty		10.1			(2.7	)		17.3			(11.8	)
	International Specialty		0.5			6.6			8.1			22
	Syndicate 1200		8.5			11.8			26.7			18
	Run-off Lines		(9.1	)		(2.5	)		(5.8	)		(1.5	)
	Total segment income before taxes		39.6			28.3			106.5			83
	Corporate and Other		(15.8	)		(18.9	)		(22.9	)		(30.9	)
	Net realized investment gains		9.1			8.3			29.6			18.7
	Total income before income taxes	$	32.9		$	17.7		$	113.2		$	70.8
Schedule of Earned Premiums by Geographic Location	'
	The table below presents earned premiums by geographic location for the three and nine months ended September 30, 2013 and 2012. For this disclosure, we determine geographic location by the country of domicile of our subsidiaries that write the business and not by the location of insureds or reinsureds from whom the business was generated.
		For the Three Months						For the Nine Months
		Ended September 30,						Ended September 30,
	(in millions)	2013			2012			2013			2012
	Bermuda	$	27.4		$	32.6		$	75.3		$	83.5
	Brazil		9.9			6.7			29.9			18.4
	Malta		0.5			0			1.3			0.1
	United Kingdom		97.7			84.9			296			231.1
	United States		195			180.1			559.7			538.7
	Total earned premiums	$	330.5		$	304.3		$	962.2		$	871.8
Identifiable Assets	'
	The following table represents identifiable assets as of September 30, 2013 and December 31, 2012:
	(in millions)	September 30,			December 31,
		2013			2012
	Excess and Surplus Lines	$	2,248.60		$	2,206.70
	Commercial Specialty		1,316.50			1,337.70
	International Specialty		785.9			731.8
	Syndicate 1200		1,670.50			1,811.40
	Run-off Lines		558			516.5
	Corporate and Other		71.1			84.8
	Total	$	6,650.60		$	6,688.90

Supplemental_Cash_Flow_Informa1

Supplemental Cash Flow Information (Tables)	9 Months Ended
	Sep. 30, 2013
Supplemental Cash Flow Elements [Abstract]	'
Schedule of Interest Paid	'
	Interest paid. Interest paid for the nine months ended September 30, was as follows:
		For the Nine Months
		Ended September 30,
	(in millions)	2013		2012
	Senior unsecured fixed rate notes	$	7	$	0
	Junior subordinated debentures		6		16.3
	Other indebtedness		0.7		2.5
	Revolving credit facility		0		0
	Total interest paid	$	13.7	$	18.8

Disclosures_about_Fair_Value_o1

Disclosures about Fair Value of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2013
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 at September 30, 2013 and December 31, 2012 is shown below:
		September 30, 2013				December 31, 2012
	(in millions)	Carrying		Fair Value		Carrying		Fair
		Amount				Amount		Value
	Junior subordinated debentures	$	193.3	$	155.2	$	193.3	$	151.8
	Senior unsecured fixed rate notes		143.8		115.2		143.8		143.2
	Other indebtedness:
	Floating rate loan stock		64.1		51.4		63		49.5
	Note payable		0.8		0.6		0.8		0.6
		$	402	$	322.4	$	400.9	$	345.1

Information_Provided_in_Connec1

Information Provided in Connection with Outstanding Debt of Subsidiaries (Tables)	9 Months Ended
	Sep. 30, 2013
Text Block [Abstract]	'
Condensed Consolidating Balance Sheet	'
	CONDENSED CONSOLIDATING BALANCE SHEET
	September 30, 2013
	(in millions)
			Argo Group			Argo Group US, Inc.			Other Subsidiaries			Consolidating			Total
			International			and Subsidiaries			and Eliminations (1)			Adjustments (2)
			Holdings, Ltd (Parent			(Subsidiary Issuer)
			Guarantor)
	Assets
	Investments		$	3.4		$	2,750.20		$	1,296.60		$	0		$	4,050.20
	Cash			0			117.5			24.1			0			141.6
	Accrued investment income			0			19.5			5.8			0			25.3
	Premiums receivable			0			150.3			276.1			0			426.4
	Reinsurance recoverables			0			1,165.40			91.8			0			1,257.20
	Goodwill and other intangible assets			0			137.9			103.2			0			241.1
	Current income taxes receivable, net			0			10.1			5.2			0			15.3
	Deferred acquisition costs, net			0			55.5			59.8			0			115.3
	Ceded unearned premiums			0			99.4			131.6			0			231
	Other assets			4.2			93.5			49.5			0			147.2
	Due from affiliates			0			0.2			(0.2	)		0			0
	Intercompany note receivable			0			102.7			(102.7	)		0			0
	Investments in subsidiaries			1,662.10			0			0			(1,662.1	)		0
	Total assets		$	1,669.70		$	4,702.20		$	1,940.80		$	(1,662.1	)	$	6,650.60
	Liabilities and Shareholders’ Equity
	Reserves for losses and loss adjustment expenses		$	0		$	2,178.70			1,044.20		$	0		$	3,222.90
	Unearned premiums			0			448.8			398.2			0			847
	Funds held and ceded reinsurance payable, net			0			638			(181.4	)		0			456.6
	Long-term debt			49			288.9			64.1			0			402
	Current income taxes payable, net			0			0			0			0			0
	Deferred tax liabilities, net			0			23.1			7.6			0			30.7
	Accrued underwriting expenses and other liabilities			14.2			98			51.8			0			164
	Due to affiliates			(1.5	)		0			0			1.5			0
	Intercompany note payable			80.6			0			0			(80.6	)		0
	Total liabilities			142.3			3,675.50			1,384.50			(79.1	)		5,123.20
	Total shareholders’ equity			1,527.40			1,026.80			556.2			(1,583.0	)		1,527.40
	Total liabilities and shareholders’ equity		$	1,669.70		$	4,702.30		$	1,940.70		$	(1,662.1	)	$	6,650.60
	(1)	Includes all other subsidiaries of Argo Group International Holdings, Ltd and all intercompany eliminations
	(2)	Includes all Argo Group parent company eliminations
	CONDENSED CONSOLIDATING BALANCE SHEET
	December 31, 2012
	(in millions)
			Argo Group			Argo Group US, Inc.			Other Subsidiaries			Consolidating			Total
			International			and Subsidiaries			and Eliminations (1)			Adjustments (2)
			Holdings, Ltd (Parent			(Subsidiary Issuer)
			Guarantor)
	Assets
	Investments		$	(0.9	)	$	2,794.70		$	1,406.90		$	0		$	4,200.70
	Cash			0			73.9			21.9			0			95.8
	Accrued investment income			0			21.3			9			0			30.3
	Premiums receivable			0			155.1			205.9			0			361
	Reinsurance recoverables			0			1,155.10			165.8			0			1,320.90
	Goodwill and other intangible assets			0			139.5			105.8			0			245.3
	Current income taxes receivable, net			0			8.7			4.2			0			12.9
	Deferred acquisition costs, net			0			54.6			44.8			0			99.4
	Ceded unearned premiums			0			85.9			107.7			0			193.6
	Other assets			4.3			82.7			42			0			129
	Due from affiliates			5			1.2			(1.2	)		(5.0	)		0
	Intercompany note receivable			0			119.6			(119.6	)		0			0
	Investments in subsidiaries			1,621.10			0			0			(1,621.1	)		0
	Total assets		$	1,629.50		$	4,692.30		$	1,993.20		$	(1,626.1	)	$	6,688.90
	Liabilities and Shareholders’ Equity
	Reserves for losses and loss adjustment expenses		$	0		$	2,226.30		$	997.2		$	0		$	3,223.50
	Unearned premiums			0			415.6			314.6			0			730.2
	Funds held and ceded reinsurance payable, net			0			598.4			47.4			0			645.8
Long-term debt			49			288.9			63			0			400.9
Current income taxes payable, net			0			0			0			0			0
Deferred tax liabilities, net			0			36.8			7			0			43.8
Accrued underwriting expenses and other liabilities			11.3			84.1			35.2			0			130.6
Due to affiliates			0			0			0			0			0
Intercompany note payable			55.1			0			0			(55.1	)		0
Total liabilities			115.4			3,650.10			1,464.40			(55.1	)		5,174.80
Total shareholders’ equity			1,514.10			1,042.20			528.8			(1,571.0	)		1,514.10
Total liabilities and shareholders’ equity		$	1,629.50		$	4,692.30		$	1,993.20		$	(1,626.1	)	$	6,688.90
(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 (Loss) Income	'
	CONDENSED CONSOLIDATING STATEMENT OF INCOME
	FOR THE THREE MONTHS ENDED SEPTEMBER 30, 2013
	(in millions)
			Argo Group International			Argo Group US, Inc. and			Other Subsidiaries			Consolidating			Total
			Holdings, Ltd (Parent			Subsidiaries			and Eliminations (1)			Adjustments (2)
			Guarantor)			(Subsidiary Issuer)
	Premiums and other revenue:
	Earned premiums		$	0		$	105.2		$	225.3		$	0		$	330.5
	Net investment income			0			16.7			7.7			(0.3	)		24.1
	Fee income, net			0			(0.6	)		0.9			0			0.3
	Net realized investment gains			0			7.2			1.9			0			9.1
	Total revenue			0			128.5			235.8			(0.3	)		364
	Expenses:
	Losses and loss adjustment expenses			0			65.9			125			0			190.9
	Other reinsurance-related expenses			0			(2.9	)		7.5			0			4.6
	Underwriting, acquisition and insurance expenses			5.8			42.1			78.8			0			126.7
	Interest expense			0.7			3.8			0.9			(0.3	)		5.1
	Foreign currency exchange (gain) loss			0			(0.1	)		3.9			0			3.8
	Total expenses			6.5			108.8			216.1			(0.3	)		331.1
	Income (loss) before income taxes			(6.5	)		19.7			19.7			0			32.9
	Provision for income taxes			0			4.4			(2.5	)		0			1.9
	Net (loss) income before equity in earnings of subsidiaries			(6.5	)		15.3			22.2			0			31
	Equity in undistributed earnings of subsidiaries			37.5			0			0			(37.5	)		0
	Net income		$	31		$	15.3		$	22.2		$	(37.5	)	$	31
	-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 (LOSS) INCOME
	FOR THE THREE MONTHS ENDED SEPTEMBER 30, 2012
	(in millions)
			Argo Group International			Argo Group US, Inc. and			Other Subsidiaries			Consolidating			Total
			Holdings, Ltd (Parent			Subsidiaries			and Eliminations (1)			Adjustments (2)
			Guarantor)			(Subsidiary Issuer)
	Premiums and other revenue:
	Earned premiums		$	0		$	108.1		$	196.2		$	0		$	304.3
	Net investment income			0			21.2			7.8			(0.1	)		28.9
	Fee income, net			0			2.4			0.6			0			3
	Net realized investment gains (losses)			(0.5	)		4.2			4.6			0			8.3
	Total revenue			(0.5	)		135.9			209.2			(0.1	)		344.5
	Expenses:
	Losses and loss adjustment expenses			0			78			108.3			0			186.3
	Other reinsurance-related expenses			0			0			7			0			7
	Underwriting, acquisition and insurance expenses			10.1			41.8			66.1			0			118
	Interest expense			0.1			4.1			1.7			(0.1	)		5.8
	Foreign currency exchange (gain) loss			0			(0.1	)		9.8			0			9.7
	Total expenses			10.2			123.8			192.9			(0.1	)		326.8
	Income (loss) before income taxes			(10.7	)		12.1			16.3			0			17.7
	Provision for income taxes			0			2.5			1.8			0			4.3
	Net income (loss) before equity in earnings of subsidiaries			(10.7	)		9.6			14.5			0			13.4
	Equity in undistributed earnings of subsidiaries			24.1			0			0			(24.1	)		0
	Net income		$	13.4		$	9.6		$	14.5		$	(24.1	)	$	13.4
	-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 NINE MONTHS ENDED SEPTEMBER 30, 2013
	(in millions)
			Argo Group International			Argo Group US, Inc. and			Other Subsidiaries			Consolidating			Total
			Holdings, Ltd (Parent			Subsidiaries			and Eliminations (1)			Adjustments (2)
			Guarantor)			(Subsidiary Issuer)
	Premiums and other revenue:
	Earned premiums		$	0		$	321.2		$	641		$	0		$	962.2
	Net investment income			0			57.2			21			(0.9	)		77.3
	Fee income, net			0			(1.9	)		2.4			0			0.5
	Net realized investment gains (losses)			0			32.8			(3.2	)		0			29.6
Total revenue			0			409.3			661.2			(0.9	)		1,069.60
Expenses:
Losses and loss adjustment expenses			0			194.9			359.2			0			554.1
Other reinsurance-related expenses			0			0			14.4			0			14.4
Underwriting, acquisition and insurance expenses			18.3			134.3			225.4			0			378
Interest expense			2.3			11.4			2.3			(0.9	)		15.1
Foreign currency exchange (gain) loss			0			0.1			(5.3	)		0			(5.2	)
Total expenses			20.6			340.7			596			(0.9	)		956.4
(Loss) income before income taxes			(20.6	)		68.6			65.2			0			113.2
Provision for income taxes			0			16			1.8			0			17.8
Net (loss) income before equity in earnings of subsidiaries			(20.6	)		52.6			63.4			0			95.4
Equity in undistributed earnings of subsidiaries			116			0			0			(116.0	)		0
Net income		$	95.4		$	52.6		$	63.4		$	(116.0	)	$	95.4
-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 (LOSS) INCOME
FOR THE NINE MONTHS ENDED SEPTEMBER 30, 2012
(in millions)
		Argo Group International			Argo Group US, Inc. and			Other Subsidiaries			Consolidating			Total
		Holdings, Ltd (Parent			Subsidiaries			and Eliminations (1)			Adjustments (2)
		Guarantor)			(Subsidiary Issuer)
Premiums and other revenue:
Earned premiums		$	0		$	327		$	544.8		$	0		$	871.8
Net investment income			0.1			65.3			25.1			(0.2	)		90.3
Fee income, net			0			2.4			2.4			0			4.8
Net realized investment gains (losses)			(0.5	)		11.8			7.4			0			18.7
Total revenue			(0.4	)		406.5			579.7			(0.2	)		985.6
Expenses:
Losses and loss adjustment expenses			0			220.9			307			0			527.9
Other reinsurance-related expenses			0			0			20.8			0			20.8
Underwriting, acquisition and insurance expenses			21.4			127.9			197			0			346.3
Interest expense			2.9			11.8			2.5			(0.2	)		17
Foreign currency exchange loss			0			0.1			2.7			0			2.8
Total expenses			24.3			360.7			530			(0.2	)		914.8
(Loss) income before income taxes			(24.7	)		45.8			49.7			0			70.8
Provision for income taxes			0			10.4			3.4			0			13.8
Net (loss) income before equity in earnings of subsidiaries			(24.7	)		35.4			46.3			0			57
Equity in undistributed earnings of subsidiaries			81.7			0			0			(81.7	)		0
Net income		$	57		$	35.4		$	46.3		$	(81.7	)	$	57
-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 NINE MONTHS ENDED SEPTEMBER 30, 2013
	(in millions)
			Argo Group			Argo Group US, Inc. and			Other Subsidiaries			Consolidating			Total
			International Holdings,			Subsidiaries			and Eliminations (1)			Adjustments (2)
			Ltd (Parent Guarantor)			(Subsidiary Issuer)
	Net cash flows from operating activities		$	(15.5	)	$	75.9			(63.4	)	$	0		$	(3.0	)
	Cash flows from investing activities:
	Proceeds from sales of investments			0			863.2			515.6			0			1,378.80
	Proceeds from maturities and mandatory calls of investments			0			139			85.2			0			224.2
	Purchases of investments			0			(942.4	)		(464.9	)		0			(1,407.3	)
	Change in short-term investments and foreign regulatory deposits			(0.8	)		(38.5	)		(36.2	)		0			(75.5	)
	Settlements of foreign currency exchange forward contracts			1.9			0			(8.3	)		0			(6.4	)
	Issuance of intercompany note, net			0			0			(24.9	)		24.9			0
	Purchase of fixed assets			0			(17.7	)		(1.4	)		0			(19.1	)
	Other, net			(0.1	)		4.4			0.2			0			4.5
	Cash provided by investing activities			1			8			65.3			24.9			99.2
	Cash flows from financing activities:
	Borrowings under intercompany note, net			24.9			0			0			(24.9	)		0
	Proceeds from issuance of senior unsecured fixed rate notes			0			0			0			0			0
	Activity under stock incentive plans			1.4			0			0			0			1.4
	Repurchase of Company’s common shares			0			(40.1	)		0			0			(40.1	)
	Excess tax expense from share-based payment arrangements			0			(0.2	)		0.4			0			0.2
	Payment of cash dividends to common shareholders			(11.8	)		0			0			0			(11.8	)
	Cash provided (used) by financing activities			14.5			(40.3	)		0.4			(24.9	)		(50.3	)
	Effect of exchange rate changes on cash			0			0			(0.1	)		0			(0.1	)
	Change in cash			(0.0	)		43.6			2.2			0			45.8
	Cash, beginning of period			0			73.9			21.9			0			95.8
	Cash, end of period		$	(0.0	)	$	117.5		$	24.1		$	0		$	141.6
	(1)	Includes all other subsidiaries of Argo Group International Holdings, Ltd and all intercompany eliminations
	(2)	Includes all Argo Group parent company eliminations
	CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS
	FOR THE NINE MONTHS ENDED SEPTEMBER 30, 2012
	(in millions)
			Argo Group			Argo Group US,			Other Subsidiaries			Consolidating			Total
			International Holdings,			Inc. and			and Eliminations (1)			Adjustments (2)
			Ltd (Parent Guarantor)			Subsidiaries
						(Subsidiary Issuer)
	Net cash flows from operating activities		$	(17.9	)	$	13.7		$	67.7		$	0		$	63.5
	Cash flows from investing activities:
	Proceeds from sales of investments			0			458.9			439.6			0			898.5
	Proceeds from maturities and mandatory calls of investments			0			253.9			111			0			364.9
	Purchases of investments			0			(712.1	)		(539.3	)		0			(1,251.4	)
	Change in short-term investments and foreign regulatory deposits			(0.1	)		(64.3	)		(34.6	)		0			(99.0	)
	Settlements of foreign currency exchange forward contracts			(0.6	)		0			0			0			(0.6	)
	Acquisitions			0			(1.4	)		0			0			(1.4	)
	Issuance of intercompany note, net			0			(10.0	)		(17.3	)		27.3			0
	Purchase of fixed assets			0			(23.6	)		(3.3	)		0			(26.9	)
	Other, net			0			2			0.7			0			2.7
	Cash provided (used) by investing activities			(0.7	)		(96.6	)		(43.2	)		27.3			(113.2	)
	Cash flows from financing activities:
	Borrowings under intercompany note, net			27.3			25			(25.0	)		(27.3	)		0
	Proceeds from issuance of senior unsecured fixed rate notes			0			139.2			0			0			139.2
	Activity under stock incentive plans			0.7			0			0			0			0.7
	Repurchase of Company’s common shares			0			(38.6	)		0			0			(38.6	)
	Excess tax expense from share-based payment arrangements			0			0			0			0			0
	Payment of cash dividends to common shareholders			(9.3	)		0			0			0			(9.3	)
	Cash (used) provided by financing activities			18.7			125.6			(25.0	)		(27.3	)		92
	Effect of exchange rate changes on cash			0			0			(0.5	)		0			(0.5	)
	Change in cash			0.1			42.7			(1.0	)		0			41.8
	Cash, beginning of period			0			85.1			15.8			0			100.9
	Cash, end of period		$	0.1		$	127.8		$	14.8		$	0		$	142.7
	(1)	Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations
(2)	Includes all Argo Group parent company eliminations

Recently_Issued_Accounting_Sta2

Recently Issued Accounting Standards - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2013
Accounting Policies [Abstract]	'
Intangible assets more likely-than-not threshold	50.00%

Investments_Schedule_of_Amorti

Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	$3,797.60	$3,895.70
Gross Unrealized Gains	289.4	325.1
Gross Unrealized Losses	36.8	20.1
Fair Value	4,050.20	4,200.70
Fixed maturities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	2,757.90	2,993.10
Gross Unrealized Gains	87.7	170.8
Gross Unrealized Losses	33.7	9.9
Fair Value	2,811.90	3,154
Fixed maturities [Member] | U.S. Governments [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	275	401.1
Gross Unrealized Gains	3	9.3
Gross Unrealized Losses	0.4	0
Fair Value	277.6	410.4
Fixed maturities [Member] | Non-U.S. Governments [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	76.2	53.1
Gross Unrealized Gains	0.4	3.6
Gross Unrealized Losses	2.4	0.1
Fair Value	74.2	56.6
Fixed maturities [Member] | Obligations of states and political subdivisions [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	597.5	551.7
Gross Unrealized Gains	24.3	44.9
Gross Unrealized Losses	13.5	0.5
Fair Value	608.3	596.1
Fixed maturities [Member] | Credit-Financial [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	385.8	382.4
Gross Unrealized Gains	15.3	25.7
Gross Unrealized Losses	2.3	0.7
Fair Value	398.8	407.4
Fixed maturities [Member] | Credit-Industrial [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	383.4	428.4
Gross Unrealized Gains	15.5	31.1
Gross Unrealized Losses	3.8	0.7
Fair Value	395.1	458.8
Fixed maturities [Member] | Credit-Utility [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	173.6	189.9
Gross Unrealized Gains	5.9	12.6
Gross Unrealized Losses	1.4	0.4
Fair Value	178.1	202.1
Fixed maturities [Member] | CMO/MBS-agency [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	233.4	375.1
Gross Unrealized Gains	10.1	20.2
Gross Unrealized Losses	1.5	0.2
Fair Value	242	395.1
Fixed maturities [Member] | CMO/MBS-non agency [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	20.3	12.5
Gross Unrealized Gains	0.8	1
Gross Unrealized Losses	0	0.2
Fair Value	21.1	13.3
Fixed maturities [Member] | CMBS [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	161.5	109.5
Gross Unrealized Gains	3.2	5.7
Gross Unrealized Losses	1.1	0.2
Fair Value	163.6	115
Fixed maturities [Member] | ABS-residential [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	7.6	10.1
Gross Unrealized Gains	0.3	0.2
Gross Unrealized Losses	0.5	0.8
Fair Value	7.4	9.5
Fixed maturities [Member] | ABS-non residential [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	100.8	81.3
Gross Unrealized Gains	0.6	1.2
Gross Unrealized Losses	0.1	0
Fair Value	101.3	82.5
Fixed maturities [Member] | Foreign denominated: Governments [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	227.5	262.1
Gross Unrealized Gains	5.2	9.2
Gross Unrealized Losses	4	4.3
Fair Value	228.7	267
Fixed maturities [Member] | Foreign Denominated: Credit [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	115.3	135.9
Gross Unrealized Gains	3.1	6.1
Gross Unrealized Losses	2.7	1.8
Fair Value	115.7	140.2
Equity securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	386.5	373.5
Gross Unrealized Gains	197.4	153
Gross Unrealized Losses	3.1	5.1
Fair Value	580.8	521.4
Other investments [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	366.9	294.8
Gross Unrealized Gains	4.3	1.2
Gross Unrealized Losses	0	5
Fair Value	371.2	291
Short-term investments [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	286.3	234.3
Gross Unrealized Gains	0	0.1
Gross Unrealized Losses	0	0.1
Fair Value	$286.30	$234.30

Investments_Additional_Informa

Investments - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Dec. 28, 2011	Jun. 18, 2011
	Security		D
			Security
Schedule of Investments [Line Items]	'	'	'	'	'	'	'
Fair value of investments, assets managed on behalf of the trade capital providers	$89.20	'	$89.20	'	$143.90	'	'
Number of securities in an unrealized loss position, total	6,362	'	6,362	'	'	'	'
Number of securities in an unrealized loss position for less than one year	'	'	1,758	'	'	'	'
Number of securities in an unrealized loss position for a period of one year or greater	'	'	25	'	'	'	'
Other-than-temporary impairment losses on fixed maturities	0	1.1	0.3	1.4	'	'	'
Other-than-temporarily losses equity portfolio	0.1	0	1.6	0.6	'	'	'
Other-than-temporary impairment losses on other investments	0	0	3.4	0	'	'	'
Forward contracts, term, days	'	'	60	'	'	'	'
Total notional amount of the Company's holdings in non-U.S. Dollar denominated higher yielding fixed maturity investments	2	2.4	2	2.4	'	'	'
Notional amount of the currency forward contracts	'	'	'	'	'	100	100
Fair value of put option	1.6	2.5	1.6	2.5	'	'	'
Realized gains from currency forward contracts	2.4	2.5	2.4	2.5	'	'	'
Realized losses from currency forward contracts	2.2	1.1	2.2	1.1	'	'	'
Amortized Cost	3,797.60	'	3,797.60	'	3,895.70	'	'
Fair Value	4,050.20	'	4,050.20	'	4,200.70	'	'
Loss and loss expense reserve	63.3	'	63.3	'	'	'	'
Underwriting business at Lloyd's	124.9	'	124.9	'	'	'	'
Currency forward contracts [Member]	'	'	'	'	'	'	'
Schedule of Investments [Line Items]	'	'	'	'	'	'	'
Notional amount of the currency forward contracts	53.2	104.9	53.2	104.9	'	'	'
Deposit with Various U.S. Insurance and Other Regulatory Agencies [Member]	'	'	'	'	'	'	'
Schedule of Investments [Line Items]	'	'	'	'	'	'	'
Amortized Cost	202.4	'	202.4	'	'	'	'
Fair Value	210.8	'	210.8	'	'	'	'
Securities Pledged As Collateral [Member]	'	'	'	'	'	'	'
Schedule of Investments [Line Items]	'	'	'	'	'	'	'
Pledged assets separately reported, securities pledged under letters of credit facilities, at amortized cost	230.4	'	230.4	'	'	'	'
Pledged assets separately reported, securities pledged under letters of credit facilities, at fair value	232.8	'	232.8	'	'	'	'
Non-U.S. Denominated Fixed Maturity Investments [Member]	'	'	'	'	'	'	'
Schedule of Investments [Line Items]	'	'	'	'	'	'	'
Total notional amount of the Company's holdings in non-U.S. Dollar denominated higher yielding fixed maturity investments	$1.80	$2.30	$1.80	$2.30	'	'	'

Investments_Schedule_of_Amorti1

Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) (USD $)	Sep. 30, 2013
In Millions, unless otherwise specified
Investments Debt And Equity Securities [Abstract]	'
Amortized Cost, Due in one year or less	$203.30
Amortized Cost, Due after one year through five years	1,170
Amortized Cost, Due after five years through ten years	607.8
Amortized Cost, Thereafter	253.2
Amortized Cost, Structured securities	523.6
Amortized Cost, Total	2,757.90
Fair Value, Due in one year or less	205.3
Fair Value, Due after one year through five years	1,196.30
Fair Value, Due after five years through ten years	629.6
Fair Value, Thereafter	245.3
Fair Value, Structured securities	535.4
Fair Value, Total	$2,811.90

Investments_Schedule_of_Aging_

Investments - Schedule of Aging of Unrealized Losses on Investments (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	$981.30	$490.30
Unrealized Losses, Less Than One Year	36.1	17
Fair Value, One Year or Greater	7.9	28.2
Unrealized Losses, One Year or Greater	0.7	3.1
Fair Value, Total	989.2	518.5
Unrealized Losses, Total	36.8	20.1
Fixed maturities [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	924.8	418.5
Unrealized Losses, Less Than One Year	33	7.7
Fair Value, One Year or Greater	7.9	21.9
Unrealized Losses, One Year or Greater	0.7	2.2
Fair Value, Total	932.7	440.4
Unrealized Losses, Total	33.7	9.9
Fixed maturities [Member] | U.S. Governments [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	36.8	11.9
Unrealized Losses, Less Than One Year	0.4	0
Fair Value, One Year or Greater	0	0
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	36.8	11.9
Unrealized Losses, Total	0.4	0
Fixed maturities [Member] | Non-US Governments [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	51.3	8.5
Unrealized Losses, Less Than One Year	2.4	0.1
Fair Value, One Year or Greater	0	0.5
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	51.3	9
Unrealized Losses, Total	2.4	0.1
Fixed maturities [Member] | Obligations of states and political subdivisions [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	216.9	28.2
Unrealized Losses, Less Than One Year	13.5	0.3
Fair Value, One Year or Greater	0	0.8
Unrealized Losses, One Year or Greater	0	0.2
Fair Value, Total	216.9	29
Unrealized Losses, Total	13.5	0.5
Fixed maturities [Member] | Credit-Financial [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	82.6	25
Unrealized Losses, Less Than One Year	2.3	0.2
Fair Value, One Year or Greater	1.3	8.6
Unrealized Losses, One Year or Greater	0	0.5
Fair Value, Total	83.9	33.6
Unrealized Losses, Total	2.3	0.7
Fixed maturities [Member] | Credit-Industrial [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	95.9	32.2
Unrealized Losses, Less Than One Year	3.6	0.4
Fair Value, One Year or Greater	2.5	3
Unrealized Losses, One Year or Greater	0.2	0.3
Fair Value, Total	98.4	35.2
Unrealized Losses, Total	3.8	0.7
Fixed maturities [Member] | Credit-Utility [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	34.9	8.4
Unrealized Losses, Less Than One Year	1.4	0.2
Fair Value, One Year or Greater	0	0.8
Unrealized Losses, One Year or Greater	0	0.2
Fair Value, Total	34.9	9.2
Unrealized Losses, Total	1.4	0.4
Fixed maturities [Member] | CMO/MBS-agency [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	44.6	26.4
Unrealized Losses, Less Than One Year	1.5	0.2
Fair Value, One Year or Greater	0.2	0.3
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	44.8	26.7
Unrealized Losses, Total	1.5	0.2
Fixed maturities [Member] | CMO/MBS-non agency [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	'	1.7
Unrealized Losses, Less Than One Year	'	0.2
Fair Value, One Year or Greater	'	0
Unrealized Losses, One Year or Greater	'	0
Fair Value, Total	'	1.7
Unrealized Losses, Total	'	0.2
Fixed maturities [Member] | CMBS [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	45.5	5.7
Unrealized Losses, Less Than One Year	1.1	0
Fair Value, One Year or Greater	0	3.5
Unrealized Losses, One Year or Greater	0	0.2
Fair Value, Total	45.5	9.2
Unrealized Losses, Total	1.1	0.2
Fixed maturities [Member] | ABS-residential [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	0	0
Unrealized Losses, Less Than One Year	0	0
Fair Value, One Year or Greater	3.9	3.9
Unrealized Losses, One Year or Greater	0.5	0.8
Fair Value, Total	3.9	3.9
Unrealized Losses, Total	0.5	0.8
Fixed maturities [Member] | ABS-non residential [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	60.2	13.7
Unrealized Losses, Less Than One Year	0.1	0
Fair Value, One Year or Greater	0	0.2
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	60.2	13.9
Unrealized Losses, Total	0.1	0
Fixed maturities [Member] | Foreign denominated: Governments [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	174.4	180.5
Unrealized Losses, Less Than One Year	4	4.3
Fair Value, One Year or Greater	0	0.3
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	174.4	180.8
Unrealized Losses, Total	4	4.3
Fixed maturities [Member] | Foreign Denominated: Credit [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	81.7	76.3
Unrealized Losses, Less Than One Year	2.7	1.8
Fair Value, One Year or Greater	0	0
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	81.7	76.3
Unrealized Losses, Total	2.7	1.8
Equity securities [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	54.3	60.2
Unrealized Losses, Less Than One Year	3.1	4.2
Fair Value, One Year or Greater	0	6.3
Unrealized Losses, One Year or Greater	0	0.9
Fair Value, Total	54.3	66.5
Unrealized Losses, Total	3.1	5.1
Other investments [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	0	11.6
Unrealized Losses, Less Than One Year	0	5
Fair Value, One Year or Greater	0	0
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	0	11.6
Unrealized Losses, Total	0	5
Short-term investments [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	2.2	0
Unrealized Losses, Less Than One Year	0	0.1
Fair Value, One Year or Greater	0	0
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	2.2	0
Unrealized Losses, Total	$0	$0.10

Investments_Schedule_of_Aging_1

Investments - Schedule of Aging of Unrealized Losses on Investments (Parenthetical) (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Investments Debt And Equity Securities [Abstract]	'	'
Unrealized losses less than one year	$0.10	$0.10
Unrealized losses one year or greater	$0.10	$0.10

Investments_Schedule_of_Compan

Investments - Schedule of Company's Gross Realized Investment Gains (Losses) (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Schedule of Investments [Line Items]	'	'	'	'
Gross realized investment and other gains	$24.30	$13.60	$71.50	$42.70
Gross realized investment and other losses	-15.2	-5.3	-41.9	-24
Net realized investment gains	9.1	8.3	29.6	18.7
Fixed maturities [Member]	'	'	'	'
Schedule of Investments [Line Items]	'	'	'	'
Gross realized investment and other gains	5.8	5.5	27.5	21
Gross realized investment and other losses	-6.1	-3	-14.5	-8.1
Equity securities [Member]	'	'	'	'
Schedule of Investments [Line Items]	'	'	'	'
Gross realized investment and other gains	7.2	0.2	15	0.4
Gross realized investment and other losses	-0.1	-0.1	-0.8	-0.4
Other investments [Member]	'	'	'	'
Schedule of Investments [Line Items]	'	'	'	'
Gross realized investment and other gains	11.2	7.7	28.9	20.8
Gross realized investment and other losses	-8.6	-1.1	-20.7	-13.2
Short-term investments [Member]	'	'	'	'
Schedule of Investments [Line Items]	'	'	'	'
Gross realized investment and other gains	0.1	0.2	0.1	0.5
Gross realized investment and other losses	-0.3	0	-0.6	-0.3
Other-than-temporary impairment losses on fixed maturities [Member]	'	'	'	'
Schedule of Investments [Line Items]	'	'	'	'
Gross realized investment and other losses	0	-1.1	-0.3	-1.4
Other-than-temporary impairment losses on equity securities [Member]	'	'	'	'
Schedule of Investments [Line Items]	'	'	'	'
Gross realized investment and other losses	-0.1	0	-1.6	-0.6
Other-than-temporary impairment losses on other investments [Member]	'	'	'	'
Schedule of Investments [Line Items]	'	'	'	'
Gross realized investment and other losses	$0	$0	($3.40)	$0

Investments_Securities_and_Let

Investments - Securities and Letters of Credit Pledged to Lloyd's as Company's Corporate Member's Capital (Detail) (USD $)	Sep. 30, 2013
In Millions, unless otherwise specified
Investments Debt And Equity Securities [Abstract]	'
Letters of credit	$124.90
Fixed maturities, at fair value	160.9
Short-term investments, at fair value	0.1
Total securities and letters of credit pledged to Lloyd's	$285.90

Investments_Financial_Assets_M

Investments - Financial Assets Measured at Fair Value on Recurring Basis (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	$4,050.20	$4,200.70
Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	3,796.10	4,048.60
Fixed maturities [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	2,811.90	3,154
Fixed maturities [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	2,811.90	3,154
Fixed maturities [Member] | U.S. Governments [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	277.6	410.4
Fixed maturities [Member] | U.S. Governments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	277.6	410.4
Fixed maturities [Member] | Non-U.S. Governments [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	74.2	56.6
Fixed maturities [Member] | Non-U.S. Governments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	74.2	56.6
Fixed maturities [Member] | Obligations of states and political subdivisions [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	608.3	596.1
Fixed maturities [Member] | Obligations of states and political subdivisions [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	608.3	596.1
Fixed maturities [Member] | Credit-Financial [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	398.8	407.4
Fixed maturities [Member] | Credit-Financial [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	398.8	407.4
Fixed maturities [Member] | Credit-Industrial [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	395.1	458.8
Fixed maturities [Member] | Credit-Industrial [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	395.1	458.8
Fixed maturities [Member] | Credit-Utility [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	178.1	202.1
Fixed maturities [Member] | Credit-Utility [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	178.1	202.1
Fixed maturities [Member] | CMO/MBS-agency [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	242	395.1
Fixed maturities [Member] | CMO/MBS-agency [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	242	395.1
Fixed maturities [Member] | CMO/MBS-non agency [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	21.1	13.3
Fixed maturities [Member] | CMO/MBS-non agency [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	21.1	13.3
Fixed maturities [Member] | CMBS [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	163.6	115
Fixed maturities [Member] | CMBS [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	163.6	115
Fixed maturities [Member] | ABS-residential [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	7.4	9.5
Fixed maturities [Member] | ABS-residential [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	7.4	9.5
Fixed maturities [Member] | ABS-non residential [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	101.3	82.5
Fixed maturities [Member] | ABS-non residential [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	101.3	82.5
Fixed maturities [Member] | Foreign denominated: Governments [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	228.7	267
Fixed maturities [Member] | Foreign denominated: Governments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	228.7	267
Fixed maturities [Member] | Foreign Denominated: Credit [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	115.7	140.2
Fixed maturities [Member] | Foreign Denominated: Credit [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	115.7	140.2
Equity securities [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	580.8	521.4
Equity securities [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	580.8	521.4
Other investments [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	371.2	291
Other investments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	114.3	132
Short-term investments [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	286.3	234.3
Short-term investments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	286.3	234.3
Other Assets [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	2.8	6.9
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	950.1	889.6
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Fixed maturities [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	142.8	219.5
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Fixed maturities [Member] | U.S. Governments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	142.8	219.5
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Fixed maturities [Member] | Non-U.S. Governments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Fixed maturities [Member] | Obligations of states and political subdivisions [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Fixed maturities [Member] | Credit-Financial [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Fixed maturities [Member] | Credit-Industrial [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Fixed maturities [Member] | Credit-Utility [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Fixed maturities [Member] | CMO/MBS-agency [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Fixed maturities [Member] | CMO/MBS-non agency [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Fixed maturities [Member] | CMBS [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Fixed maturities [Member] | ABS-residential [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Fixed maturities [Member] | ABS-non residential [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Fixed maturities [Member] | Foreign denominated: Governments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Fixed maturities [Member] | Foreign Denominated: Credit [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Equity securities [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	523.3	469
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Other investments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Short-term investments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	284	201.1
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Other Assets [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	2,841.90	3,150.30
Significant Other Observable Inputs (Level 2) [Member] | Fixed maturities [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	2,669.10	2,934.50
Significant Other Observable Inputs (Level 2) [Member] | Fixed maturities [Member] | U.S. Governments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	134.8	190.9
Significant Other Observable Inputs (Level 2) [Member] | Fixed maturities [Member] | Non-U.S. Governments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	74.2	56.6
Significant Other Observable Inputs (Level 2) [Member] | Fixed maturities [Member] | Obligations of states and political subdivisions [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	608.3	596.1
Significant Other Observable Inputs (Level 2) [Member] | Fixed maturities [Member] | Credit-Financial [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	398.8	407.4
Significant Other Observable Inputs (Level 2) [Member] | Fixed maturities [Member] | Credit-Industrial [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	395.1	458.8
Significant Other Observable Inputs (Level 2) [Member] | Fixed maturities [Member] | Credit-Utility [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	178.1	202.1
Significant Other Observable Inputs (Level 2) [Member] | Fixed maturities [Member] | CMO/MBS-agency [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	242	395.1
Significant Other Observable Inputs (Level 2) [Member] | Fixed maturities [Member] | CMO/MBS-non agency [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	21.1	13.3
Significant Other Observable Inputs (Level 2) [Member] | Fixed maturities [Member] | CMBS [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	163.6	115
Significant Other Observable Inputs (Level 2) [Member] | Fixed maturities [Member] | ABS-residential [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	7.4	9.5
Significant Other Observable Inputs (Level 2) [Member] | Fixed maturities [Member] | ABS-non residential [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	101.3	82.5
Significant Other Observable Inputs (Level 2) [Member] | Fixed maturities [Member] | Foreign denominated: Governments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	228.7	267
Significant Other Observable Inputs (Level 2) [Member] | Fixed maturities [Member] | Foreign Denominated: Credit [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	115.7	140.2
Significant Other Observable Inputs (Level 2) [Member] | Equity securities [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	56.2	50.6
Significant Other Observable Inputs (Level 2) [Member] | Other investments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	114.3	132
Significant Other Observable Inputs (Level 2) [Member] | Short-term investments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	2.3	33.2
Significant Other Observable Inputs (Level 2) [Member] | Other Assets [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	4.1	8.7
Significant Unobservable Inputs (Level 3) [Member] | Fixed maturities [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Significant Unobservable Inputs (Level 3) [Member] | Fixed maturities [Member] | U.S. Governments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Significant Unobservable Inputs (Level 3) [Member] | Fixed maturities [Member] | Non-U.S. Governments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Significant Unobservable Inputs (Level 3) [Member] | Fixed maturities [Member] | Obligations of states and political subdivisions [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Significant Unobservable Inputs (Level 3) [Member] | Fixed maturities [Member] | Credit-Financial [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Significant Unobservable Inputs (Level 3) [Member] | Fixed maturities [Member] | Credit-Industrial [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Significant Unobservable Inputs (Level 3) [Member] | Fixed maturities [Member] | Credit-Utility [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Significant Unobservable Inputs (Level 3) [Member] | Fixed maturities [Member] | CMO/MBS-agency [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Significant Unobservable Inputs (Level 3) [Member] | Fixed maturities [Member] | CMO/MBS-non agency [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Significant Unobservable Inputs (Level 3) [Member] | Fixed maturities [Member] | CMBS [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Significant Unobservable Inputs (Level 3) [Member] | Fixed maturities [Member] | ABS-residential [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Significant Unobservable Inputs (Level 3) [Member] | Fixed maturities [Member] | ABS-non residential [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Significant Unobservable Inputs (Level 3) [Member] | Fixed maturities [Member] | Foreign denominated: Governments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Significant Unobservable Inputs (Level 3) [Member] | Fixed maturities [Member] | Foreign Denominated: Credit [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Significant Unobservable Inputs (Level 3) [Member] | Equity securities [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	1.3	1.8
Significant Unobservable Inputs (Level 3) [Member] | Other investments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Significant Unobservable Inputs (Level 3) [Member] | Short-term investments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	0	0
Significant Unobservable Inputs (Level 3) [Member] | Other Assets [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	$2.80	$6.90

Investments_Schedule_of_Reconc

Investments - Schedule of Reconciliation of Beginning and Ending Balances for Investments Categorized as Level 3 (Detail) (USD $)	9 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]	'	'
Beginning balance	$8.70	$12.10
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Included in net income	0.1	0.1
Included in other comprehensive income	0	0
Purchases	0	0
Issuances	0	0
Sales	-0.6	-1.4
Settlements	-4.1	-2.1
Ending balance	4.1	8.7
Amount of total gains or losses for the period included in net income attributable to the change in unrealized gains or losses relating to assets still held at end of period	0	0
Equity securities [Member]	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]	'	'
Beginning balance	1.8	2.4
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Included in net income	0.1	0
Included in other comprehensive income	0	0
Purchases	0	0
Issuances	0	0
Sales	-0.6	-0.6
Settlements	0	0
Ending balance	1.3	1.8
Amount of total gains or losses for the period included in net income 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	6.9	9
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Included in net income	0	0
Included in other comprehensive income	0	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	-4.1	-2.1
Ending balance	2.8	6.9
Amount of total gains or losses for the period included in net income attributable to the change in unrealized gains or losses relating to assets still held at end of period	0	0
Credit Financial [Member]	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]	'	'
Beginning balance	'	0.7
Transfers into Level 3	'	0
Transfers out of Level 3	'	0
Included in net income	'	0.1
Included in other comprehensive income	'	0
Purchases	'	0
Issuances	'	0
Sales	'	-0.8
Settlements	'	0
Ending balance	'	0
Amount of total gains or losses for the period included in net income attributable to the change in unrealized gains or losses relating to assets still held at end of period	'	$0

Shareholders_Equity_Additional

Shareholders Equity - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended			1 Months Ended		9 Months Ended	1 Months Ended	3 Months Ended	9 Months Ended
In Millions, except Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	7-May-13	Feb. 18, 2011	Feb. 18, 2011	Sep. 30, 2013	Nov. 06, 2013	Sep. 30, 2013	Sep. 30, 2013
						2007 Repurchase Authorization [Member]	2011 Repurchase Authorization [Member]	2011 Repurchase Authorization [Member]	2013 Repurchase Authorization [Member]	Open Market [Member]	Open Market [Member]
									Subsequent Event [Member]
Equity [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Stock dividend declared	'	'	'	'	10.00%	'	'	'	'	'	'
Additional stock issued as dividend	'	'	2,447,839	'	'	'	'	'	'	'	'
Dividend payable, date of record	2-Sep-13	4-Sep-13	'	'	'	'	'	'	'	'	'
Dividend payable, Date	16-Sep-13	18-Sep-13	'	'	'	'	'	'	'	'	'
Dividend declared per common share	$0.15	$0.12	$0.45	$0.36	'	'	'	'	'	'	'
Cash dividends paid - common shares, total	$4	$3	$11.80	$9.30	'	'	'	'	'	'	'
Common stock repurchase authorization, maximum	'	'	'	'	'	150	150	'	150	'	'
Common shares repurchased	195,412	'	948,988	'	'	'	'	7,408,601	'	94,809	376,839
Common stock repurchased, average cost per share	'	'	'	'	'	'	'	$32.96	'	'	'
Common shares repurchased, total cost	8.6	'	38.7	'	'	'	'	244.2	'	4.2	15.8
Common stock repurchase authorization for future repurchases	'	'	'	'	'	'	'	$32.20	'	'	'

Shareholders_Equity_Schedule_o

Shareholders Equity - Schedule of Repurchase of Shares (Detail) (USD $)	3 Months Ended	9 Months Ended
In Millions, except Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013
Schedule Of Share Repurchase Programs [Line Items]	'	'
Number of Shares Repurchased	195,412	948,988
Total Cost	$8.60	$38.70
12/14/2012 [Member]	'	'
Schedule Of Share Repurchase Programs [Line Items]	'	'
Date Trading Plan Initiated	'	14-Dec-12
2013 Purchase Period	'	'01/01/13-01/17/13
Trading plans purchase period date range start	'	'01/01/13
Trading plans purchase period date range end	'	'01/17/13
Number of Shares Repurchased	'	171,480
Average Price of Shares Repurchased	'	$35.41
Total Cost	'	6
Repurchase Authorization Year	'	'2011
03/15/2013 [Member]	'	'
Schedule Of Share Repurchase Programs [Line Items]	'	'
Date Trading Plan Initiated	'	15-Mar-13
2013 Purchase Period	'	'03/16/13-05/01/13
Trading plans purchase period date range start	'	'03/16/13
Trading plans purchase period date range end	'	'05/01/13
Number of Shares Repurchased	'	205,452
Average Price of Shares Repurchased	'	$40.75
Total Cost	'	8.3
Repurchase Authorization Year	'	'2011
06/14/2013 [Member]	'	'
Schedule Of Share Repurchase Programs [Line Items]	'	'
Date Trading Plan Initiated	'	14-Jun-13
2013 Purchase Period	'	'06/18/13-07/31/13
Trading plans purchase period date range start	'	'06/18/13
Trading plans purchase period date range end	'	'07/31/13
Number of Shares Repurchased	'	195,217
Average Price of Shares Repurchased	'	$43.47
Total Cost	'	$8.50
Repurchase Authorization Year	'	'2011

Shareholders_Equity_Schedule_o1

Shareholders Equity - Schedule of Repurchase of Shares (Parenthetical) (Detail) (USD $)	3 Months Ended	9 Months Ended		12 Months Ended
In Millions, except Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Dec. 31, 2012
			Rule 10b5-1 [Member]	Rule 10b5-1 [Member]
Schedule Of Share Repurchase Programs [Line Items]	'	'	'	'
Number of Shares Repurchased	195,412	948,988	'	117,663
Total Cost	$8.60	$38.70	$9.90	$3.90
Total Number of Shares Repurchased	'	'	289,143	'
Total Average Price of Shares Repurchased	'	'	$34.58	'

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		9 Months Ended
In Millions, except Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share [Abstract]	'	'	'	'
Net income	$31	$13.40	$95.40	$57
Weighted average common shares outstanding - basic	26,682,176	27,751,140	26,961,792	28,265,456
Effect of dilutive securities	'	'	'	'
Equity compensation awards	1,137,374	582,656	1,031,543	447,668
Weighted average common shares outstanding - diluted	27,819,550	28,333,796	27,993,335	28,713,124
Net income per common share - basic	$1.16	$0.48	$3.54	$2.02
Net income per common share - diluted	$1.12	$0.47	$3.41	$1.98

Net_Income_Per_Common_Share_Ad

Net Income Per Common Share - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Earnings Per Share [Abstract]	'	'	'	'	'
Treasury shares	7,408,601	6,248,209	7,408,601	6,248,209	6,459,613
Antidilutive securities excluded from computation of diluted net income per share, shares	58,784	972,175	58,784	972,175	'

Accumulated_Other_Comprehensiv2

Accumulated Other Comprehensive Income - Changes in Accumulated Other Comprehensive Income (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Accumulated Other Comprehensive Income Loss Net Of Tax [Abstract]	'	'	'	'
Beginning balance, Foreign currency translation adjustments	'	'	($8.70)	'
Other comprehensive income before reclassifications, Foreign currency translation adjustments	'	'	-2.2	'
Amounts reclassified from accumulated other comprehensive income, Foreign currency translation adjustments	'	'	0	'
Net current-period other comprehensive income, Foreign currency translation adjustments	'	'	-2.2	'
Ending balance, Foreign currency translation adjustments	-10.9	'	-10.9	'
Beginning balance, Unrealized holding gains on securities	'	'	204.1	'
Other comprehensive income before reclassifications, Unrealized holding gains on securities	'	'	-23.9	'
Amounts reclassified from accumulated other comprehensive income, Unrealized holding gains on securities	-1.3	'	-11.6	'
Net current-period other comprehensive income, Unrealized holding gains on securities	'	'	-35.5	'
Ending balance, Unrealized holding gains on securities	168.6	'	168.6	'
Beginning balance, Defined benefit pension plans	'	'	-5.9	'
Other comprehensive income before reclassifications, Defined benefit pension plans	'	'	0	'
Amounts reclassified from accumulated other comprehensive income, Defined benefit pension plans	'	'	0	'
Net current-period other comprehensive income, Defined benefit pension plans	'	'	0	'
Ending balance, Defined benefit pension plans	-5.9	'	-5.9	'
Beginning balance	'	'	189.5	'
Other comprehensive income before reclassifications	'	'	-26.1	'
Amounts reclassified from accumulated other comprehensive income	'	'	-11.6	'
Other comprehensive income (loss), net of tax	16.2	46.6	-37.7	64.3
Ending balance	$151.80	'	$151.80	'

Accumulated_Other_Comprehensiv3

Accumulated Other Comprehensive Income - Amounts Reclassified from Accumulated Other Comprehensive Income (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Unrealized gains and losses on securities	'	'	'	'
Net realized investment gains	($3.30)	($1.70)	($18.30)	($11.10)
Provision for income taxes	2	0.4	6.7	3.2
Net of taxes	($1.30)	'	($11.60)	'

Income_Taxes_Income_Tax_Provis

Income Taxes - Income Tax Provision (Benefit) (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Income Tax Disclosure [Abstract]	'	'	'	'
Current tax provision	$3.50	$5.70	$13.40	$11.20
Future tax deductions	-2.4	-2.1	2	0.8
Valuation allowance change	0.8	0.7	2.4	1.8
Income tax provision	$1.90	$4.30	$17.80	$13.80

Income_Taxes_Schedule_of_PreTa

Income Taxes - Schedule of Pre-Tax Income (Loss) and Effective Income Tax Rates (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Income Tax Examination [Line Items]	'	'	'	'
Pre-tax income (loss)	$32.90	$17.70	$113.20	$70.80
Bermuda [Member]	'	'	'	'
Income Tax Examination [Line Items]	'	'	'	'
Pre-tax income (loss)	14.2	8	34.2	18.2
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
United States [Member]	'	'	'	'
Income Tax Examination [Line Items]	'	'	'	'
Pre-tax income (loss)	19.6	12.1	68.6	45.8
Effective Tax Rate	22.90%	21.00%	23.50%	22.80%
United Kingdom [Member]	'	'	'	'
Income Tax Examination [Line Items]	'	'	'	'
Pre-tax income (loss)	1.6	0.1	18.3	13.8
Effective Tax Rate	-164.70%	1217.40%	9.20%	24.00%
Belgium [Member]	'	'	'	'
Income Tax Examination [Line Items]	'	'	'	'
Pre-tax income (loss)	0.1	0.1	0.1	0.2
Effective Tax Rate	91.60%	20.00%	26.90%	33.60%
Brazil [Member]	'	'	'	'
Income Tax Examination [Line Items]	'	'	'	'
Pre-tax income (loss)	-1.5	-1.9	-6.2	-5.4
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
Dubai [Member]	'	'	'	'
Income Tax Examination [Line Items]	'	'	'	'
Pre-tax income (loss)	-0.4	0	-0.4	0.1
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
Ireland [Member]	'	'	'	'
Income Tax Examination [Line Items]	'	'	'	'
Pre-tax income (loss)	-0.1	0	-0.1	-0.1
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
Malta [Member]	'	'	'	'
Income Tax Examination [Line Items]	'	'	'	'
Pre-tax income (loss)	-0.6	-0.7	-1.3	-1.8
Effective Tax Rate	0.00%	0.00%	0.00%	0.00%
Switzerland [Member]	'	'	'	'
Income Tax Examination [Line Items]	'	'	'	'
Pre-tax income (loss)	$0	$0	$0	$0
Effective Tax Rate	19.70%	25.00%	21.00%	24.00%

Income_Taxes_Schedule_of_PreTa1

Income Taxes - Schedule of Pre-Tax Income (Loss) and Effective Income Tax Rates (Parenthetical) (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Income Tax Examination [Line Items]	'	'	'	'
Pre-tax income (loss)	$32.90	$17.70	$113.20	$70.80
Maximum [Member] | Switzerland [Member]	'	'	'	'
Income Tax Examination [Line Items]	'	'	'	'
Pre-tax income (loss)	'	$0.10	'	$0.10

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		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Income Tax Examination [Line Items]	'	'	'	'
Income tax provision at expected rate	$6.20	$3.30	$25.20	$17
Tax-exempt interest	-1.3	-1.2	-4.1	-4.2
Dividends received deduction	-0.7	-0.4	-1.8	-1.3
Valuation allowance change	0.8	0.7	2.4	1.8
Other permanent adjustments, net	-0.5	0.1	-1.4	0.8
Adjustment for annualized rate	0.3	0	-0.1	0
United States state tax benefit	-0.1	0	-0.4	-0.2
Other foreign adjustments	0	0	0	0.1
Deferred tax rate reduction	-1.3	-0.6	-1.3	-0.6
Foreign exchange adjustments	-1.7	2.4	-1.3	0.4
Foreign withholding taxes	0.2	0	0.6	0
Income tax provision (benefit)	1.9	4.3	17.8	13.8
Foreign Country [Member]	'	'	'	'
Income Tax Examination [Line Items]	'	'	'	'
Income tax provision (benefit)	-2.6	1.8	1.7	3.4
United States Federal [Member]	'	'	'	'
Income Tax Examination [Line Items]	'	'	'	'
Income tax provision (benefit)	4.4	2.3	16.1	10.6
United States State [Member]	'	'	'	'
Income Tax Examination [Line Items]	'	'	'	'
Foreign withholding taxes	0.2	0	0.6	0
Income tax provision (benefit)	($0.10)	$0.20	($0.60)	($0.20)

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	9 Months Ended					9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2012
		Malta [Member]	Brazil [Member]	ARIS [Member]	PXRE Reinsurance Company [Member]	December 31, 2025 [Member]	December 31, 2027 [Member]
Income Tax Contingency [Line Items]	'	'	'	'	'	'	'
Total net deferred tax assets	$24.20	'	'	'	'	'	'
Capital loss carryforward	30	'	'	'	'	'	'
Net operating loss carryforward	18.7	'	'	1	'	'	'
Loss carryforwards, amounts expiring if not utilized	'	'	'	'	'	17.2	1.5
Valuation allowance	54.9	'	'	'	'	'	'
Loss carryforwards per year	2.8	'	'	'	'	'	'
Reduced valuation allowance	'	'	'	'	0.7	'	'
Increased valuation allowance	'	$0.40	$2.70	'	'	'	'

ShareBased_Compensation_Additi

Share-Based Compensation - Additional Information (Detail) (USD $)

9 Months Ended

9 Months Ended

9 Months Ended

In Millions, except Share data, unless otherwise specified

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Nov. 30, 2007

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2013

Maximum [Member]

2007 Plan [Member]

2007 Plan [Member]

2007 Plan [Member]

2007 Plan [Member]

2007 Plan [Member]

2007 Plan [Member]

Cash-Settled SARs Activity [Member]

Cash-Settled SARs Activity [Member]

Cash-Settled SARs Activity [Member]

Cash-Settled SARs Activity [Member]

Deferred Compensation Plan [Member]

Deferred Compensation Plan [Member]

Amended Plan [Member]

Restricted Share Units [Member]

Restricted Share Units [Member]

Restricted Share Units [Member]

Restricted Share Units [Member]

Stock-Settled SARs Activity [Member]

Stock-Settled SARs Activity [Member]

Stock-Settled SARs Activity [Member]

Stock-Settled SARs Activity [Member]

Maximum [Member]

Maximum [Member]

Minimum [Member]

Minimum [Member]

Maximum [Member]

Minimum [Member]

Maximum [Member]

Minimum [Member]

Maximum [Member]

Minimum [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

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Share-based compensation arrangement by share-based payment award, award vesting period, maximum, years

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'5 years

'5 years

'1 year

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'4 years

'1 year

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'4 years

'2 years

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'4 years

'1 year

Share-based compensation arrangement by share-based payment award, number of shares authorized

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4,500,000

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6,250,000

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Share-based compensation arrangement by share-based payment award, terms of award

'Share options and SARs will count as one share for the purposes of the limits under the 2007 Plan; 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 2007 Plan.

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Count of restricted shares, settle in common shares

2.75

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Term of shares options, (in years)

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'seven years

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Share-based compensation arrangement by share-based payment award, compensation expense

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$0.50

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$2.20

$1.40

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$2.10

$1.70

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Employee service share-based compensation, unrecognized compensation costs on nonvested awards

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13.5

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6.2

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4.9

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'

Deferred compensation arrangement with individual, compensation expense

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$12.70

$3.60

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$1.50

$0.90

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Deferred compensation arrangement with individual, description

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'Under the Directors Plan, non-employee directors can elect each year to defer payment of 50% or 100% of their cash compensation payable during the next calendar year.

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Deferred compensation arrangement with individual, employer contribution percentage

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75.00%

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Deferred compensation arrangement with individual, interest rate

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'two percent above the prime rate

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Share-based compensation arrangement by share-based payment award, discount from market price

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100.00%

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50.00%

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Deferred compensation arrangement with individual, distributions period, months

'6 months

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Deferred compensation arrangement with individual, shares authorized for issuance

1,650

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Sharebased_Compensation_Fair_V

Share-based Compensation - Fair Value Assumptions (Detail)	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]	'	'
Risk-free rate of return	1.44%	1.09%
Expected dividend yields	1.55%	1.67%
Expected award life (years)	'4 years 11 months 1 day	'5 years 11 days
Expected volatility	31.10%	32.60%

Sharebased_Compensation_Summar

Share-based Compensation - Summary of Option Activity (Detail) (2007 Plan [Member], USD $)	9 Months Ended
	Sep. 30, 2013
2007 Plan [Member]	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'
Shares, Outstanding at January 1, 2013	418,325
Granted, Shares	0
Exercised, shares	-151,286
Expired or forfeited, Shares	-45,051
Shares, Outstanding at September 30, 2013	221,988
Weighted-Average Exercise Price, Outstanding at January 1, 2013	$33.56
Granted, Weighted-Average Exercise Price	$0
Exercised, Weighted-Average Exercise Price	$31.62
Expired or forfeited, Weighted-Average Exercise Price	$34.21
Weighted-Average Exercise Price, Outstanding at September 30, 2013	$34.74

Sharebased_Compensation_Summar1

Share-based Compensation - Summary of Restricted Share Units Activity Under 2007 Plan (Detail) (Restricted Share Units [Member], USD $)	9 Months Ended
	Sep. 30, 2013
Restricted Share Units [Member]	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'
Shares, Outstanding at January 1, 2013	236,139
Granted, Shares	117,788
Vested and issued, Shares	-54,034
Expired or forfeited, Shares	-41,077
Shares, Outstanding at September 30, 2013	258,816
Weighted-Average Exercise Price, Outstanding at January 1, 2013	$29.76
Granted, Weighted-Average Grant Date Fair Value	$37.05
Vested and issued, Weighted-Average Grant Date Fair Value	$29.80
Expired or forfeited, Weighted-Average Grant Date Fair Value	$35.62
Weighted-Average Exercise Price, Outstanding at September 30, 2013	$32.71

Sharebased_Compensation_Summar2

Share-based Compensation - Summary of Stock-Settled SARs Activity Under 2007 Plan (Detail) (Stock-Settled SARs Activity [Member], USD $)	9 Months Ended
	Sep. 30, 2013
Stock-Settled SARs Activity [Member]	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'
Shares, Outstanding at January 1, 2013	1,261,448
Granted, Shares	332,004
Exercised, shares	-167,693
Expired or forfeited, Shares	-124,711
Shares, Outstanding at September 30, 2013	1,301,048
Weighted-Average Exercise Price, Outstanding at January 1, 2013	$27.04
Granted, Weighted-Average Exercise Price	$37.21
Exercised, Weighted-Average Exercise Price	$26.69
Expired or forfeited, Weighted-Average Exercise Price	$29.27
Weighted-Average Exercise Price, Outstanding at September 30, 2013	$29.46

Sharebased_Compensation_Summar3

Share-based Compensation - Summary of Option Activity Under Amended Plan (Detail) (Cash-Settled SARs Activity [Member], USD $)	9 Months Ended
	Sep. 30, 2013
Cash-Settled SARs Activity [Member]	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'
Shares, Outstanding at January 1, 2013	1,886,124
Granted, Shares	556,598
Exercised, shares	-451,792
Expired or forfeited, Shares	-175,284
Shares, Outstanding at September 30, 2013	1,815,646
Weighted-Average Exercise Price, Outstanding at January 1, 2013	$29.75
Granted, Weighted-Average Exercise Price	$39.67
Exercised, Weighted-Average Exercise Price	$29.46
Expired or forfeited, Weighted-Average Exercise Price	$29.85
Weighted-Average Exercise Price, Outstanding at September 30, 2013	$32.85

Sharebased_Compensation_Summar4

Share-based Compensation - Summary of Non-Vested Shares Activity Under Amended Plan (Detail) (Amended Plan [Member], USD $)	9 Months Ended
	Sep. 30, 2013
Amended Plan [Member]	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'
Shares, Outstanding at January 1, 2013	210,097
Granted, Shares	0
Exercised, shares	-115,555
Expired or forfeited, Shares	-41,814
Shares, Outstanding at September 30, 2013	52,728
Weighted-Average Exercise Price, Outstanding at January 1, 2013	$33.13
Granted, Weighted-Average Exercise Price	$0
Exercised, Weighted-Average Exercise Price	$27.75
Expired or forfeited, Weighted-Average Exercise Price	$47.71
Weighted-Average Exercise Price, Outstanding at September 30, 2013	$33.37

Reserves_for_Losses_and_Loss_A2

Reserves for Losses and Loss Adjustment Expenses - Additional Information (Detail) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]	'	'
Paid loss recoverables excluded from reinsurance recoverables on unpaid losses and LAE	$142.70	$106.50
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	-21.5	-17.8
Brazil [Member]	'	'
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]	'	'
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	0.8	'
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	9.1	'
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	0.4	18.6
General 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	11.4	24.9
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	26.8	33.5
Automobile 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	2.4	2.4
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	'	3.1
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	6.7	2.4
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	0.6	4.5
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	4.1	4.4
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	9.6	6
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	2	'
General and Products Liability Lines of Business [Member] | 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	35.9	23.6
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	6.4	5.5
Unfavorable Development [Member] | Automobile 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	'	3.8
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	5.8	7.7
Marine Industry Loss Warranty [Member]	'	'
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]	'	'
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	2.6	'
Professional Liability 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	3	'
Assumed Contract [Member]	'	'
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]	'	'
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	$1.20	'

Reserves_for_Losses_and_Loss_A3

Reserves for Losses and Loss Adjustment Expenses - Reinsurance Recoverables (Detail) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Insurance [Abstract]	'	'
Net reserves beginning of the year	$2,110.90	$2,336.70
Net reserves from assumed retroactive insurance contract	0	13
Losses and LAE incurred during current calendar year, net of reinsurance:	'	'
Current accident year	575.6	545.7
Prior accident years	-21.5	-17.8
Losses and LAE incurred during calendar year, net of reinsurance	554.1	527.9
Losses and LAE payments made during current calendar year, net of reinsurance:	'	'
Current accident year	131.7	111.9
Prior accident years	429	450.1
Losses and LAE payments made during current calendar year, net of reinsurance:	560.7	562
Change in participation interest	10.4	-3.1
Foreign exchange adjustments	-6.3	5.4
Net reserves - end of period	2,108.40	2,317.90
Reinsurance recoverables on unpaid losses and LAE, end of period	1,114.50	910
Gross reserves - end of period	$3,222.90	$3,227.90

Derivative_Instruments_Additio

Derivative Instruments - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Jun. 18, 2011	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 28, 2011	Dec. 31, 2011	Dec. 31, 2012
							Contract
Derivative Instruments And Hedging Activities Disclosure [Abstract]	'	'	'	'	'	'	'	'
Number of reinsurance contracts	'	'	'	'	'	'	2	'
Coverage amount	$100	'	'	'	'	$100	'	'
Coverage period, months	'18 months	'	'	'	'	'24 months	'	'
Other reinsurance-related expenses	'	4.6	7	14.4	20.8	'	'	'
Other assets	'	$2.90	'	$2.90	'	'	'	$6.90

Underwriting_Acquisition_and_I2

Underwriting, Acquisition and Insurance Expenses - Underwriting, Acquisition and Insurance Expenses (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Other Income And Expenses [Abstract]	'	'	'	'
Commissions	$57.20	$55.30	$174.70	$156
General expenses	65.4	60.2	200.8	182.2
Premium taxes, boards and bureaus	6.8	7.1	19.7	19.2
Underwriting Acquisition and Insurance Expenses, Total	129.4	122.6	395.2	357.4
Net deferral of policy acquisition costs	-2.7	-4.6	-17.2	-11.1
Total underwriting, acquisition and insurance expenses	$126.70	$118	$378	$346.30

Underwriting_Acquisition_and_I3

Underwriting, Acquisition and Insurance Expenses - Additional Information (Detail) (Cash-Settled SARs Activity [Member], USD $)	3 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013
Cash-Settled SARs Activity [Member]	'	'
Underwriting Acquisition and Insurance Expense [Line Items]	'	'
Expense associated with cash settled stock appreciation rights	$1.70	$12.70

Segment_Information_Revenue_an

Segment Information - Revenue and (Loss) Income Before Income Taxes for Each Segment (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]	'	'	'	'
Earned premiums	$330.50	$304.30	$962.20	$871.80
Net investment income	24.1	28.9	77.3	90.3
Fee income, net	0.3	3	0.5	4.8
Total segment income before taxes	39.6	28.3	106.5	83
Net realized investment gains	9.1	8.3	29.6	18.7
Total revenue	364	344.5	1,069.60	985.6
Net realized investment gains	9.1	8.3	29.6	18.7
Total income before income taxes	32.9	17.7	113.2	70.8
Excess and Surplus Lines [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Earned premiums	120.6	100.8	334.4	295.5
Net investment income	10.2	12.5	32.3	38.5
Total segment income before taxes	29.6	15.1	60.2	56.3
Commercial Specialty [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Earned premiums	74.6	78.6	225.1	241.8
Net investment income	5.6	6.7	17.6	20.6
Total segment income before taxes	10.1	-2.7	17.3	-11.8
International Specialty [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Earned premiums	37.5	33.4	106.6	90.8
Net investment income	2.4	3.1	6.4	10
Total segment income before taxes	0.5	6.6	8.1	22
Syndicate 1200 [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Earned premiums	97.3	90.8	295.6	242.3
Net investment income	2.5	3.8	8.1	11.4
Total segment income before taxes	8.5	11.8	26.7	18
Run-Off Lines [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Earned premiums	0.5	0.7	0.5	1.4
Net investment income	2.6	2.8	8.2	9.8
Total segment income before taxes	-9.1	-2.5	-5.8	-1.5
Corporate and Other [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Net investment income	0.8	0	4.7	0
Net realized investment gains	-15.8	-18.9	-22.9	-30.9
Net realized investment gains	($15.80)	($18.90)	($22.90)	($30.90)

Segment_Information_Schedule_o

Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]	'	'	'	'
Total earned premiums	$330.50	$304.30	$962.20	$871.80
Bermuda [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Total earned premiums	27.4	32.6	75.3	83.5
Brazil [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Total earned premiums	9.9	6.7	29.9	18.4
Malta [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Total earned premiums	0.5	0	1.3	0.1
United Kingdom [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Total earned premiums	97.7	84.9	296	231.1
United States [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Total earned premiums	$195	$180.10	$559.70	$538.70

Segment_Information_Identifiab

Segment Information - Identifiable Assets (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Identifiable Assets [Line Items]	'	'
Identifiable assets	$6,650.60	$6,688.90
Excess and Surplus Lines [Member]	'	'
Identifiable Assets [Line Items]	'	'
Identifiable assets	2,248.60	2,206.70
Commercial Specialty [Member]	'	'
Identifiable Assets [Line Items]	'	'
Identifiable assets	1,316.50	1,337.70
International Specialty [Member]	'	'
Identifiable Assets [Line Items]	'	'
Identifiable assets	785.9	731.8
Syndicate 1200 [Member]	'	'
Identifiable Assets [Line Items]	'	'
Identifiable assets	1,670.50	1,811.40
Run-Off Lines [Member]	'	'
Identifiable Assets [Line Items]	'	'
Identifiable assets	558	516.5
Corporate and Other [Member]	'	'
Identifiable Assets [Line Items]	'	'
Identifiable assets	$71.10	$84.80

Segment_Information_Additional

Segment Information - Additional Information (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Segment Reporting [Abstract]	'	'
Assets associated with trade capital providers	$378.90	$505.70

Supplemental_Cash_Flow_Informa2

Supplemental Cash Flow Information - Additional Information (Detail) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Income Taxes Paid Net [Abstract]	'	'
Income taxes paid	$14.10	$18.10

Supplemental_Cash_Flow_Informa3

Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Interest Paid [Abstract]	'	'
Senior unsecured fixed rate notes	$7	$0
Junior subordinated debentures	6	16.3
Other indebtedness	0.7	2.5
Revolving credit facility	0	0
Total interest paid	$13.70	$18.80

Disclosures_about_Fair_Value_o2

Disclosures about Fair Value of Financial Instruments - Additional Information (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Financial Instruments Owned and Pledged as Collateral [Line Items]	'	'
Premiums receivable	$426.40	$361
Reinsurance recoverable	1,257.20	1,320.90
Over 90 Days [Member]	'	'
Financial Instruments Owned and Pledged as Collateral [Line Items]	'	'
Premiums receivable	17	24.3
Reinsurance recoverable	17.9	18
Premiums receivable, allowance for doubtful accounts	4.3	2.5
Reinsurance recoverables, allowance for doubtful accounts	2.3	2.2
Premiums receivable, secured by collateral	0.6	0.4
Reinsurance recoverables, secured by collateral	$1.60	$0.30

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 $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Carrying Amount	$402	$400.90
Fair Value	322.4	345.1
Junior Subordinated Debentures [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Carrying Amount	193.3	193.3
Fair Value	155.2	151.8
Senior Notes [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Carrying Amount	143.8	143.8
Fair Value	115.2	143.2
Other Indebtedness [Member] | Floating Rate Loan Stock [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Carrying Amount	64.1	63
Fair Value	51.4	49.5
Other Indebtedness [Member] | Notes Payable [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Carrying Amount	0.8	0.8
Fair Value	$0.60	$0.60

Information_Provided_in_Connec2

Information Provided in Connection With Outstanding Debt of Subsidiaries - Additional Information (Detail) (Senior Notes [Member], USD $)	9 Months Ended
	Sep. 30, 2012
Senior Notes [Member]	'
Debt Instrument [Line Items]	'
Principal amount of Agro Group	$143,750,000
Interest rate of principal amount	6.50%
Percentage of amount redeemed	100.00%

Information_Provided_in_Connec3

Information Provided in Connection with Outstanding Debt of Subsidiaries - Condensed Consolidating Balance Sheet (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
In Millions, unless otherwise specified
Assets	'	'	'	'
Total Investments	$4,050.20	$4,200.70	'	'
Cash	141.6	95.8	142.7	100.9
Accrued investment income	25.3	30.3	'	'
Premiums receivable	426.4	361	'	'
Reinsurance recoverables	1,257.20	1,320.90	'	'
Goodwill and other intangible assets	241.1	245.3	'	'
Current income taxes receivable, net	15.3	12.9	'	'
Deferred acquisition costs, net	115.3	99.4	'	'
Ceded unearned premiums	231	193.6	'	'
Accrued investment income and other assets	147.2	129	'	'
Due from affiliates	0	0	'	'
Intercompany note receivable	0	0	'	'
Investments in subsidiaries	0	0	'	'
Total assets	6,650.60	6,688.90	'	'
Liabilities and Shareholders' Equity	'	'	'	'
Reserves for losses and loss adjustment expenses	3,222.90	3,223.50	'	'
Unearned premiums	847	730.2	'	'
Funds held and ceded reinsurance payable, net	456.6	645.8	'	'
Long-term debt	402	400.9	'	'
Current income taxes payable, net	0	0	'	'
Deferred tax liabilities, net	30.7	43.8	'	'
Accrued underwriting expenses and other liabilities	164	130.6	'	'
Due to affiliates	0	0	'	'
Intercompany note payable	0	0	'	'
Total liabilities	5,123.20	5,174.80	'	'
Total shareholders' equity	1,527.40	1,514.10	'	'
Total liabilities and shareholders' equity	6,650.60	6,688.90	'	'
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]	'	'	'	'
Assets	'	'	'	'
Total Investments	3.4	-0.9	'	'
Cash	0	0	0.1	0
Accrued investment income	0	0	'	'
Premiums receivable	0	0	'	'
Reinsurance recoverables	0	0	'	'
Goodwill and other intangible assets	0	0	'	'
Current income taxes receivable, net	0	0	'	'
Deferred acquisition costs, net	0	0	'	'
Ceded unearned premiums	0	0	'	'
Accrued investment income and other assets	4.2	4.3	'	'
Due from affiliates	0	5	'	'
Intercompany note receivable	0	0	'	'
Investments in subsidiaries	1,662.10	1,621.10	'	'
Total assets	1,669.70	1,629.50	'	'
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	0	'	'
Deferred tax liabilities, net	0	0	'	'
Accrued underwriting expenses and other liabilities	14.2	11.3	'	'
Due to affiliates	-1.5	0	'	'
Intercompany note payable	80.6	55.1	'	'
Total liabilities	142.3	115.4	'	'
Total shareholders' equity	1,527.40	1,514.10	'	'
Total liabilities and shareholders' equity	1,669.70	1,629.50	'	'
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]	'	'	'	'
Assets	'	'	'	'
Total Investments	2,750.20	2,794.70	'	'
Cash	117.5	73.9	127.8	85.1
Accrued investment income	19.5	21.3	'	'
Premiums receivable	150.3	155.1	'	'
Reinsurance recoverables	1,165.40	1,155.10	'	'
Goodwill and other intangible assets	137.9	139.5	'	'
Current income taxes receivable, net	10.1	8.7	'	'
Deferred acquisition costs, net	55.5	54.6	'	'
Ceded unearned premiums	99.4	85.9	'	'
Accrued investment income and other assets	93.5	82.7	'	'
Due from affiliates	0.2	1.2	'	'
Intercompany note receivable	102.7	119.6	'	'
Investments in subsidiaries	0	0	'	'
Total assets	4,702.20	4,692.30	'	'
Liabilities and Shareholders' Equity	'	'	'	'
Reserves for losses and loss adjustment expenses	2,178.70	2,226.30	'	'
Unearned premiums	448.8	415.6	'	'
Funds held and ceded reinsurance payable, net	638	598.4	'	'
Long-term debt	288.9	288.9	'	'
Current income taxes payable, net	0	0	'	'
Deferred tax liabilities, net	23.1	36.8	'	'
Accrued underwriting expenses and other liabilities	98	84.1	'	'
Due to affiliates	0	0	'	'
Intercompany note payable	0	0	'	'
Total liabilities	3,675.50	3,650.10	'	'
Total shareholders' equity	1,026.80	1,042.20	'	'
Total liabilities and shareholders' equity	4,702.20	4,692.30	'	'
Other Subsidiaries and Eliminations [Member]	'	'	'	'
Assets	'	'	'	'
Total Investments	1,296.60	1,406.90	'	'
Cash	24.1	21.9	14.8	15.8
Accrued investment income	5.8	9	'	'
Premiums receivable	276.1	205.9	'	'
Reinsurance recoverables	91.8	165.8	'	'
Goodwill and other intangible assets	103.2	105.8	'	'
Current income taxes receivable, net	5.2	4.2	'	'
Deferred acquisition costs, net	59.8	44.8	'	'
Ceded unearned premiums	131.6	107.7	'	'
Accrued investment income and other assets	49.5	42	'	'
Due from affiliates	-0.2	-1.2	'	'
Intercompany note receivable	-102.7	-119.6	'	'
Investments in subsidiaries	0	0	'	'
Total assets	1,940.80	1,993.20	'	'
Liabilities and Shareholders' Equity	'	'	'	'
Reserves for losses and loss adjustment expenses	1,044.20	997.2	'	'
Unearned premiums	398.2	314.6	'	'
Funds held and ceded reinsurance payable, net	-181.4	47.4	'	'
Long-term debt	64.1	63	'	'
Current income taxes payable, net	0	0	'	'
Deferred tax liabilities, net	7.6	7	'	'
Accrued underwriting expenses and other liabilities	51.8	35.2	'	'
Due to affiliates	0	0	'	'
Intercompany note payable	0	0	'	'
Total liabilities	1,384.60	1,464.40	'	'
Total shareholders' equity	556.2	528.8	'	'
Total liabilities and shareholders' equity	1,940.80	1,993.20	'	'
Consolidating Adjustments [Member]	'	'	'	'
Assets	'	'	'	'
Total Investments	0	0	'	'
Cash	0	0	0	0
Accrued investment income	0	0	'	'
Premiums receivable	0	0	'	'
Reinsurance recoverables	0	0	'	'
Goodwill and other intangible assets	0	0	'	'
Current income taxes receivable, net	0	0	'	'
Deferred acquisition costs, net	0	0	'	'
Ceded unearned premiums	0	0	'	'
Accrued investment income and other assets	0	0	'	'
Due from affiliates	0	-5	'	'
Intercompany note receivable	0	0	'	'
Investments in subsidiaries	-1,662.10	-1,621.10	'	'
Total assets	-1,662.10	-1,626.10	'	'
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	0	0	'	'
Current income taxes payable, net	0	0	'	'
Deferred tax liabilities, net	0	0	'	'
Accrued underwriting expenses and other liabilities	0	0	'	'
Due to affiliates	1.5	0	'	'
Intercompany note payable	-80.6	-55.1	'	'
Total liabilities	-79.1	-55.1	'	'
Total shareholders' equity	-1,583	-1,571	'	'
Total liabilities and shareholders' equity	($1,662.10)	($1,626.10)	'	'

Information_Provided_in_Connec4

Information Provided in Connection with Outstanding Debt of Subsidiaries - Condensed Consolidating Statement of (Loss) Income (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Premiums and other revenue:	'	'	'	'
Earned premiums	$330.50	$304.30	$962.20	$871.80
Net investment income	24.1	28.9	77.3	90.3
Fee income, net	0.3	3	0.5	4.8
Net realized investment gains (losses)	9.1	8.3	29.6	18.7
Total revenue	364	344.5	1,069.60	985.6
Expenses:	'	'	'	'
Losses and loss adjustment expenses	190.9	186.3	554.1	527.9
Other reinsurance-related expenses	4.6	7	14.4	20.8
Underwriting, acquisition and insurance expenses	126.7	118	378	346.3
Interest expense	5.1	5.8	15.1	17
Foreign currency exchange (gain) loss	3.8	9.7	-5.2	2.8
Total expenses	331.1	326.8	956.4	914.8
(Loss) income before income taxes	32.9	17.7	113.2	70.8
Provision for income taxes	1.9	4.3	17.8	13.8
Net (loss) income before equity in earnings of subsidiaries	31	13.4	95.4	57
Equity in undistributed earnings of subsidiaries	0	0	0	0
Net income	31	13.4	95.4	57
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]	'	'	'	'
Premiums and other revenue:	'	'	'	'
Earned premiums	0	0	0	0
Net investment income	0	0	0	0.1
Fee income, net	0	0	0	0
Net realized investment gains (losses)	0	-0.5	0	-0.5
Total revenue	0	-0.5	0	-0.4
Expenses:	'	'	'	'
Losses and loss adjustment expenses	0	0	0	0
Other reinsurance-related expenses	0	0	0	0
Underwriting, acquisition and insurance expenses	5.8	10.1	18.3	21.4
Interest expense	0.7	0.1	2.3	2.9
Foreign currency exchange (gain) loss	0	0	0	0
Total expenses	6.5	10.2	20.6	24.3
(Loss) income before income taxes	-6.5	-10.7	-20.6	-24.7
Provision for income taxes	0	0	0	0
Net (loss) income before equity in earnings of subsidiaries	-6.5	-10.7	-20.6	-24.7
Equity in undistributed earnings of subsidiaries	37.5	24.1	116	81.7
Net income	31	13.4	95.4	57
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]	'	'	'	'
Premiums and other revenue:	'	'	'	'
Earned premiums	105.2	108.1	321.2	327
Net investment income	16.7	21.2	57.2	65.3
Fee income, net	-0.6	2.4	-1.9	2.4
Net realized investment gains (losses)	7.2	4.2	32.8	11.8
Total revenue	128.5	135.9	409.3	406.5
Expenses:	'	'	'	'
Losses and loss adjustment expenses	65.9	78	194.9	220.9
Other reinsurance-related expenses	-2.9	0	0	0
Underwriting, acquisition and insurance expenses	42.1	41.8	134.3	127.9
Interest expense	3.8	4.1	11.4	11.8
Foreign currency exchange (gain) loss	-0.1	-0.1	0.1	0.1
Total expenses	108.8	123.8	340.7	360.7
(Loss) income before income taxes	19.7	12.1	68.6	45.8
Provision for income taxes	4.4	2.5	16	10.4
Net (loss) income before equity in earnings of subsidiaries	15.3	9.6	52.6	35.4
Equity in undistributed earnings of subsidiaries	0	0	0	0
Net income	15.3	9.6	52.6	35.4
Other Subsidiaries and Eliminations [Member]	'	'	'	'
Premiums and other revenue:	'	'	'	'
Earned premiums	225.3	196.2	641	544.8
Net investment income	7.7	7.8	21	25.1
Fee income, net	0.9	0.6	2.4	2.4
Net realized investment gains (losses)	1.9	4.6	-3.2	7.4
Total revenue	235.8	209.2	661.2	579.7
Expenses:	'	'	'	'
Losses and loss adjustment expenses	125	108.3	359.2	307
Other reinsurance-related expenses	7.5	7	14.4	20.8
Underwriting, acquisition and insurance expenses	78.8	66.1	225.4	197
Interest expense	0.9	1.7	2.3	2.5
Foreign currency exchange (gain) loss	3.9	9.8	-5.3	2.7
Total expenses	216.1	192.9	596	530
(Loss) income before income taxes	19.7	16.3	65.2	49.7
Provision for income taxes	-2.5	1.8	1.8	3.4
Net (loss) income before equity in earnings of subsidiaries	22.2	14.5	63.4	46.3
Equity in undistributed earnings of subsidiaries	0	0	0	0
Net income	22.2	14.5	63.4	46.3
Consolidating Adjustments [Member]	'	'	'	'
Premiums and other revenue:	'	'	'	'
Earned premiums	0	0	0	0
Net investment income	-0.3	-0.1	-0.9	-0.2
Fee income, net	0	0	0	0
Net realized investment gains (losses)	0	0	0	0
Total revenue	-0.3	-0.1	-0.9	-0.2
Expenses:	'	'	'	'
Losses and loss adjustment expenses	0	0	0	0
Other reinsurance-related expenses	0	0	0	0
Underwriting, acquisition and insurance expenses	0	0	0	0
Interest expense	-0.3	-0.1	-0.9	-0.2
Foreign currency exchange (gain) loss	0	0	0	0
Total expenses	-0.3	-0.1	-0.9	-0.2
(Loss) income before income taxes	0	0	0	0
Provision for income taxes	0	0	0	0
Net (loss) income before equity in earnings of subsidiaries	0	0	0	0
Equity in undistributed earnings of subsidiaries	-37.5	-24.1	-116	-81.7
Net income	($37.50)	($24.10)	($116)	($81.70)

Information_Provided_in_Connec5

Information Provided in Connection with Outstanding Debt of Subsidiaries - Condensed Consolidating Statement of Cash Flows (Detail) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Condensed Financial Statements, Captions [Line Items]	'	'
Net cash flows from operating activities	($3)	$63.50
Cash flows from investing activities:	'	'
Proceeds from sales of investments	1,378.80	898.5
Proceeds from maturities and mandatory calls of investments	224.2	364.9
Purchases of investments	-1,407.30	-1,251.40
Change in short-term investments and foreign regulatory deposits	-75.5	-99
Settlements of foreign currency exchange forward contracts	-6.4	-0.6
Acquisitions	0	-1.4
Issuance of intercompany note, net	0	0
Purchase of fixed assets	-19.1	-26.9
Purchases of fixed assets and other, net	4.5	2.7
Cash provided (used) by investing activities	99.2	-113.2
Cash flows from financing activities:	'	'
Borrowings under intercompany note, net	0	0
Proceeds from issuance of senior unsecured fixed rate notes	0	139.2
Activity under stock incentive plans	1.4	0.7
Repurchase of Company's common shares	-40.1	-38.6
Excess tax expense from share-based payment arrangements	0.2	0
Payment of cash dividends to common shareholders	-11.8	-9.3
Cash (used) provided by financing activities	-50.3	92
Effect of exchange rate changes on cash	-0.1	-0.5
Change in cash	45.8	41.8
Cash, beginning of period	95.8	100.9
Cash, end of period	141.6	142.7
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]	'	'
Condensed Financial Statements, Captions [Line Items]	'	'
Net cash flows from operating activities	-15.5	-17.9
Cash flows from investing activities:	'	'
Proceeds from sales of investments	0	0
Proceeds from maturities and mandatory calls of investments	0	0
Purchases of investments	0	0
Change in short-term investments and foreign regulatory deposits	-0.8	-0.1
Settlements of foreign currency exchange forward contracts	1.9	-0.6
Acquisitions	'	0
Issuance of intercompany note, net	0	0
Purchase of fixed assets	0	0
Purchases of fixed assets and other, net	-0.1	0
Cash provided (used) by investing activities	1	-0.7
Cash flows from financing activities:	'	'
Borrowings under intercompany note, net	24.9	27.3
Proceeds from issuance of senior unsecured fixed rate notes	0	0
Activity under stock incentive plans	1.4	0.7
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	-11.8	-9.3
Cash (used) provided by financing activities	14.5	18.7
Effect of exchange rate changes on cash	0	0
Change in cash	0	0.1
Cash, beginning of period	0	0
Cash, end of period	0	0.1
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]	'	'
Condensed Financial Statements, Captions [Line Items]	'	'
Net cash flows from operating activities	75.9	13.7
Cash flows from investing activities:	'	'
Proceeds from sales of investments	863.2	458.9
Proceeds from maturities and mandatory calls of investments	139	253.9
Purchases of investments	-942.4	-712.1
Change in short-term investments and foreign regulatory deposits	-38.5	-64.3
Settlements of foreign currency exchange forward contracts	0	0
Acquisitions	'	-1.4
Issuance of intercompany note, net	0	-10
Purchase of fixed assets	-17.7	-23.6
Purchases of fixed assets and other, net	4.4	2
Cash provided (used) by investing activities	8	-96.6
Cash flows from financing activities:	'	'
Borrowings under intercompany note, net	0	25
Proceeds from issuance of senior unsecured fixed rate notes	0	139.2
Activity under stock incentive plans	0	0
Repurchase of Company's common shares	-40.1	-38.6
Excess tax expense from share-based payment arrangements	-0.2	0
Payment of cash dividends to common shareholders	0	0
Cash (used) provided by financing activities	-40.3	125.6
Effect of exchange rate changes on cash	0	0
Change in cash	43.6	42.7
Cash, beginning of period	73.9	85.1
Cash, end of period	117.5	127.8
Other Subsidiaries and Eliminations [Member]	'	'
Condensed Financial Statements, Captions [Line Items]	'	'
Net cash flows from operating activities	-63.4	67.7
Cash flows from investing activities:	'	'
Proceeds from sales of investments	515.6	439.6
Proceeds from maturities and mandatory calls of investments	85.2	111
Purchases of investments	-464.9	-539.3
Change in short-term investments and foreign regulatory deposits	-36.2	-34.6
Settlements of foreign currency exchange forward contracts	-8.3	0
Acquisitions	'	0
Issuance of intercompany note, net	-24.9	-17.3
Purchase of fixed assets	-1.4	-3.3
Purchases of fixed assets and other, net	0.2	0.7
Cash provided (used) by investing activities	65.3	-43.2
Cash flows from financing activities:	'	'
Borrowings under intercompany note, net	0	-25
Proceeds from issuance of senior unsecured fixed rate notes	0	0
Activity under stock incentive plans	0	0
Repurchase of Company's common shares	0	0
Excess tax expense from share-based payment arrangements	0.4	0
Payment of cash dividends to common shareholders	0	0
Cash (used) provided by financing activities	0.4	-25
Effect of exchange rate changes on cash	-0.1	-0.5
Change in cash	2.2	-1
Cash, beginning of period	21.9	15.8
Cash, end of period	24.1	14.8
Consolidating Adjustments [Member]	'	'
Condensed Financial Statements, Captions [Line Items]	'	'
Net cash flows from operating activities	0	0
Cash flows from investing activities:	'	'
Proceeds from sales of investments	0	0
Proceeds from maturities and mandatory calls of investments	0	0
Purchases of investments	0	0
Change in short-term investments and foreign regulatory deposits	0	0
Settlements of foreign currency exchange forward contracts	0	0
Acquisitions	'	0
Issuance of intercompany note, net	24.9	27.3
Purchase of fixed assets	0	0
Purchases of fixed assets and other, net	0	0
Cash provided (used) by investing activities	24.9	27.3
Cash flows from financing activities:	'	'
Borrowings under intercompany note, net	-24.9	-27.3
Proceeds from issuance of senior unsecured fixed rate notes	0	0
Activity under stock incentive plans	0	0
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	0	0
Cash (used) provided by financing activities	-24.9	-27.3
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