Argo Group International (ARGO-PA) 10-Q 8 May 14 2014 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2014	Apr. 28, 2014
Document And Entity Information [Abstract]	'	'
Document Type	'10-Q	'
Amendment Flag	'false	'
Document Period End Date	31-Mar-14	'
Document Fiscal Year Focus	'2014	'
Document Fiscal Period Focus	'Q1	'
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	'	26,169,903

CONSOLIDATED_BALANCE_SHEETS

CONSOLIDATED BALANCE SHEETS (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Assets	'	'
Available-for-sale (cost: 2014 - $2,739.0; 2013 - $2,760.1)	$2,801.80	$2,814.40	[1]
Equity securities, at fair value (cost: 2014 - $345.9; 2013 - $346.9)	530.3	534.3	[1]
Other investments (cost: 2014 - $399.7; 2013 - $377.4)	403.7	378.9	[1]
Short-term investments, at fair value (cost: 2014 - $382.7; 2013 - $351.6)	382.7	351.6	[1]
Total investments	4,118.50	4,079.20	[1]
Cash	149.5	157.4	[1]
Accrued investment income	25	25.7	[1]
Premiums receivable	371.1	348.4	[1]
Reinsurance recoverables	978.1	1,263.50	[1]
Goodwill	153.8	153.8	[1]
Intangible assets, net of accumulated amortization	84.7	86	[1]
Current income taxes receivable, net	10.4	0	[1]
Deferred acquisition costs, net	117.5	113.9	[1]
Ceded unearned premiums	251.9	196.3	[1]
Other assets	170.9	166.8	[1]
Total assets	6,431.40	6,591	[1]
Liabilities and Shareholders' Equity	'	'
Reserves for losses and loss adjustment expenses	3,042.40	3,230.30	[1]
Unearned premiums	786.3	779.1	[1]
Accrued underwriting expenses	128.2	122.3	[1]
Ceded reinsurance payable, net	354.3	354.7	[1]
Funds held	37.8	43.7	[1]
Senior unsecured fixed rate notes	143.8	143.8	[1]
Other indebtedness	66.5	66.3	[1]
Junior subordinated debentures	193.3	193.3	[1]
Current income taxes payable, net	0	5.2	[1]
Deferred tax liabilities, net	34.2	28.7	[1]
Other liabilities	49.7	60.6	[1]
Total liabilities	4,836.50	5,028	[1]
Commitments and contingencies (Note 15)	'	'	[1]
Shareholders' equity:	'	'
Common shares - $1.00 par, 500,000,000 shares authorized; 34,180,904 and 34,066,889 shares issued and outstanding at March 31, 2014 and December 31, 2013, respectively	34.2	34.1	[1]
Additional paid-in capital	829.5	827.3	[1]
Treasury shares (7,723,529 and 7,558,345 shares at March 31, 2014 and December 31, 2013, respectively)	-258.1	-250.6	[1]
Retained earnings	840.4	804.4	[1]
Accumulated other comprehensive income, net of taxes	148.9	147.8	[1]
Total shareholders' equity	1,594.90	1,563	[1]
Total liabilities and shareholders' equity	$6,431.40	$6,591	[1]
[1]	Derived from audited consolidated financial statements.

CONSOLIDATED_BALANCE_SHEETS_Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, except Share data, unless otherwise specified
Statement Of Financial Position [Abstract]	'	'
Available-for-sale cost	$2,739	$2,760.10	[1]
Equity securities cost	345.9	346.9	[1]
Other investments cost	399.7	377.4	[1]
Short-term investments, Cost	$382.70	$351.60	[1]
Common shares, par value	$1	$1	[1]
Common shares, shares authorized	500,000,000	500,000,000	[1]
Common shares, shares issued	34,180,904	34,066,889	[1]
Common shares, shares outstanding	34,180,904	34,066,889	[1]
Treasury shares, shares	7,723,529	7,558,345	[1]
[1]	Derived from audited consolidated financial statements.

CONSOLIDATED_STATEMENTS_OF_INC

CONSOLIDATED STATEMENTS OF INCOME (Unaudited) (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Premiums and other revenue:	'	'
Earned premiums	$325.70	$304.20
Net investment income	23.3	27.9
Net realized investment gains and other	11.1	9.5
Total revenue	360.1	341.6
Expenses:	'	'
Losses and loss adjustment expenses	182.5	170.5
Other reinsurance-related expenses	0	5.1
Underwriting, acquisition and insurance expenses	128.7	126.7
Interest expense	5	4.9
Fee expense, net	1.4	0
Foreign currency exchange gain	-0.2	-3.1
Total expenses	317.4	304.1
Income before income taxes	42.7	37.5
Provision for income taxes	2.5	4.8
Net income	40.2	32.7
Net income per common share:	'	'
Basic	$1.52	$1.20
Diluted	$1.49	$1.16
Dividend declared per common share:	$0.15	$0.14
Weighted average common shares:	'	'
Basic	26,514,481	27,247,009
Diluted	26,966,461	28,157,073
Net realized investment gains and other before other-than-temporary impairment losses	11.1	10.9
Other-than-temporary impairment losses recognized in earnings	'	'
Other-than-temporary impairment losses on equity securities	0	-1.4
Impairment losses recognized in earnings	0	-1.4
Net realized investment gains and other	$11.10	$9.50

CONSOLIDATED_STATEMENTS_OF_COM

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Statement Of Income And Comprehensive Income [Abstract]	'	'
Net income	$40.20	$32.70
Other comprehensive income:	'	'
Foreign currency translation adjustments	-0.2	0
Unrealized gains on securities:	'	'
Gains arising during the period	5.1	24.2
Reclassification adjustment for gains included in net income	-0.1	-0.7
Other comprehensive income before tax	4.8	23.5
Unrealized gains on securities:	'	'
Gains arising during the period	3.8	10.4
Reclassification adjustment for gains included in net income	-0.1	0.1
Income tax provision related to other comprehensive income	3.7	10.5
Other comprehensive income, net of tax	1.1	13
Comprehensive income	$41.30	$45.70

CONSOLIDATED_STATEMENTS_OF_CAS

CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014		Mar. 31, 2013
Cash flows from operating activities:	'		'
Net income	$40.20		$32.70
Adjustments to reconcile net income to net cash (used) provided by operating activities:	'		'
Amortization and depreciation	10.4		9.7
Share-based payments expense	4.4		8.6
Excess tax benefit from share-based payment arrangements	-0.1		0
Deferred income tax provision, net	1.1		3
Net realized investment and other gains	-11.1		-9.5
Change in:	'		'
Accrued investment income	1.7		3.4
Receivables	261		77.1
Deferred acquisition costs	0		-0.4
Ceded unearned premiums	-55.2		-37.3
Reserves for losses and loss adjustment expenses	-182.5		-17.8
Unearned premiums	3.3		7.1
Ceded reinsurance payable and funds held	-6.4		-133.2
Income taxes	-15.5		-0.7
Accrued underwriting expenses	6.7		2.2
Other, net	-14.4		-28.9
Cash provided (used) by operating activities	43.6		-84
Cash flows from investing activities:	'		'
Sales of fixed maturity investments	243.4		425.3
Maturities and mandatory calls of fixed maturity investments	78.4		115.1
Sales of equity securities	14.9		5.6
Sales of other investments	5		5.3
Purchases of fixed maturity investments	-290.4		-378
Purchases of equity securities	-7.5		-38.3
Purchases of other investments	-9.4		-26.5
Change in foreign regulatory deposits and voluntary pools	-9		-1.2
Change in short-term investments	-44.9		-5.2
Settlements of foreign currency exchange forward contracts	-1.4		-1.9
Purchases of fixed assets	-6		-4.2
Other, net	-18.1		-1.3
Cash (used) provided by investing activities	-45		94.7
Cash flows from financing activities:	'		'
Activity under stock incentive plans	0.8		1.3
Repurchase of Company's common shares	-3.7		-12.9
Excess tax expense from share-based payment arrangements	0.1		0
Payment of cash dividends to common shareholders	-4		-3.7
Cash used by financing activities	-6.8		-15.3
Effect of exchange rate changes on cash	0.3		0
Change in cash	-7.9		-4.6
Cash, beginning of period	157.4	[1]	95.8
Cash, end of period	$149.50		$91.20
[1]	Derived from audited consolidated financial statements.

Basis_of_Presentation

Basis of Presentation	3 Months Ended
	Mar. 31, 2014
Accounting Policies [Abstract]	'
Basis of Presentation	'
	1. Basis of Presentation
	The accompanying consolidated financial statements of Argo Group International Holdings, Ltd. (“Argo Group,” “we” or the “Company”) and its subsidiaries have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) for interim financial information and with the instructions for Form 10-Q and Article 10 of Regulation S-X. The preparation of interim financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenues and expenses during the reporting period. The major estimates reflected in our consolidated financial statements include, but are not limited to, reserves for losses and loss adjustment expenses; reinsurance recoverables, including the reinsurance recoverables allowance for doubtful accounts; estimates of written and earned premiums; reinsurance premium receivable; fair value of investments and assessment of potential impairment; valuation of goodwill and intangibles and our deferred tax asset valuation allowance. Actual results could differ from those estimates. Certain financial information that normally is included in annual financial statements, including certain financial statement footnotes, prepared in accordance with GAAP, is not required for interim reporting purposes and has been condensed or omitted. These statements should be read in conjunction with the consolidated financial statements and notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2013, filed with the Securities and Exchange Commission on February 28, 2014.
	The interim financial information as of, and for the three months ended, March 31, 2014 and 2013 is unaudited. However, in the opinion of management, the interim information includes all adjustments, consisting of normal recurring accruals, necessary for a fair presentation of the results presented for the interim periods. The operating results for the interim periods are not necessarily indicative of the results to be expected for the full year. All significant intercompany amounts have been eliminated in consolidation. Certain amounts in prior years’ financial statements have been reclassified to conform to the current presentation.
	10% Stock Dividend
	On 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 our common shares. All references to share and per share amounts in this document and related disclosures have been adjusted to reflect the stock dividend for all periods presented.

Recently_Issued_Accounting_Sta

Recently Issued Accounting Standards	3 Months Ended
	Mar. 31, 2014
Accounting Changes And Error Corrections [Abstract]	'
Recently Issued Accounting Standards	'
	2. Recently Issued Accounting Standards
	In February 2013, the Financial Accounting Standards Board (“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. The adoption of this amendment did not have a material impact on our financial results or 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 or 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. The adoption of this amendment did not have an impact on our financial results or disclosures.
	In July 2013, the FASB issued amendments to “Income Taxes” (Topic 740) that will require the presentation of certain unrecognized tax benefits as reductions to deferred tax assets rather than liabilities in our Consolidated Balance Sheets when a net operating loss carryforward, similar tax loss or tax credit carryforward exists. The new standard requires adoption on a prospective basis in the first quarter of 2015; however, early adoption is permitted. We do not anticipate this update will have an impact on our financial results and disclosures.
	In April 2014, the FASB issued amendments to “Presentation of Financial Statements (Topic 205) and Property, Plant and Equipment (Topic 360): Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity.” The amended guidance requires that a disposal representing a strategic shift that has (or will have) a major effect on an entity’s financial results or a business activity classified as held for sale should be reported as discontinued operations. The amendments also expand the disclosure requirements for discontinued operations and add new disclosures for individually significant dispositions that do not qualify as discontinued operations. The amendments are effective prospectively for fiscal years, and interim reporting periods within those years, beginning after December 15, 2014. Early adoption is permitted only for disposals that have not been previously reported. We do not anticipate this update will have an impact on our financial results and disclosures.

Investments

Investments	3 Months Ended
	Mar. 31, 2014
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 March 31, 2014 and December 31, 2013 were as follows:
						Gross			Gross
	March 31, 2014		Amortized			Unrealized			Unrealized			Fair
	(in millions)		Cost			Gains			Losses			Value
	Fixed maturities
	USD denominated:
	U.S. Governments		$	223.6		$	2.1		$	0.4		$	225.3
	Non-U.S. Governments			70.4			0.7			0.7			70.4
	Obligations of states and political subdivisions			504.2			23.2			4.3			523.1
	Credit-Financial			455.9			15.8			1.3			470.4
	Credit-Industrial			418			14.3			1.2			431.1
	Credit-Utility			146.1			5.6			0.3			151.4
	Structured securities:
	CMO/MBS-agency (1)			200.1			8.9			1.1			207.9
	CMO/MBS-non agency			18.4			1			0			19.4
	CMBS (2)			164.5			3.3			0.5			167.3
	ABS-residential (3)			6.3			0.3			0.4			6.2
	ABS-non residential			147.3			0.6			0.1			147.8
	Foreign denominated:
	Governments			187.8			8.4			8.4			187.8
	Credit			196.4			5.2			7.9			193.7
	Total fixed maturities			2,739.00			89.4			26.6			2,801.80
	Equity securities			345.9			186.3			1.9			530.3
	Other investments			399.7			4			0			403.7
	Short-term investments			382.7			0			0			382.7
	Total investments		$	3,867.30		$	279.7		$	28.5		$	4,118.50
						Gross			Gross
	December 31, 2013		Amortized			Unrealized			Unrealized			Fair
	(in millions)		Cost			Gains			Losses			Value
	Fixed maturities
	USD denominated:
	U.S. Governments		$	273.4		$	2.4		$	0.5		$	275.3
	Non-U.S. Governments			60.9			0.3			1			60.2
	Obligations of states and political subdivisions			563.7			22.9			8.8			577.8
	Credit-Financial			426.3			15.4			1.8			439.9
	Credit-Industrial			385.5			13.8			2.2			397.1
	Credit-Utility			162			5.4			0.8			166.6
	Structured securities:
	CMO/MBS-agency (1)			219.4			9.2			1.7			226.9
	CMO/MBS-non agency			19.4			0.8			0			20.2
	CMBS (2)			162.7			3.1			1			164.8
	ABS-residential (3)			6.8			0.3			0.5			6.6
	ABS-non residential			116.8			0.5			0.1			117.2
	Foreign denominated:
	Governments			207.7			4.9			5.9			206.7
	Credit			155.5			4.4			4.8			155.1
	Total fixed maturities			2,760.10			83.4			29.1			2,814.40
	Equity securities			346.9			188.6			1.2			534.3
	Other investments			377.4			2.8			1.3			378.9
	Short-term investments			351.6			0			0			351.6
	Total investments		$	3,836.00		$	274.8		$	31.6		$	4,079.20
	(1)	Collateralized mortgage obligations/mortgage-backed securities (“CMO/MBS”).
	(2)	Commercial mortgage-backed securities (“CMBS”).
	(3)	Asset-backed securities (“ABS”).
	Included in “Total investments” in our Consolidated Balance Sheets at March 31, 2014 and December 31, 2013 is $69.0 million and $87.3 million, respectively, of assets 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 March 31, 2014, by contractual maturity, were as follows:
			Amortized			Fair
	(in millions)		Cost			Value
	Due in one year or less		$	228.7		$	229.8
	Due after one year through five years			1,267.40			1,286.20
Due after five years through ten years			533			561.7
Thereafter			173.3			175.5
Structured securities			536.6			548.6
Total		$	2,739.00		$	2,801.80
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 in fixed maturities, equity securities, other investments and short-term investments as of March 31, 2014 and December 31, 2013 is presented below:
		Less Than One Year						One Year or Greater					Total
March 31, 2014		Fair			Unrealized			Fair			Unrealized		Fair			Unrealized
(in millions)		Value			Losses			Value			Losses		Value			Losses
Fixed maturities
USD denominated:
U.S. Governments		$	34.6		$	0.4		$	0		$	0	$	34.6		$	0.4
Non-U.S. Governments			31.7			0.6			0.7			0.1		32.4			0.7
Obligations of states and political subdivisions (2)			142.8			4.3			3.8			0		146.6			4.3
Credit-Financial (2)			84			1.3			3			0		87			1.3
Credit-Industrial (2)			90.8			1.2			2.1			0		92.9			1.2
Credit-Utility			18.9			0.2			1.3			0.1		20.2			0.3
Structured securities:
CMO/MBS-agency			39.5			0.6			5.5			0.5		45			1.1
CMBS			15.6			0.4			1.8			0.1		17.4			0.5
ABS-residential			0			0			3.6			0.4		3.6			0.4
ABS-non residential (2)			59.2			0.1			1			0		60.2			0.1
Foreign denominated:
Governments			155.9			8.4			0			0		155.9			8.4
Credit			132.4			7.9			0			0		132.4			7.9
Total fixed maturities			805.4			25.4			22.8			1.2		828.2			26.6
Equity securities			29.4			1.9			0			0		29.4			1.9
Short-term investments (1)			2.6			0			0			0		2.6			0
Total		$	837.4		$	27.3		$	22.8		$	1.2	$	860.2		$	28.5
		Less Than One Year						One Year or Greater					Total
December 31, 2013		Fair			Unrealized			Fair			Unrealized		Fair			Unrealized
(in millions)		Value			Losses			Value			Losses		Value			Losses
Fixed maturities
USD denominated:
U.S. Governments		$	55.3		$	0.5		$	0		$	0	$	55.3		$	0.5
Non-U.S. Governments			36.3			1			0			0		36.3			1
Obligations of states and political subdivisions (2)			154.6			8.8			3.1			0		157.7			8.8
Credit-Financial			88.7			1.7			2.4			0.1		91.1			1.8
Credit-Industrial			85.7			2			2.9			0.2		88.6			2.2
Credit-Utility			21.2			0.7			1.9			0.1		23.1			0.8
Structured securities:
CMO/MBS-agency			58.4			1.2			5.6			0.5		64			1.7
CMBS (2)			24.2			1			1.1			0		25.3			1
ABS-residential			0			0			3.6			0.5		3.6			0.5
ABS-non residential (2)			59.1			0.1			2			0		61.1			0.1
Foreign denominated:
Governments			170.5			5.9			0			0		170.5			5.9
Credit			125.3			4.8			0			0		125.3			4.8
Total fixed maturities			879.3			27.7			22.6			1.4		901.9			29.1
Equity securities			25.5			1.2			0			0		25.5			1.2
Other investments			(1.4	)		1.3			0			0		(1.4	)		1.3
Short-term investments (1)			4.5			0			0			0		4.5			0
Total		$	907.9		$	30.2		$	22.6		$	1.4	$	930.5		$	31.6
(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 5,914 securities, of which 1,215 were in an unrealized loss position for less than one year and 52 were in an unrealized loss position for a period one year or greater as of March 31, 2014. 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 March 31, 2014.
We regularly evaluate our investments for other-than-temporary impairment. For fixed maturity securities, the evaluation for a credit loss is generally based on the present value of expected cash flows of the security as compared to the amortized book value. For structured securities, frequency and severity of loss inputs are used in projecting future cash flows of the securities. Loss frequency is measured as the credit default rate, which includes such factors as loan-to-value ratios and credit scores of borrowers. Loss severity includes such factors as trends in real estate values and proceeds at foreclosure. We also recognize other-than-temporary losses on our fixed maturity securities that we intend to sell.
We did not recognize any other-than-temporary losses on our fixed maturities portfolio for the three months ended March 31, 2014 and 2013, 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 did not recognize any other-than-temporary losses for the three months ended March 31, 2014. We recognized other-than-temporary losses on our equity portfolio of $1.4 million for the three months ended March 31, 2013. In situations where we did not recognize other-than-temporary losses on investments in our equity portfolio, we have evaluated the near-term prospects of the investment in relation to the severity and duration of the impairment and based on that evaluation, have the ability and intent to hold these investments until a recovery of the cost basis.
Realized Gains and Losses
The following table presents the Company’s gross realized investment gains (losses) for the three months ended March 31:
		For the Three Months
		Ended March 31,
(in millions)		2014			2013
Realized gains
Fixed maturities		$	4.6		$	11.5
Equity securities			6.5			0.5
Other investments			12.7			11.1
Short-term investments			0			0.1
Gross realized investment gains			23.8			23.2
Realized losses
Fixed maturities			(3.5	)		(6.5	)
Equity securities			(0.1	)		0
Other investments			(4.9	)		(5.7	)
Short-term investments			(0.2	)		(0.1	)
Other assets			(4.0	)		0
Other-than-temporary impairment losses on equity securities			0			(1.4	)
Gross realized investment and other losses			(12.7	)		(13.7	)
Net realized investment gains and other		$	11.1		$	9.5
As of March 31, 2013, we hedged $1.7 million of certain holdings in non-U.S. Dollar denominated fixed maturity investments with $1.5 million of foreign exchange contracts. We did not have any fixed maturity investments hedged with foreign exchange contracts at March 31, 2014. The net realized effect on income was not significant for the three months ended March 31, 2013.
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 our Consolidated Balance Sheets in “Other investments.” The realized and unrealized gains and losses are included in “Net realized investment gains and other” in our Consolidated Statements of Income. The notional amount of the currency forward contracts was $46.6 million and $73.7 million as of March 31, 2014 and 2013, respectively. The fair value of the currency forward contracts was a gain of $1.2 million and a loss of $0.6 million as of March 31, 2014 and 2013, respectively. For the three months ended March 31, 2014 and 2013, we recognized $2.3 million and $1.6 million in realized gains and $1.3 million and $2.3 million in realized losses, respectively, from the currency forward contracts.
Regulatory Deposits, Pledged Securities and Letters of Credit
At March 31, 2014, the amortized cost and fair value of investments on deposit with U.S. and various regulatory agencies for regulatory purposes were $197.9 million and $205.2 million, respectively. At December 31, 2013, the amortized cost and fair value of investments on deposit with U.S. and various regulatory agencies for regulatory purposes were $196.7 million and $203.4 million, respectively.
At March 31, 2014, investments with an amortized cost of $79.4 million and fair value of $80.4 million were pledged as collateral in support of irrevocable letters of credit in the amount of $51.3 million issued under the terms of certain reinsurance agreements in respect of reported loss and loss expense reserves.
At December 31, 2013, investments with an amortized cost of $96.6 million and fair value of $97.5 million were pledged as collateral in support of irrevocable letters of credit in the amount of $60.2 million issued under the terms of certain reinsurance agreements in respect of reported loss and loss expense reserves.
Our Corporate member’s capital supporting our Lloyd’s business consisted of the following:
		March 31,			December 31,
(in millions)		2014			2013
Fixed maturities, at fair value		$	251.3		$	250.7
Short-term investments, at fair value			0			0.1
Total securities pledged to Lloyd’s		$	251.3		$	250.8
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 March 31, 2014. A description of the valuation techniques we use to measure assets at fair value is as follows:
Fixed Maturities (Available-for-Sale) Levels 1 and 2:
	•		United States Treasury securities are typically valued using Level 1 inputs. For these securities, we obtain fair value measurements from third-party pricing services using quoted prices (unadjusted) in active markets at the reporting date.
	•		United States Government agencies, non-U.S. Government securities, obligations of states and political subdivisions, credit securities and foreign denominated securities are reported at fair value using Level 2 inputs. For these securities, we obtain fair value measurements from third-party pricing services. Observable data may include dealer quotes, market spreads, yield curves, live trading levels, trade execution data, credit information and the security’s terms and conditions, among other things.
	•		CMO/MBS agency, CMO/MBS non-agency, CMBS, ABS residential and ABS non-residential securities are reported at fair value using Level 2 inputs. For these securities, we obtain fair value measurements from third-party pricing services. Observable data may include dealer quotes, market spreads, cash flows, yield curves, live trading levels, trade execution data, market consensus prepayment speeds, credit information and the security’s terms and conditions, among other things.
Equity Securities Level 1: Equity securities are principally reported at fair value using Level 1 inputs. For these securities, we obtain fair value measurements from a third party pricing service using quoted prices (unadjusted) in active markets at the reporting date.
Equity Securities Level 2: We own interests in equity 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 had one reinsurance contract deemed a derivative in 2013. 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 4 “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 months ended March 31, 2014 or 2013.
Based on an analysis of the inputs, our financial assets measured at fair value on a recurring basis at March 31, 2014 and December 31, 2013 have been categorized as follows:
					Fair Value Measurements at
					Reporting Date Using
(in millions)		March 31, 2014			Level 1 (a)			Level 2 (b)			Level 3 (c)
Fixed maturities
USD denominated:
U.S. Governments		$	225.3		$	105.5		$	119.8		$	0
Non-U.S. Governments			70.4			0			70.4			0
Obligations of states and political subdivisions			523.1			0			523.1			0
Credit-Financial			470.4			0			470.4			0
Credit-Industrial			431.1			0			431.1			0
Credit-Utility			151.4			0			151.4			0
Structured securities:
CMO/MBS-agency			207.9			0			207.9			0
CMO/MBS-non agency			19.4			0			19.4			0
CMBS			167.3			0			167.3			0
ABS-residential			6.2			0			6.2			0
ABS-non residential			147.8			0			147.8			0
Foreign denominated:
Governments			187.8			0			187.8			0
Credit			193.7			0			193.7			0
Total fixed maturities			2,801.80			105.5			2,696.30			0
Equity securities			530.3			472.6			56.5			1.2
Other investments			127.2			0			127.2			0
Short-term investments			382.7			378.9			3.8			0
		$	3,842.00		$	957		$	2,883.80		$	1.2
					Fair Value Measurements at
					Reporting Date Using
(in millions)		December 31, 2013			Level 1 (a)			Level 2 (b)			Level 3 (c)
Fixed maturities
USD denominated:
U.S. Governments		$	275.3		$	143.8		$	131.5		$	0
Non-U.S. Governments			60.2			0			60.2			0
Obligations of states and political subdivisions			577.8			0			577.8			0
Credit-Financial			439.9			0			439.9			0
Credit-Industrial			397.1			0			397.1			0
Credit-Utility			166.6			0			166.6			0
Structured securities:
CMO/MBS-agency			226.9			0			226.9			0
CMO/MBS-non agency			20.2			0			20.2			0
CMBS			164.8			0			164.8			0
ABS-residential			6.6			0			6.6			0
ABS-non residential			117.2			0			117.2			0
Foreign denominated:
Governments			206.7			0			206.7			0
Credit			155.1			0			155.1			0
Total fixed maturities			2,814.40			143.8			2,670.60			0
Equity securities			534.3			476.7			56.3			1.3
Other investments			117.8			0			117.8			0
Short-term investments			351.6			347			4.6			0
Other assets			2.6			0			0			2.6
		$	3,820.70		$	967.5		$	2,849.30		$	3.9
(a)	Quoted prices in active markets for identical assets
(b)	Significant other observable inputs
(c)	Significant unobservable inputs
The fair value measurements in the tables above do not agree to “Total investments” on our 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 March 31, 2014 and December 31, 2013 are as follows:
Fair Value Measurements Using Unobservable Inputs (Level 3)
(in millions)		Equity			Other			Total
		Securities			Assets
Beginning balance, January 1, 2014		$	1.3		$	2.6		$	3.9
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 (loss)			0			0			0
Included in other comprehensive income (loss)			0			0			0
Purchases, issuances, sales, and settlements
Purchases			0			0			0
Issuances			0			0			0
Sales			(0.1	)		0			(0.1	)
Settlements			0			(2.6	)		(2.6	)
Ending balance, March 31, 2014		$	1.2		$	0		$	1.2
Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at March 31, 2014		$	0		$	0		$	0
(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 (loss)			0.1			0			0.1
Included in other comprehensive income (loss)			0			0			0
Purchases, issuances, sales, and settlements
Purchases			0			0			0
Issuances			0			0			0
Sales			(0.6	)		0			(0.6	)
Settlements			0			(4.3	)		(4.3	)
Ending balance, December 31, 2013		$	1.3		$	2.6		$	3.9
Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at December 31, 2013		$	0		$	0		$	0
At March 31, 2014 and December 31, 2013, 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.

Derivative_Instruments

Derivative Instruments	3 Months Ended
	Mar. 31, 2014
Derivative Instruments And Hedging Activities Disclosure [Abstract]	'
Derivative Instruments	'
	4. Derivative Instruments
	Through our subsidiary Argo Re, Ltd. (“Argo Re”), we entered into a reinsurance contract 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 contract entered into on December 28, 2011 and effective January 1, 2012, provided coverage of $100 million for hurricanes and earthquakes (including fire) in the U.S. and covered losses for the first and subsequent events on a per-occurrence basis over a 24-month coverage period. This contract expired in December 2013. 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 resulted in a loss to the note holders. This transaction ignored the effects of inuring reinsurance, creating the remote possibility of a double recovery on covered events, and was therefore deemed to be a derivative.
	We recorded the contract 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 our Consolidated Statements of Income. As there is no quoted fair value available for this derivative, 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 months ended March 31, 2013 was $5.1 million, which was incurred due to the change in the fair value of the derivative, principally due to the passage of the transaction’s risk coverage term. Included in “Other assets” in our Consolidated Balance Sheets at December 31, 2013 was $2.6 million, which represented the fair value of the contract at that date. As the contract expired in December 2013, no such expense was incurred during the first quarter of 2014, nor was there an asset on our Consolidated Balance Sheets at March 31, 2014.
	The special purpose reinsurance company that is the counterparty to this transaction is a variable interest entity under the provisions of Accounting Standards Codification (“ASC”) Topic 810-10, “Consolidation.” Argo Group is not the primary beneficiary of this entity and is therefore not required to consolidate it in our consolidated financial statements.
	4. Derivative Instruments
	Through our subsidiary Argo Re, Ltd. (“Argo Re”), we entered into a reinsurance contract 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 contract entered into on December 28, 2011 and effective January 1, 2012, provided coverage of $100 million for hurricanes and earthquakes (including fire) in the U.S. and covered losses for the first and subsequent events on a per-occurrence basis over a 24-month coverage period. This contract expired in December 2013. 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 resulted in a loss to the note holders. This transaction ignored the effects of inuring reinsurance, creating the remote possibility of a double recovery on covered events, and was therefore deemed to be a derivative.
	We recorded the contract 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 our Consolidated Statements of Income. As there is no quoted fair value available for this derivative, 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 months ended March 31, 2013 was $5.1 million, which was incurred due to the change in the fair value of the derivative, principally due to the passage of the transaction’s risk coverage term. Included in “Other assets” in our Consolidated Balance Sheets at December 31, 2013 was $2.6 million, which represented the fair value of the contract at that date. As the contract expired in December 2013, no such expense was incurred during the first quarter of 2014, nor was there an asset on our Consolidated Balance Sheets at March 31, 2014.
	The special purpose reinsurance company that is the counterparty to this transaction is a variable interest entity under the provisions of Accounting Standards Codification (“ASC”) Topic 810-10, “Consolidation.” Argo Group is not the primary beneficiary of this entity and is therefore not required to consolidate it in our consolidated financial statements.

Reserves_for_Losses_and_Loss_A

Reserves for Losses and Loss Adjustment Expenses	3 Months Ended
	Mar. 31, 2014
Insurance [Abstract]	'
Reserves for Losses and Loss Adjustment Expenses	'
	5. Reserves for Losses and Loss Adjustment Expenses
	The following table provides a reconciliation of reserves for losses and loss adjustment expenses (“LAE”):
			For the Three Months Ended
	(in millions)		2014			2013
	Net reserves beginning of the year		$	2,107.60		$	2,110.90
	Add:
	Losses and LAE incurred during current calendar year, net of reinsurance:
	Current accident year			191.4			175
	Prior accident years			(8.9	)		(4.5	)
	Losses and LAE incurred during calendar year, net of reinsurance			182.5			170.5
	Deduct:
	Losses and LAE payments made during current calendar year, net of reinsurance:
	Current accident year			27.9			21.9
	Prior accident years			150.1			159.7
	Losses and LAE payments made during current calendar year, net of reinsurance			178			181.6
	Add:
	Change in participation interest (1)			24.7			10.4
	Foreign exchange adjustments			1.7			(6.5	)
	Net reserves - end of period			2,138.50			2,103.70
	Add:
	Reinsurance recoverables on unpaid losses and LAE, end of period			903.9			1,101.30
	Gross reserves - end of period		$	3,042.40		$	3,205.00
	(1)	Amount represents (decrease) increase in reserves due to change in syndicate participation.
	On December 31, 2012, Syndicate 1200 entered into a retroactive whole account quota share contract covering the 2009 and prior years of account. The counterparty was another syndicate within the Lloyd’s market. As a result of this transaction, reserves for losses and LAE at December 31, 2012 were net of $192.2 million ceded under this quota share agreement. Subject to a Split Reinsurance to Close arrangement previously approved by the Prudential Regulatory Authority, the liabilities assumed by the whole account quota share provider were legally transferred to that quota share provider on January 18, 2014, with an inception date of January 1, 2014.
	Reserves for losses and loss adjustment expenses represent the estimated indemnity cost and related adjustment expenses necessary to investigate and settle claims. Such estimates are based upon individual case estimates for reported claims, estimates from ceding companies for reinsurance assumed and actuarial estimates for losses that have been incurred but not yet reported to the insurer. Any change in probable ultimate liabilities is reflected in current operating results.
	Included in losses and LAE for the three months ended March 31, 2014 was $8.9 million in favorable prior years’ loss reserve development comprised of the following: $8.0 million of net favorable development in the Excess and Surplus Lines segment primarily caused by favorable development of $21.6 million in the general and products liability lines, partially offset by $9.4 million of unfavorable development in commercial automobile and $4.2 million of unfavorable development in property lines; $2.0 million of net unfavorable development in the Commercial Specialty segment, primarily driven by $6.7 million of unfavorable development in general liability due to increases in claim severity, offset by $4.3 million of favorable development in workers compensation and $1.1 million of favorable development in short-tail lines; $0.4 million of net unfavorable development in the International Specialty segment in our Brazil unit; $8.8 million of net favorable development in the Syndicate 1200 segment driven by favorable development in marine and energy lines, as well as favorable development in professional indemnity and general liability; $5.5 million of unfavorable development in the Run-off Lines segment primarily caused by $4.5 million of unfavorable development in workers compensation driven by increasing medical costs on older claims, as well as $1.0 million in asbestos liability on assumed business.
	Included in losses and LAE for the three months ended March 31, 2013 was $4.5 million in favorable prior years’ loss reserve development comprised of the following: $5.2 million of net favorable development in the Excess and Surplus Lines segment primarily caused by favorable development of $5.9 million in the general and products liability lines and $3.3 million in commercial multi-peril liability, partially offset with $4.0 million of unfavorable development in commercial automobile; $1.1 million of net unfavorable development in the Commercial Specialty segment, primarily driven by $2.2 million of unfavorable development in general liability due to increases in claim severity offset by $1.0 million of favorable development in the surety line of business at Rockwood Casualty Insurance Company (“Rockwood”); $0.9 million of net unfavorable development in the International Specialty segment driven by $1.3 million of unfavorable development for the property catastrophe reinsurance unit, primarily due to crop losses, partially offset by $0.5 million of favorable casualty losses for professional lines; $2.2 million of net favorable development in the Syndicate 1200 segment driven by favorable development in property facultative and North American binder business; $0.9 million of unfavorable development in the Run-off Lines segment primarily related to commutations.
	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.

Other_Indebtedness

Other Indebtedness	3 Months Ended
	Mar. 31, 2014
Text Block [Abstract]	'
Other Indebtedness	'
	6. Other Indebtedness
	Borrowing Under Revolving Credit Facility
	On March 7, 2014, the Borrowers entered into a $175.0 million Credit Agreement (“New Credit Agreement”) with JPMorgan Chase Bank, N.A., as administrative agent. The New Credit Agreement replaced and terminated the original $150.0 million Credit Agreement. The New Credit Agreement provides for a $175.0 million revolving credit facility with a maturity date of March 7, 2018 unless extended in accordance with the terms of the New Credit Agreement. Borrowings under the New Credit Agreement may be used for general corporate purposes, including working capital and permitted acquisitions, and each of the Borrowers has agreed to be jointly and severally liable for the obligations of the other Borrowers under the New Credit Agreement.
	Revolving loans can either be designated as “ABR Borrowings” or “Eurocurrency Borrowings” at the time of borrowing by the Borrowers. “ABR Borrowings” will bear interest at a rate per annum equal to the greatest of (a) the Prime Rate, (b) the Federal Funds Effective Rate plus  1⁄2 of 1% and (c) the Adjusted Eurocurrency Rate for a one month interest period plus 1%, plus an applicable margin of 0.375% to 1.25%. “Eurocurrency Borrowings” will bear interest at a rate per annum equal to LIBOR plus an applicable margin of 1.375% to 2.25%. The Adjusted Eurocurrency Rate for Eurocurrency borrowings in U.S. Dollars will be equal to LIBOR times a reserve percentage established in accordance with Regulation D of the U.S. Federal Reserve System.
	The New Credit Agreement contains customary events of default. If an event of default occurs and is continuing, the Borrowers might be required immediately to repay all amounts outstanding under the New Credit Agreement. Lenders holding at least a majority of the loans and commitments under the New Credit Agreement may elect to accelerate the maturity of the loans and/or terminate the commitments under the New Credit Agreement upon the occurrence and during the continuation of an event of default.
	Included in the New Credit Agreement is a provision that allows up to $17.5 million of the revolving credit facility to be used for letters of credit (“LOC’s”), subject to availability. On March 7, 2014, the $0.2 million LOC outstanding under the original $150.0 million Credit Agreement was transferred to the New Credit Agreement. As of March 31, 2014 and December 31, 2013, there were no borrowings outstanding and $0.2 million in LOC’s against the Credit Facility.
	Other Debt
	As part of the ARIS Title Insurance Corporation (“ARIS”) acquisition, at March 31, 2014 and December 31, 2013, we had a note payable for $0.8 million, of which $0.2 million was on a recourse basis. The note has a variable interest rate of 2.00% above 30-day LIBOR, with the variable interest rate being reset quarterly and subject to certain interest rate ceilings. The interest rate at December 31, 2012 was 2.21%. Interest payments are payable quarterly. The note payable matured pursuant to its terms on December 16, 2013. Interest accrued through the maturity date has been paid as scheduled, and the principal amount remains unpaid and outstanding as of March 31, 2014. The parties entered into negotiations to restructure this obligation on mutually acceptable terms in view of the fact that our investment in ARIS has yet to generate meaningful returns. An agreement was not reached by March 31, 2014, and the holder of the note has initiated proceedings to recover the unpaid principal.

Disclosures_about_Fair_Value_o

Disclosures about Fair Value of Financial Instruments	3 Months Ended
	Mar. 31, 2014
Fair Value Disclosures [Abstract]	'
Disclosures about Fair Value of Financial Instruments	'
	7. Disclosures about Fair Value of Financial Instruments
	Cash. The carrying amount approximates fair value.
	Investment securities and short-term investments. See Note 3, “Investments,” for additional information.
	Premiums receivable and reinsurance recoverables on paid losses. The carrying value of current receivables approximates fair value. At March 31, 2014 and December 31, 2013, the carrying values of premiums receivable over 90 days were $19.7 million and $9.8 million, respectively. Included in “Reinsurance recoverables” in our Consolidated Balance Sheets at March 31, 2014 and December 31, 2013, are amounts that are due from third party trade capital providers associated with the operations of Argo International. Upon settlement, the receivable is offset against the liability also reflected in our accompanying Consolidated Balance Sheets. At March 31, 2014 and December 31, 2013, the payable was in excess of the receivable. Of our reinsurance recoverable on paid losses, excluding amounts attributable to Argo International’s third party trade capital providers, at March 31, 2014 and December 31, 2013, the carrying values over 90 days were $15.1 million and $22.2 million, respectively. Our methodology for establishing our allowances for doubtful accounts includes specifically identifying all potential uncollectible balances regardless of aging. Any of the over 90 day balances, where collectability was deemed questionable, have been included in the allowances. At March 31, 2014 and December 31, 2013, the allowance for doubtful accounts for premiums receivable was $4.6 million and $4.5 million, respectively, and the allowance for doubtful accounts for reinsurance recoverables on paid losses was $2.2 million and $1.6 million, respectively. Premiums receivable over 90 days were secured by collateral in the amount of $0.5 million and $0.4 million at March 31, 2014 and December 31, 2013, respectively. Reinsurance recoverables on paid losses over 90 days were secured by collateral in the amount of $0.4 million and $0.3 million at March 31, 2014 and December 31, 2013, respectively.
	At March 31, 2014 and December 31, 2013, 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 March 31, 2014 and December 31, 2013 is shown below:
		March 31, 2014				December 31, 2013
	(in millions)	Carrying		Fair Value		Carrying		Fair Value
		Amount				Amount
	Junior subordinated debentures	$	193.3	$	155.8	$	193.3	$	155.5
	Senior unsecured fixed rate notes		143.8		132.3		143.8		116.2
	Other indebtedness:
	Floating rate loan stock		65.7		53		65.5		52.7
	Note payable		0.8		0.6		0.8		0.6
		$	403.6	$	341.7	$	403.4	$	325

Shareholders_Equity

Shareholders' Equity	3 Months Ended
	Mar. 31, 2014
Equity [Abstract]	'
Shareholders' Equity	'
	8. Shareholders’ Equity
	On February 18, 2014, our Board of Directors declared a quarterly cash dividend in the amount of $0.15 on each share of common stock outstanding. On March 17, 2014, we paid $4.0 million to our shareholders of record on March 3, 2014.
	On February 15, 2013, our Board of Directors declared a quarterly cash dividend in the amount of $0.15 on each shares of common stock outstanding or $0.14 on each share outstanding adjusted for the 10% stock dividend. On March 15, 2013, we paid $3.7 million to our shareholders of record on March 1, 2013.
	On November 13, 2007, our Board of Directors authorized the repurchase of up to $150.0 million of Argo Group’s common shares. 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 superseded the November 13, 2007 repurchase program such that no further amounts could be repurchased under the repurchase program approved in 2007. On November 5, 2013, our Board of Directors authorized the repurchase of up to $150.0 million of our common shares (“2013 Repurchase Authorization”). The 2013 Repurchase Authorization supersedes the 2011 Repurchase Authorization such that no further amounts could be repurchased under the 2011 Repurchase Authorization. As of March 31, 2014, availability under the 2013 Repurchase Authorization for future repurchases of our common shares was $136.1 million.
	For the three months ended March 31, 2014, we repurchased a total of 165,184 common shares for $7.5 million. A summary of activity from January 1, 2014 through March 31, 2014 follows.
	For the three months ended March 31, 2014, there were no common shares repurchased on the open market. In 2014, we repurchased shares under Securities Exchange Act of 1934 Rule 10b5-1 trading plans as follows:
	Date Trading	2014		Number of		Average Price of		Total Cost		Repurchase
	Plan Initiated	Purchase Period		Shares Repurchased		Shares Repurchased		(in millions)		Authorization Year
	12/17/13		02/04/14-02/12/14		15,789	$	43.28	$	0.7		2013
	3/18/14		03/18/14-04/28/14		436,867	$	45.81	$	20		2013

Accumulated_Other_Comprehensiv

Accumulated Other Comprehensive Income	3 Months Ended
	Mar. 31, 2014
Equity [Abstract]	'
Accumulated Other Comprehensive Income	'
	9. Accumulated Other Comprehensive Income
	In February 2013, the FASB issued Accounting Standards Update 2013-02 that amends ASC 220, “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. 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 three months ended March 31 is presented in the following tables:
	(in millions)	Foreign Currency			Unrealized			Defined Benefit			Total
		Translation			Holding Gains on			Pension Plans
		Adjustments			Securities
	Beginning balance, January 1, 2014	$	(11.5	)	$	163.9		$	(4.6	)	$	147.8
	Other comprehensive income before reclassifications		(0.2	)		1.3			0			1.1
	Amounts reclassified from accumulated other comprehensive income		0			0			0			0
	Net current-period other comprehensive income		(0.2	)		1.3			0			1.1
	Ending balance, March 31, 2014	$	(11.7	)	$	165.2		$	(4.6	)	$	148.9
	(in millions)	Foreign Currency			Unrealized			Defined Benefit			Total
		Translation			Holding Gains on			Pension Plans
		Adjustments			Securities
	Beginning balance, January 1, 2013	$	(8.7	)	$	204.1		$	(5.9	)	$	189.5
	Other comprehensive income before reclassifications		0			13.8			0			13.8
	Amounts reclassified from accumulated other comprehensive income		0			(0.8	)		0			(0.8	)
	Net current-period other comprehensive income		0			13			0			13
	Ending balance, March 31, 2013	$	(8.7	)	$	217.1		$	(5.9	)	$	202.5
	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:
		For the Three Months
		Ended March 31,
	(in millions)	2014			2013
	Unrealized gains and losses on securities
	Net realized investment gains	$	(0.1	)	$	(0.7	)
	Provision for income taxes		0.1			(0.1	)
	Net of taxes	$	0		$	(0.8	)

Net_Income_Per_Common_Share

Net Income Per Common Share	3 Months Ended
	Mar. 31, 2014
Earnings Per Share [Abstract]	'
Net Income Per Common Share	'
	10. Net Income Per Common Share
	The following table presents the calculation of net income per common share on a basic and diluted basis:
		For the Three Months
		Ended March 31,
	(in millions, except number of shares and per share amounts)	2014		2013
	Net income	$	40.2	$	32.7
	Weighted average common shares outstanding - basic		26,514,481		27,247,009
	Effect of dilutive securities
	Equity compensation awards		451,980		910,064
	Weighted average common shares outstanding - diluted		26,966,461		28,157,073
	Net income per common share - basic	$	1.52	$	1.2
	Net income per common share - diluted	$	1.49	$	1.16
	Excluded from the weighted average common shares outstanding calculation at March 31, 2014 and 2013 are 7,723,529 shares and 6,785,438 shares, respectively, which are held as treasury shares. The shares are excluded as of their repurchase date. For the three months ended March 31, 2014, equity compensation awards to purchase 3,300 shares of common stock were excluded from the computation of diluted net income per common share as these instruments were anti-dilutive. These instruments expire at varying times from 2014 through 2020. For the three months ended March 31, 2013, equity compensation awards to purchase 214,453 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 2015.

Supplemental_Cash_Flow_Informa

Supplemental Cash Flow Information	3 Months Ended
	Mar. 31, 2014
Supplemental Cash Flow Elements [Abstract]	'
Supplemental Cash Flow Information	'
	11. Supplemental Cash Flow Information
	Income taxes paid. We paid income taxes of $15.6 million and $2.5 million during the three months ended March 31, 2014 and 2013, respectively.
	Interest paid. Interest paid for the three months ended March 31, was as follows:
		For the Three Months
		Ended March 31,
	(in millions)	2014		2013
	Senior unsecured fixed rate notes	$	2.3	$	2.3
	Junior subordinated debentures		2		1.9
	Other indebtedness		0.9		0.9
	Total interest paid	$	5.2	$	5.1

Sharebased_Compensation

Share-based Compensation	3 Months Ended
	Mar. 31, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]	'
Share-based Compensation	'
	12. 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 expected dividend yield is based on our history and expected dividend payouts. The following table summarizes the assumptions we used for the three months ended March 31, 2014 and 2013:
		2014			2013
	Risk-free rate of return		1.74	%		0.8	%
	Expected dividend yields		1.49	%		1.64	%
	Expected award life (years)		4.85			5.07
	Expected volatility		24.91	%		31.9	%
	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. We are prohibited from repricing the options. The term of the share options cannot exceed seven years from the grant date. Options outstanding under this plan vested over a one to five year period, subject to continued employment. As of March 31, 2014, all options under this plan were fully vested. No expense related to these options was recognized for each of the three months ended March 31, 2014 and 2013, as such expenses were fully recognized in prior periods.
	A summary of restricted share activity under the 2007 Plan as of March 31, 2014 and changes during the three months then ended is as follows:
					Weighted-Average
					Grant Date
		Shares			Fair Value
	Outstanding at January 1, 2014		257,975		$	32.97
	Granted		4,373		$	40.08
	Vested and issued		(57,205	)	$	31.58
	Expired or forfeited		(1,036	)	$	40.08
	Outstanding at March 31, 2014		204,107		$	33.48
	The restricted shares vest over two to four years. Expense recognized under this plan for the restricted shares was $0.7 million and $1.0 million for the three months ended March 31, 2014 and 2013, respectively. Compensation expense for all share-based compensation awards is included in “Underwriting, acquisition and insurance expense” in the accompanying Consolidated Statements of Income. As of March 31, 2014, there was $4.8 million of total unrecognized compensation cost related to restricted share compensation arrangements granted by Argo Group.
	A summary of stock-settled SARs activity under the 2007 Plan as of March 31, 2014 and changes during the three months then ended is as follows:
					Weighted-Average
		Shares			Exercise Price
	Outstanding at January 1, 2014		1,288,698		$	29.47
	Granted		28,069		$	36.44
	Exercised		(58,093	)	$	28.01
	Expired or forfeited		(11,237	)	$	36.44
	Outstanding at March 31, 2014		1,247,437		$	29.63
	The stock-settled SARs vest over a one to four year period. Upon exercise of the stock-settled SARs, the employee is entitled to receive shares of our common stock equal to the appreciation of the stock as compared to the exercise price. Expense recognized for the stock-settled SARs was $0.6 million and $0.7 million for the three months ended March 31, 2014 and 2013, respectively. As of March 31, 2014, there was $3.3 million of total unrecognized compensation cost related to stock-settled SARs outstanding.
	A summary of cash-settled SARs activity under the 2007 Plan as of March 31, 2014 and changes during the three months then ended is as follows:
					Weighted-Average
		Shares			Exercise Price
	Outstanding at January 1, 2014		1,743,758		$	33
	Granted		3,574		$	39.75
	Exercised		(81,093	)	$	30.15
	Expired or forfeited		(19,906	)	$	35.9
	Outstanding at March 31, 2014		1,646,333		$	33.09
	The cash-settled SARs vest over a one to four year period. Upon exercise of the cash-settled SARs, the employee is entitled to receive cash payment for the appreciation in the value of our common stock over the exercise price. We account for the cash-settled SARs as liability awards, which require the awards to be revalued at each reporting period. Expense recognized for the cash-settled SARs was $2.7 million and $6.5 million for the three months ended March 31, 2014 and 2013, respectively. As of March 31, 2014, there was $7.2 million of total unrecognized compensation cost related to cash-settled SARs outstanding.
	Argo Group International Holdings Ltd. Deferred Compensation Plan for Non-Employee Directors
	Until December 16, 2013, the non-employee members of our Board of Directors were entitled to participate in the Argo Group International Holdings, Ltd. Deferred Compensation Plan for Non-Employee Directors (“Directors Plan”), a non-funded and non-qualified deferred compensation plan. Under the Directors Plan, non-employee directors can elect each year to defer payment of 0%, 50% or 100% of their cash compensation payable during the next calendar year. While no further deferrals will be permitted under the Directors Plan from and after December 16, 2013, and certain amounts thereunder were paid out prior to the end of 2013, additional deferred amounts remain subject to the terms of the Directors Plan and will be paid out in accordance with the terms of the Directors Plan. Deferred amounts are credited with the interest earned at a rate of two percent above the prime commercial lending rate, to be reset 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. The Directors Plan provided 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. In conjunction with the termination of the Directors Plan, all cash balances and related interest for the years 2010 through 2013 were settled resulting in $1.9 million in cash payments for the year ended December 31, 2013. Remaining distributions from the Directors Plan will occur six months after the non-employee director ceases to be a member of the Board, the date on which a change in control (as defined in the Directors Plan) occurs or December 1, 2017, whichever comes first, and will be made in cash. The non-employee directors are responsible for all tax requirements on the deferred compensation and any related earnings. Under the Directors Plan, our recorded compensation expense was nil and $0.6 million for the three months ended March 31, 2014 and 2013, respectively. The period over period reduction in compensation expense is due to the aforementioned termination and partial payout of the cash deferral component of the Directors Plan.
	Argonaut Group’s Amended and Restated Stock Incentive Plan
	The historical 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. Grants under the Amended Plan have a legal life of seven to eleven years. As of March 31, 2014, all awards under this plan were fully vested. No expense related to these awards was recognized for each of the three months ended March 31, 2014 and 2013, as such expenses were fully recognized in prior periods.

Underwriting_Acquisition_and_I

Underwriting, Acquisition and Insurance Expenses	3 Months Ended
	Mar. 31, 2014
Text Block [Abstract]	'
Underwriting, Acquisition and Insurance Expenses	'
	13. Underwriting, Acquisition and Insurance Expenses
	Underwriting, acquisition and insurance expenses were as follows:
		For the Three Months
		Ended March 31,
	(in millions)	2014			2013
	Commissions	$	54.9		$	51.9
	General expenses		68.8			68.7
	Premium taxes, boards and bureaus		6.6			6.8
			130.3			127.4
	Net deferral of policy acquisition costs		(1.6	)		(0.7	)
	Total underwriting, acquisition and insurance expenses	$	128.7		$	126.7
	Included in general expenses for the three months ended March 31, 2014 and 2013, was $4.3 million and $8.6 million, respectively, in additional expense for our total equity compensation.

Income_Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2014
Income Tax Disclosure [Abstract]	'
Income Taxes	'
	14. Income Taxes
	We are incorporated under the laws of Bermuda and, under current Bermuda law, are not obligated to pay any taxes in Bermuda based upon income or capital gains. We have received an undertaking from the Supervisor of Insurance in Bermuda pursuant to the provisions of the Exempted Undertakings Tax Protection Act, 2011, which exempts us from any Bermuda taxes computed on profits, income or any capital asset, gain or appreciation or any tax in the nature of estate duty or inheritance tax, at least until the year 2035.
	We do not consider ourselves to be engaged in a trade or business in the United States or the United Kingdom and, accordingly, do not expect to be subject to direct United States or United Kingdom income taxation.
	We have subsidiaries based in the United Kingdom that are subject to the tax laws of that country. Under current law, these subsidiaries are taxed at the applicable corporate tax rates. Six of the United Kingdom subsidiaries are deemed to be engaged in business in the United States, and therefore, are subject to United States corporate tax in respect of a proportion of their United States underwriting business only. Relief is available against the United Kingdom tax liabilities in respect of overseas taxes paid that arise from the underwriting business. Corporate income tax losses incurred in the United Kingdom can be carried forward, for application against future income, indefinitely. Our United Kingdom subsidiaries file separate United Kingdom income tax returns.
	We have subsidiaries based in the United States that are subject to United States tax laws. Under current law, these subsidiaries are taxed at the applicable corporate tax rates. Our United States subsidiaries file a consolidated United States federal income tax return.
	We also have operations in Belgium, Switzerland, Brazil, France, Malta, Spain and Ireland, which also are subject to income taxes imposed by the jurisdiction in which they operate. We have operations in the United Arab Emirates, which are not subject to income tax under the laws of that country.
	Our income tax provision includes the following components:
			For the Three Months
			Ended March 31,
	(in millions)		2014			2013
	Current tax provision		$	1.4		$	1.8
	Deferred tax provision related to:
	Future tax deductions			30.2			2.4
	Valuation allowance change			(29.1	)		0.6
	Income tax provision		$	2.5		$	4.8
	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 months ended March 31, 2014 and 2013, pre-tax income (loss) attributable to our operations and the operations’ effective tax rates were as follows:
			For the Three Months Ended March 31,
	(in millions)		2014						2013
			Pre-tax			Effective Tax			Pre-tax			Effective Tax
			income (loss)			Rate			income (loss)			Rate
	Bermuda		$	21.9			0	%	$	16.1			0	%
	United States			14.8			20.6	%		17.5			18.3	%
	United Kingdom			7.3			-7.7	%		6			25.5	%
	Belgium			0	(1)		18.1	%		0	(1)		-67.6	%
	Brazil			0			0	%		(1.8	)		0	%
	Dubai			(0.9	)		0	%		0			0	%
	Malta			(0.4	)		0	%		(0.3	)		0	%
	Switzerland			0	(1)		-19.8	%		0	(1)		24.5	%
	Pre-tax income (loss)		$	42.7					$	37.5
	(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 is as follows:
			For the Three Months
			Ended March 31,
	(in millions)		2014			2013
	Income tax provision at expected rate		$	4.7		$	6.7
	Tax effect of:
	Tax-exempt interest			(1.2	)		(1.4	)
	Dividends received deduction			(0.6	)		(0.6	)
	Valuation allowance change			(29.1	)		0.6
	Other permanent adjustments, net			0.1			(0.9	)
	Adjustment for annualized rate			0.8			0.4
	United States state tax benefit			0.1			(0.2	)
	PXRE Reinsurance capital loss carryforward			29.8			0
	Foreign exchange adjustments			(2.2	)		0.2
	Foreign withholding taxes			0.1			0
	Income tax provision		$	2.5		$	4.8
	Income tax (benefit) provision - Foreign		$	(0.6	)	$	1.6
	Income tax provision - United States Federal			2.8			3.5
	Income tax provision (benefit) - United States State			0.2			(0.3	)
	Foreign withholding tax - United States			0.1			0
	Income tax provision		$	2.5		$	4.8
	We recognize potential accrued interest and penalties within our global operations in “Interest expense” and “Underwriting, acquisition and insurance expenses,” respectively, in our Consolidated Statements of Income.
	Our net deferred tax assets (liabilities) are supported by taxes paid in previous periods, reversal of the taxable temporary differences and recognition of future income. Management regularly evaluates the recoverability of the deferred tax assets and makes any necessary adjustments to them based upon any changes in management’s expectations of future taxable income. Realization of deferred tax assets is dependent upon our generation of sufficient taxable income in the future to recover tax benefits that cannot be recovered from taxes paid in the carryback period, which is generally two years for net operating losses and three years for capital losses for our United States operations. At March 31, 2014, we had a total net deferred tax liability of $7.0 million prior to any valuation allowance. Management has concluded that a valuation allowance is required for a portion of the tax effected net operating loss carryforward of $18.5 million from PXRE Corporation and for the tax effected net operating loss carryforward of $1.0 million from ARIS. The capital loss carryforward generated from the sale of PXRE Reinsurance Company expired on December 31, 2013. Of the PXRE Corporation loss carryforwards, $17.0 million will expire if not used by December 31, 2025 and $1.5 million will expire if not used by December 31, 2027. Of the ARIS loss carryforward, $0.2 million will expire if not used by December 31, 2027, $0.4 million will expire if not used by December 31, 2028 and $0.4 million will expire if not used by December 31, 2029. The valuation allowances have been established as Internal Revenue Code Section 382 limits the application of net operating loss and net capital loss carryforwards following an ownership change. The loss carryforwards available per year are $2.8 million as required by Internal Revenue Code Section 382. Further, due to cumulative losses since inception, management has concluded that a valuation allowance is required for the full amount of the tax effected net operating losses generated by our Brazil and Malta entities. Accordingly, a valuation allowance of $27.2 million is required as of March 31, 2014 of which $16.6 million relates to the PXRE Corporation and ARIS loss carryforwards, $8.8 million relates to Brazil operations and $1.8 million relates to Malta operations. For the three months ended March 31, 2014, the valuation allowance was reduced by $29.8 million pertaining to the expiration of the PXRE Reinsurance Company capital loss carryforward, reduced by $0.2 million pertaining to the PXRE Corporation and ARIS loss carryforwards, increased by $0.8 million pertaining to the Brazil operations and increased by $0.1 million pertaining to the Malta operations.
	We had no material unrecognized tax benefits as of March 31, 2014 and 2013. Our United States subsidiaries are no longer subject to U.S. federal and state income tax examinations by tax authorities for years before 2010. 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.

Commitments_and_Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2014
Commitments And Contingencies Disclosure [Abstract]	'
Commitments and Contingencies	'
	15. 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.
	Argo Group has contractual commitments to invest up to $26.3 million related to its limited partnership investments at March 31, 2014. These commitments will be funded as required by the partnership agreements.

Segment_Information

Segment Information	3 Months Ended
	Mar. 31, 2014
Segment Reporting [Abstract]	'
Segment Information	'
	16. 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 months ended March 31, 2014 and 2013 were as follows:
		For the Three Months
		Ended March 31,
	(in millions)	2014			2013
	Revenue:
	Earned premiums
	Excess and Surplus Lines	$	117.4		$	105.1
	Commercial Specialty		71.7			75.2
	International Specialty		37			32.7
	Syndicate 1200		99.3			90.9
	Run-off Lines		0.3			0.3
	Total earned premiums		325.7			304.2
	Net investment income
	Excess and Surplus Lines		9.3			10.9
	Commercial Specialty		4.7			5.9
	International Specialty		1.8			2.3
	Syndicate 1200		3.7			2.8
	Run-off Lines		2.5			2.8
	Corporate and Other		1.3			3.2
	Total net investment income		23.3			27.9
	Net realized investment gains and other		11.1			9.5
	Total revenue	$	360.1		$	341.6
		For the Three Months
		Ended March 31,
	(in millions)	2014			2013
	Income (loss) before income taxes
	Excess and Surplus Lines	$	20.1		$	13.9
	Commercial Specialty		1.7			5.7
	International Specialty		6.8			4.8
	Syndicate 1200		15.8			8.4
	Run-off Lines		(5.9	)		(0.4	)
	Total segment income before taxes		38.5			32.4
	Corporate and Other		(6.9	)		(4.4	)
	Net realized investment gains and other		11.1			9.5
	Total income before income taxes	$	42.7		$	37.5
	The table below presents earned premiums by geographic location for the three months ended March 31, 2014 and 2013. 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
		Ended March 31,
	(in millions)	2014			2013
	Bermuda	$	24.4		$	25.8
	Brazil		12.6			10
	Malta		0.5			0.4
	United Kingdom		98.9			87
	United States		189.3			181
	Total earned premiums	$	325.7		$	304.2
	The following table presents our identifiable assets.
		March 31,			December 31,
	(in millions)	2014			2013
	Excess and Surplus Lines	$	2,310.10		$	2,252.20
	Commercial Specialty		1,332.60			1,333.40
	International Specialty		781.2			743.6
	Syndicate 1200		1,390.90			1,638.80
	Run-off Lines		556.1			555.9
	Corporate and Other		60.5			67.1
	Total	$	6,431.40		$	6,591.00
	Included in total assets at March 31, 2014 and December 31, 2013 are $246.4 million and $386.8 million, respectively, in assets associated with trade capital providers.

Information_Provided_in_Connec

Information Provided in Connection with Outstanding Debt of Subsidiaries	3 Months Ended
	Mar. 31, 2014
Text Block [Abstract]	'
Information Provided in Connection with Outstanding Debt of Subsidiaries	'
	17. Information Provided in Connection with Outstanding Debt of Subsidiaries
	The following tables present condensed consolidating financial information at March 31, 2014 and December 31, 2013 and for the three months ended March 31, 2014 and 2013, 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
	March 31, 2014
	(in millions)
			Argo Group			Argo Group US, Inc.			Other Subsidiaries and			Consolidating			Total
			International			and Subsidiaries			Eliminations (1)			Adjustments (2)
			Holdings, Ltd (Parent			(Subsidiary Issuer)
			Guarantor)
	Assets
	Investments		$	4.6		$	2,797.90		$	1,316.00		$	0		$	4,118.50
	Cash			0			118.4			31.1			0			149.5
	Accrued investment income			0			18.6			6.4			0			25
	Premiums receivable			0			153.5			217.6			0			371.1
	Reinsurance recoverables			0			1,165.90			(187.8	)		0			978.1
	Goodwill and other intangible assets, net			0			136.9			101.6			0			238.5
	Current income taxes receivable, net			0			5.6			4.8			0			10.4
	Deferred acquisition costs, net			0			54.4			63.1			0			117.5
	Ceded unearned premiums			0			106.8			145.1			0			251.9
	Other assets			6.4			107.4			57.1			0			170.9
	Due from affiliates			0.2			4.7			(4.7	)		(0.2	)		0
	Intercompany note receivable			0			88			(88.0	)		0			0
	Investments in subsidiaries			1,666.10			0			0			(1,666.1	)		0
	Total assets		$	1,677.30		$	4,758.10		$	1,662.30		$	(1,666.3	)	$	6,431.40
	Liabilities and Shareholders’ Equity
	Reserves for losses and loss adjustment expenses		$	0		$	2,174.10		$	868.3		$	0		$	3,042.40
	Unearned premiums			0			421.8			364.5			0			786.3
	Funds held and ceded reinsurance payable, net			0			682.2			(290.1	)		0			392.1
	Long-term debt			49			288.9			65.7			0			403.6
	Deferred tax liabilities, net			0			27.6			6.6			0			34.2
	Accrued underwriting expenses and other liabilities			14.2			97			66.7			0			177.9
	Intercompany note payable			19.2			0			0			(19.2	)		0
	Total liabilities			82.4			3,691.60			1,081.70			(19.2	)		4,836.50
	Total shareholders’ equity			1,594.90			1,066.50			580.6			(1,647.1	)		1,594.90
	Total liabilities and shareholders’ equity		$	1,677.30		$	4,758.10		$	1,662.30		$	(1,666.3	)	$	6,431.40
	CONDENSED CONSOLIDATING BALANCE SHEET
	December 31, 2013
	(in millions)
			Argo Group			Argo Group US, Inc.			Other Subsidiaries and			Consolidating			Total
			International			and Subsidiaries			Eliminations (1)			Adjustments (2)
			Holdings, Ltd (Parent			(Subsidiary Issuer)
			Guarantor)
	Assets
	Investments		$	1.1		$	2,785.10		$	1,293.00		$	0		$	4,079.20
	Cash			0			132.1			25.3			0			157.4
	Accrued investment income			0			19.7			6			0			25.7
	Premiums receivable			0			150.8			197.6			0			348.4
	Reinsurance recoverables			0			1,145.90			117.6			0			1,263.50
	Goodwill and other intangible assets, net			0			137.4			102.4			0			239.8
	Current income taxes receivable, net			0			0.5			(0.5	)		0			0
	Deferred acquisition costs, net			0			55.6			58.3			0			113.9
	Ceded unearned premiums			0			88.2			108.1			0			196.3
	Other assets			4.6			97.8			64.4			0			166.8
	Due from affiliates			3.1			1.5			(1.5	)		(3.1	)		0
	Intercompany note receivable			0			87.4			(87.4	)		0			0
	Investments in subsidiaries			1,625.20			0			0			(1,625.2	)		0
	Total assets		$	1,634.00		$	4,702.00		$	1,883.30		$	(1,628.3	)	$	6,591.00
	Liabilities and Shareholders’ Equity
	Reserves for losses and loss adjustment expenses		$	0		$	2,157.00		$	1,073.30		$	0		$	3,230.30
	Unearned premiums			0			425.5			353.6			0			779.1
	Funds held and ceded reinsurance payable, net			0			642.4			(244.0	)		0			398.4
	Long-term debt			49			288.9			65.5			0			403.4
	Current income taxes payable, net			0			8.9			(3.7	)		0			5.2
Deferred tax liabilities, net			0			22.8			5.9			0			28.7
Accrued underwriting expenses and other liabilities			15.4			98.2			69.3			0			182.9
Intercompany note payable			6.6			0			0			(6.6	)		0
Total liabilities			71			3,643.70			1,319.90			(6.6	)		5,028.00
Total shareholders’ equity			1,563.00			1,058.30			563.4			(1,621.7	)		1,563.00
Total liabilities and shareholders’ equity		$	1,634.00		$	4,702.00		$	1,883.30		$	(1,628.3	)	$	6,591.00
(1)	Includes all other subsidiaries of Argo Group International Holdings, Ltd and all intercompany eliminations
(2)	Includes all Argo Group parent company eliminations
CONDENSED CONSOLIDATING STATEMENT OF INCOME
FOR THE THREE MONTHS ENDED MARCH 31, 2014
(in millions)
		Argo Group 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		$	112		$	213.7		$	0		$	325.7
Net investment income			(0.1	)		15			8.4			0			23.3
Net realized investment gains and other			0			11.5			(0.4	)		0			11.1
Total revenue			(0.1	)		138.5			221.7			0			360.1
Expenses:
Losses and loss adjustment expenses			0			72.5			110			0			182.5
Underwriting, acquisition and insurance expenses			6			46.3			76.4			0			128.7
Interest expense			0.6			3.8			0.7			(0.1	)		5
Fee expense, net			0			1.3			0.1			0			1.4
Foreign currency exchange gain			0			(0.1	)		(0.1	)		0			(0.2	)
Total expenses			6.6			123.8			187.1			(0.1	)		317.4
Income (loss) before income taxes			(6.7	)		14.7			34.6			0.1			42.7
Provision for income taxes			0			3			(0.5	)		0			2.5
Net (loss) income before equity in earnings of subsidiaries			(6.7	)		11.7			35.1			0.1			40.2
Equity in undistributed earnings of subsidiaries			46.9			0			0			(46.9	)		0
Net income		$	40.2		$	11.7		$	35.1		$	(46.8	)	$	40.2
(1)	Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations
(2)	Includes all Argo Group parent company eliminations
CONDENSED CONSOLIDATING STATEMENT OF INCOME
FOR THE THREE MONTHS ENDED MARCH 31, 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		$	107.7		$	196.5		$	0		$	304.2
Net investment income			0			20.9			7.3			(0.3	)		27.9
Fee (expense) income, net			0			(0.3	)		0.3			0			0
Net realized investment gains and other			0			7.6			1.9			0			9.5
Total revenue			0			135.9			206			(0.3	)		341.6
Expenses:
Losses and loss adjustment expenses			0			64.3			106.2			0			170.5
Other reinsurance-related expenses			0			1.5			3.6			0			5.1
Underwriting, acquisition and insurance expenses			6.2			48.7			71.8			0			126.7
Interest expense			0.8			3.7			0.7			(0.3	)		4.9
Foreign currency exchange (gain) loss			(0.1	)		0.2			(3.2	)		0			(3.1	)
Total expenses			6.9			118.4			179.1			(0.3	)		304.1
Income (loss) before income taxes			(6.9	)		17.5			26.9			0			37.5
Provision for income taxes			0			3.2			1.6			0			4.8
Net income (loss) before equity in earnings of subsidiaries			(6.9	)		14.3			25.3			0			32.7
Equity in undistributed earnings of subsidiaries			39.6			0			0			(39.6	)		0
Net income		$	32.7		$	14.3		$	25.3		$	(39.6	)	$	32.7
(1)	Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations
(2)	Includes all Argo Group parent company eliminations
CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS
FOR THE THREE MONTHS ENDED MARCH 31, 2014
(in millions)
		Argo Group			Argo Group US, Inc.			Other Subsidiaries			Consolidating			Total
		International Holdings,			and Subsidiaries			and Eliminations (1)			Adjustments (2)
		Ltd (Parent Guarantor)			(Subsidiary Issuer)
Net cash flows from operating activities		$	(9.4	)	$	(11.2	)		64.2		$	0		$	43.6
Cash flows from investing activities:
Proceeds from sales of investments			0			163.3			100			0			263.3
Proceeds from maturities and mandatory calls of investments			0			51.3			27.1			0			78.4
Purchases of investments			0			(178.6	)		(128.7	)		0			(307.3	)
Change in short-term investments and foreign regulatory deposits			(0.9	)		(29.8	)		(23.2	)		0			(53.9	)
Settlements of foreign currency exchange forward contracts			0.9			0			(2.3	)		0			(1.4	)
Issuance of intercompany note, net			0			0			(12.5	)		12.5			0
Other, net			0.1			(5.1	)		(19.1	)		0			(24.1	)
Cash provided (used) by investing activities			0.1			1.1			(58.7	)		12.5			(45.0	)
Cash flows from financing activities:
Borrowings under intercompany note, net			12.5			0			0			(12.5	)		0
Activity under stock incentive plans			0.8			0			0			0			0.8
Repurchase of Company’s common shares			0			(3.7	)		0			0			(3.7	)
Excess tax expense from share-based payment arrangements			0			0.1			0			0			0.1
Payment of cash dividends to common shareholders			(4.0	)		0			0			0			(4.0	)
Cash provided (used) by financing activities			9.3			(3.6	)		0			(12.5	)		(6.8	)
Effect of exchange rate changes on cash			0			0			0.3			0			0.3
Change in cash			0			(13.7	)		5.8			0			(7.9	)
Cash, beginning of period			0			132.1			25.3			0			157.4
Cash, end of period		$	0		$	118.4		$	31.1		$	0		$	149.5
(1)	Includes all other subsidiaries of Argo Group International Holdings, Ltd and all intercompany eliminations
(2)	Includes all Argo Group parent company eliminations
CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS
FOR THE THREE MONTHS ENDED MARCH 31, 2013
(in millions)
		Argo Group			Argo Group US, Inc.			Other Subsidiaries			Consolidating			Total
		International Holdings,			and Subsidiaries			and Eliminations (1)			Adjustments (2)
		Ltd (Parent Guarantor)			(Subsidiary Issuer)
Net cash flows from operating activities		$	(0.5	)	$	2.7		$	(86.2	)	$	0		$	(84.0	)
Cash flows from investing activities:
Proceeds from sales of investments			0			199			237.2			0			436.2
Proceeds from maturities and mandatory calls of investments			0			86.7			28.4			0			115.1
Purchases of investments			0			(282.9	)		(159.9	)		0			(442.8	)
Change in short-term investments and foreign regulatory deposits			(2.2	)		28.6			(32.8	)		0			(6.4	)
Settlements of foreign currency exchange forward contracts			0.2			0			(2.1	)		0			(1.9	)
Issuance of intercompany note, net			0			(10.0	)		5.1			4.9			0
Other, net			0			(6.2	)		0.7			0			(5.5	)
Cash (used) provided by investing activities			(2.0	)		15.2			76.6			4.9			94.7
Cash flows from financing activities:
Borrowings under intercompany note, net			4.9			0			0			(4.9	)		0
Proceeds from issuance of senior unsecured fixed rate notes			1.3			0			(1.3	)		0			0
Activity under stock incentive plans			0			0			1.3			0			1.3
Repurchase of Company’s common shares			0			(12.9	)		0			0			(12.9	)
Payment of cash dividends to common shareholders			(3.7	)		0			0			0			(3.7	)
Cash provided (used) by financing activities			2.5			(12.9	)		0			(4.9	)		(15.3	)
Effect of exchange rate changes on cash			0			0			0			0			0
Change in cash			0			5			(9.6	)		0			(4.6	)
Cash, beginning of period			0			73.9			21.9			0			95.8
Cash, end of period		$	0		$	78.9		$	12.3		$	0		$	91.2
(1)	Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations
(2)	Includes all Argo Group parent company eliminations

Basis_of_Presentation_Policies

Basis of Presentation (Policies)	3 Months Ended
	Mar. 31, 2014
Accounting Policies [Abstract]	'
Basis of Presentation	'
	Basis of Presentation
	The accompanying consolidated financial statements of Argo Group International Holdings, Ltd. (“Argo Group,” “we” or the “Company”) and its subsidiaries have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) for interim financial information and with the instructions for Form 10-Q and Article 10 of Regulation S-X. The preparation of interim financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenues and expenses during the reporting period. The major estimates reflected in our consolidated financial statements include, but are not limited to, reserves for losses and loss adjustment expenses; reinsurance recoverables, including the reinsurance recoverables allowance for doubtful accounts; estimates of written and earned premiums; reinsurance premium receivable; fair value of investments and assessment of potential impairment; valuation of goodwill and intangibles and our deferred tax asset valuation allowance. Actual results could differ from those estimates. Certain financial information that normally is included in annual financial statements, including certain financial statement footnotes, prepared in accordance with GAAP, is not required for interim reporting purposes and has been condensed or omitted. These statements should be read in conjunction with the consolidated financial statements and notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2013, filed with the Securities and Exchange Commission on February 28, 2014.
	The interim financial information as of, and for the three months ended, March 31, 2014 and 2013 is unaudited. However, in the opinion of management, the interim information includes all adjustments, consisting of normal recurring accruals, necessary for a fair presentation of the results presented for the interim periods. The operating results for the interim periods are not necessarily indicative of the results to be expected for the full year. All significant intercompany amounts have been eliminated in consolidation. Certain amounts in prior years’ financial statements have been reclassified to conform to the current presentation.
10% Stock Dividend	'
	10% Stock Dividend
	On 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 our common shares. All references to share and per share amounts in this document and related disclosures have been adjusted to reflect the stock dividend for all periods presented.
Recently Issued Accounting Standards	'
	Recently Issued Accounting Standards
	In February 2013, the Financial Accounting Standards Board (“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. The adoption of this amendment did not have a material impact on our financial results or 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 or 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. The adoption of this amendment did not have an impact on our financial results or disclosures.
	In July 2013, the FASB issued amendments to “Income Taxes” (Topic 740) that will require the presentation of certain unrecognized tax benefits as reductions to deferred tax assets rather than liabilities in our Consolidated Balance Sheets when a net operating loss carryforward, similar tax loss or tax credit carryforward exists. The new standard requires adoption on a prospective basis in the first quarter of 2015; however, early adoption is permitted. We do not anticipate this update will have an impact on our financial results and disclosures.
	In April 2014, the FASB issued amendments to “Presentation of Financial Statements (Topic 205) and Property, Plant and Equipment (Topic 360): Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity.” The amended guidance requires that a disposal representing a strategic shift that has (or will have) a major effect on an entity’s financial results or a business activity classified as held for sale should be reported as discontinued operations. The amendments also expand the disclosure requirements for discontinued operations and add new disclosures for individually significant dispositions that do not qualify as discontinued operations. The amendments are effective prospectively for fiscal years, and interim reporting periods within those years, beginning after December 15, 2014. Early adoption is permitted only for disposals that have not been previously reported. We do not anticipate this update will have an impact on our financial results and disclosures.

Investments_Tables

Investments (Tables)	3 Months Ended
	Mar. 31, 2014
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 March 31, 2014 and December 31, 2013 were as follows:
						Gross			Gross
	March 31, 2014		Amortized			Unrealized			Unrealized			Fair
	(in millions)		Cost			Gains			Losses			Value
	Fixed maturities
	USD denominated:
	U.S. Governments		$	223.6		$	2.1		$	0.4		$	225.3
	Non-U.S. Governments			70.4			0.7			0.7			70.4
	Obligations of states and political subdivisions			504.2			23.2			4.3			523.1
	Credit-Financial			455.9			15.8			1.3			470.4
	Credit-Industrial			418			14.3			1.2			431.1
	Credit-Utility			146.1			5.6			0.3			151.4
	Structured securities:
	CMO/MBS-agency (1)			200.1			8.9			1.1			207.9
	CMO/MBS-non agency			18.4			1			0			19.4
	CMBS (2)			164.5			3.3			0.5			167.3
	ABS-residential (3)			6.3			0.3			0.4			6.2
	ABS-non residential			147.3			0.6			0.1			147.8
	Foreign denominated:
	Governments			187.8			8.4			8.4			187.8
	Credit			196.4			5.2			7.9			193.7
	Total fixed maturities			2,739.00			89.4			26.6			2,801.80
	Equity securities			345.9			186.3			1.9			530.3
	Other investments			399.7			4			0			403.7
	Short-term investments			382.7			0			0			382.7
	Total investments		$	3,867.30		$	279.7		$	28.5		$	4,118.50
						Gross			Gross
	December 31, 2013		Amortized			Unrealized			Unrealized			Fair
	(in millions)		Cost			Gains			Losses			Value
	Fixed maturities
	USD denominated:
	U.S. Governments		$	273.4		$	2.4		$	0.5		$	275.3
	Non-U.S. Governments			60.9			0.3			1			60.2
	Obligations of states and political subdivisions			563.7			22.9			8.8			577.8
	Credit-Financial			426.3			15.4			1.8			439.9
	Credit-Industrial			385.5			13.8			2.2			397.1
	Credit-Utility			162			5.4			0.8			166.6
	Structured securities:
	CMO/MBS-agency (1)			219.4			9.2			1.7			226.9
	CMO/MBS-non agency			19.4			0.8			0			20.2
	CMBS (2)			162.7			3.1			1			164.8
	ABS-residential (3)			6.8			0.3			0.5			6.6
	ABS-non residential			116.8			0.5			0.1			117.2
	Foreign denominated:
	Governments			207.7			4.9			5.9			206.7
	Credit			155.5			4.4			4.8			155.1
	Total fixed maturities			2,760.10			83.4			29.1			2,814.40
	Equity securities			346.9			188.6			1.2			534.3
	Other investments			377.4			2.8			1.3			378.9
	Short-term investments			351.6			0			0			351.6
	Total investments		$	3,836.00		$	274.8		$	31.6		$	4,079.20
	(1)	Collateralized mortgage obligations/mortgage-backed securities (“CMO/MBS”).
	(2)	Commercial mortgage-backed securities (“CMBS”).
	The amortized cost, gross unrealized gains, gross unrealized losses and fair value of investments as of March 31, 2014 and December 31, 2013 were as follows:
						Gross			Gross
	March 31, 2014		Amortized			Unrealized			Unrealized			Fair
	(in millions)		Cost			Gains			Losses			Value
	Fixed maturities
	USD denominated:
	U.S. Governments		$	223.6		$	2.1		$	0.4		$	225.3
	Non-U.S. Governments			70.4			0.7			0.7			70.4
	Obligations of states and political subdivisions			504.2			23.2			4.3			523.1
	Credit-Financial			455.9			15.8			1.3			470.4
Credit-Industrial			418			14.3			1.2			431.1
Credit-Utility			146.1			5.6			0.3			151.4
Structured securities:
CMO/MBS-agency (1)			200.1			8.9			1.1			207.9
CMO/MBS-non agency			18.4			1			0			19.4
CMBS (2)			164.5			3.3			0.5			167.3
ABS-residential (3)			6.3			0.3			0.4			6.2
ABS-non residential			147.3			0.6			0.1			147.8
Foreign denominated:
Governments			187.8			8.4			8.4			187.8
Credit			196.4			5.2			7.9			193.7
Total fixed maturities			2,739.00			89.4			26.6			2,801.80
Equity securities			345.9			186.3			1.9			530.3
Other investments			399.7			4			0			403.7
Short-term investments			382.7			0			0			382.7
Total investments		$	3,867.30		$	279.7		$	28.5		$	4,118.50
					Gross			Gross
December 31, 2013		Amortized			Unrealized			Unrealized			Fair
(in millions)		Cost			Gains			Losses			Value
Fixed maturities
USD denominated:
U.S. Governments		$	273.4		$	2.4		$	0.5		$	275.3
Non-U.S. Governments			60.9			0.3			1			60.2
Obligations of states and political subdivisions			563.7			22.9			8.8			577.8
Credit-Financial			426.3			15.4			1.8			439.9
Credit-Industrial			385.5			13.8			2.2			397.1
Credit-Utility			162			5.4			0.8			166.6
Structured securities:
CMO/MBS-agency (1)			219.4			9.2			1.7			226.9
CMO/MBS-non agency			19.4			0.8			0			20.2
CMBS (2)			162.7			3.1			1			164.8
ABS-residential (3)			6.8			0.3			0.5			6.6
ABS-non residential			116.8			0.5			0.1			117.2
Foreign denominated:
Governments			207.7			4.9			5.9			206.7
Credit			155.5			4.4			4.8			155.1
Total fixed maturities			2,760.10			83.4			29.1			2,814.40
Equity securities			346.9			188.6			1.2			534.3
Other investments			377.4			2.8			1.3			378.9
Short-term investments			351.6			0			0			351.6
Total investments		$	3,836.00		$	274.8		$	31.6		$	4,079.20
(1)	Collateralized mortgage obligations/mortgage-backed securities (“CMO/MBS”).
(2)	Commercial mortgage-backed securities (“CMBS”).
(3)	Asset-backed securities (“ABS”).
Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity	'
	The amortized cost and fair values of fixed maturity investments as of March 31, 2014, by contractual maturity, were as follows:
			Amortized			Fair
	(in millions)		Cost			Value
	Due in one year or less		$	228.7		$	229.8
	Due after one year through five years			1,267.40			1,286.20
	Due after five years through ten years			533			561.7
	Thereafter			173.3			175.5
	Structured securities			536.6			548.6
	Total		$	2,739.00		$	2,801.80
Schedule of Aging of Unrealized Losses on Company's Investments in Fixed Maturities, Equity Securities and Other Investments	'
	An aging of unrealized losses on our investments in fixed maturities, equity securities, other investments and short-term investments as of March 31, 2014 and December 31, 2013 is presented below:
			Less Than One Year						One Year or Greater					Total
	March 31, 2014		Fair			Unrealized			Fair			Unrealized		Fair			Unrealized
	(in millions)		Value			Losses			Value			Losses		Value			Losses
	Fixed maturities
	USD denominated:
	U.S. Governments		$	34.6		$	0.4		$	0		$	0	$	34.6		$	0.4
	Non-U.S. Governments			31.7			0.6			0.7			0.1		32.4			0.7
	Obligations of states and political subdivisions (2)			142.8			4.3			3.8			0		146.6			4.3
	Credit-Financial (2)			84			1.3			3			0		87			1.3
	Credit-Industrial (2)			90.8			1.2			2.1			0		92.9			1.2
	Credit-Utility			18.9			0.2			1.3			0.1		20.2			0.3
	Structured securities:
	CMO/MBS-agency			39.5			0.6			5.5			0.5		45			1.1
	CMBS			15.6			0.4			1.8			0.1		17.4			0.5
	ABS-residential			0			0			3.6			0.4		3.6			0.4
	ABS-non residential (2)			59.2			0.1			1			0		60.2			0.1
	Foreign denominated:
	Governments			155.9			8.4			0			0		155.9			8.4
	Credit			132.4			7.9			0			0		132.4			7.9
	Total fixed maturities			805.4			25.4			22.8			1.2		828.2			26.6
	Equity securities			29.4			1.9			0			0		29.4			1.9
	Short-term investments (1)			2.6			0			0			0		2.6			0
	Total		$	837.4		$	27.3		$	22.8		$	1.2	$	860.2		$	28.5
			Less Than One Year						One Year or Greater					Total
	December 31, 2013		Fair			Unrealized			Fair			Unrealized		Fair			Unrealized
	(in millions)		Value			Losses			Value			Losses		Value			Losses
	Fixed maturities
	USD denominated:
	U.S. Governments		$	55.3		$	0.5		$	0		$	0	$	55.3		$	0.5
	Non-U.S. Governments			36.3			1			0			0		36.3			1
	Obligations of states and political subdivisions (2)			154.6			8.8			3.1			0		157.7			8.8
	Credit-Financial			88.7			1.7			2.4			0.1		91.1			1.8
	Credit-Industrial			85.7			2			2.9			0.2		88.6			2.2
	Credit-Utility			21.2			0.7			1.9			0.1		23.1			0.8
	Structured securities:
	CMO/MBS-agency			58.4			1.2			5.6			0.5		64			1.7
	CMBS (2)			24.2			1			1.1			0		25.3			1
	ABS-residential			0			0			3.6			0.5		3.6			0.5
	ABS-non residential (2)			59.1			0.1			2			0		61.1			0.1
	Foreign denominated:
	Governments			170.5			5.9			0			0		170.5			5.9
	Credit			125.3			4.8			0			0		125.3			4.8
	Total fixed maturities			879.3			27.7			22.6			1.4		901.9			29.1
	Equity securities			25.5			1.2			0			0		25.5			1.2
	Other investments			(1.4	)		1.3			0			0		(1.4	)		1.3
	Short-term investments (1)			4.5			0			0			0		4.5			0
	Total		$	907.9		$	30.2		$	22.6		$	1.4	$	930.5		$	31.6
	(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 months ended March 31:
			For the Three Months
			Ended March 31,
	(in millions)		2014			2013
	Realized gains
	Fixed maturities		$	4.6		$	11.5
	Equity securities			6.5			0.5
	Other investments			12.7			11.1
	Short-term investments			0			0.1
	Gross realized investment gains			23.8			23.2
	Realized losses
	Fixed maturities			(3.5	)		(6.5	)
	Equity securities			(0.1	)		0
	Other investments			(4.9	)		(5.7	)
	Short-term investments			(0.2	)		(0.1	)
	Other assets			(4.0	)		0
	Other-than-temporary impairment losses on equity securities			0			(1.4	)
	Gross realized investment and other losses			(12.7	)		(13.7	)
	Net realized investment gains and other		$	11.1		$	9.5
Securities and Letters of Credit Pledged to Lloyd's as Company's Corporate Member's Capital	'
	Our Corporate member’s capital supporting our Lloyd’s business consisted of the following:
			March 31,			December 31,
	(in millions)		2014			2013
	Fixed maturities, at fair value		$	251.3		$	250.7
	Short-term investments, at fair value			0			0.1
	Total securities pledged to Lloyd’s		$	251.3		$	250.8
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 March 31, 2014 and December 31, 2013 have been categorized as follows:
						Fair Value Measurements at
						Reporting Date Using
	(in millions)		March 31, 2014			Level 1 (a)			Level 2 (b)			Level 3 (c)
	Fixed maturities
	USD denominated:
	U.S. Governments		$	225.3		$	105.5		$	119.8		$	0
	Non-U.S. Governments			70.4			0			70.4			0
	Obligations of states and political subdivisions			523.1			0			523.1			0
	Credit-Financial			470.4			0			470.4			0
	Credit-Industrial			431.1			0			431.1			0
	Credit-Utility			151.4			0			151.4			0
	Structured securities:
	CMO/MBS-agency			207.9			0			207.9			0
	CMO/MBS-non agency			19.4			0			19.4			0
	CMBS			167.3			0			167.3			0
	ABS-residential			6.2			0			6.2			0
	ABS-non residential			147.8			0			147.8			0
	Foreign denominated:
	Governments			187.8			0			187.8			0
	Credit			193.7			0			193.7			0
	Total fixed maturities			2,801.80			105.5			2,696.30			0
	Equity securities			530.3			472.6			56.5			1.2
	Other investments			127.2			0			127.2			0
	Short-term investments			382.7			378.9			3.8			0
			$	3,842.00		$	957		$	2,883.80		$	1.2
						Fair Value Measurements at
						Reporting Date Using
	(in millions)		December 31, 2013			Level 1 (a)			Level 2 (b)			Level 3 (c)
	Fixed maturities
	USD denominated:
	U.S. Governments		$	275.3		$	143.8		$	131.5		$	0
	Non-U.S. Governments			60.2			0			60.2			0
	Obligations of states and political subdivisions			577.8			0			577.8			0
	Credit-Financial			439.9			0			439.9			0
	Credit-Industrial			397.1			0			397.1			0
	Credit-Utility			166.6			0			166.6			0
	Structured securities:
	CMO/MBS-agency			226.9			0			226.9			0
	CMO/MBS-non agency			20.2			0			20.2			0
	CMBS			164.8			0			164.8			0
	ABS-residential			6.6			0			6.6			0
	ABS-non residential			117.2			0			117.2			0
	Foreign denominated:
	Governments			206.7			0			206.7			0
	Credit			155.1			0			155.1			0
	Total fixed maturities			2,814.40			143.8			2,670.60			0
	Equity securities			534.3			476.7			56.3			1.3
	Other investments			117.8			0			117.8			0
	Short-term investments			351.6			347			4.6			0
	Other assets			2.6			0			0			2.6
			$	3,820.70		$	967.5		$	2,849.30		$	3.9
	(a)	Quoted prices in active markets for identical assets
	(b)	Significant other observable inputs
	(c)	Significant unobservable inputs
Schedule of Reconciliation of Beginning and Ending Balances for Investments Categorized as Level 3	'
	A reconciliation of the beginning and ending balances for the investments categorized as Level 3 at March 31, 2014 and December 31, 2013 are as follows:
	Fair Value Measurements Using Unobservable Inputs (Level 3)
	(in millions)		Equity			Other			Total
			Securities			Assets
	Beginning balance, January 1, 2014		$	1.3		$	2.6		$	3.9
	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 (loss)			0			0			0
	Included in other comprehensive income (loss)			0			0			0
	Purchases, issuances, sales, and settlements
	Purchases			0			0			0
	Issuances			0			0			0
	Sales			(0.1	)		0			(0.1	)
	Settlements			0			(2.6	)		(2.6	)
	Ending balance, March 31, 2014		$	1.2		$	0		$	1.2
	Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at March 31, 2014		$	0		$	0		$	0
	(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 (loss)			0.1			0			0.1
	Included in other comprehensive income (loss)			0			0			0
	Purchases, issuances, sales, and settlements
	Purchases			0			0			0
	Issuances			0			0			0
	Sales			(0.6	)		0			(0.6	)
	Settlements			0			(4.3	)		(4.3	)
	Ending balance, December 31, 2013		$	1.3		$	2.6		$	3.9
	Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at December 31, 2013		$	0		$	0		$	0

Reserves_for_Losses_and_Loss_A1

Reserves for Losses and Loss Adjustment Expenses (Tables)	3 Months Ended
	Mar. 31, 2014
Insurance [Abstract]	'
Reinsurance Recoverables	'
	The following table provides a reconciliation of reserves for losses and loss adjustment expenses (“LAE”):
			For the Three Months Ended
	(in millions)		2014			2013
	Net reserves beginning of the year		$	2,107.60		$	2,110.90
	Add:
	Losses and LAE incurred during current calendar year, net of reinsurance:
	Current accident year			191.4			175
	Prior accident years			(8.9	)		(4.5	)
	Losses and LAE incurred during calendar year, net of reinsurance			182.5			170.5
	Deduct:
	Losses and LAE payments made during current calendar year, net of reinsurance:
	Current accident year			27.9			21.9
	Prior accident years			150.1			159.7
	Losses and LAE payments made during current calendar year, net of reinsurance			178			181.6
	Add:
	Change in participation interest (1)			24.7			10.4
	Foreign exchange adjustments			1.7			(6.5	)
	Net reserves - end of period			2,138.50			2,103.70
	Add:
	Reinsurance recoverables on unpaid losses and LAE, end of period			903.9			1,101.30
	Gross reserves - end of period		$	3,042.40		$	3,205.00
	(1)	Amount represents (decrease) increase in reserves due to change in syndicate participation.

Disclosures_about_Fair_Value_o1

Disclosures about Fair Value of Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2014
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 March 31, 2014 and December 31, 2013 is shown below:
		March 31, 2014				December 31, 2013
	(in millions)	Carrying		Fair Value		Carrying		Fair Value
		Amount				Amount
	Junior subordinated debentures	$	193.3	$	155.8	$	193.3	$	155.5
	Senior unsecured fixed rate notes		143.8		132.3		143.8		116.2
	Other indebtedness:
	Floating rate loan stock		65.5		52.8		65.5		52.7
	Note payable		0.8		0.6		0.8		0.6
		$	403.4	$	341.5	$	403.4	$	325

Shareholders_Equity_Tables

Shareholders' Equity (Tables)	3 Months Ended
	Mar. 31, 2014
Equity [Abstract]	'
Schedule of Repurchase of Shares	'
	In 2014, we repurchased shares under Securities Exchange Act of 1934 Rule 10b5-1 trading plans as follows:
	Date Trading	2014		Number of		Average Price of		Total Cost		Repurchase
	Plan Initiated	Purchase Period		Shares Repurchased		Shares Repurchased		(in millions)		Authorization Year
	12/17/13		02/04/14-02/12/14		15,789	$	43.28	$	0.7		2013
	3/18/14		03/18/14-04/28/14		436,867	$	45.81	$	20		2013

Accumulated_Other_Comprehensiv1

Accumulated Other Comprehensive Income (Tables)	3 Months Ended
	Mar. 31, 2014
Equity [Abstract]	'
Changes in Accumulated Other Comprehensive Income (Loss)	'
	A summary of changes in accumulated other comprehensive income (loss), net of taxes (where applicable) by component for the three months ended March 31 is presented in the following tables:
	(in millions)	Foreign Currency			Unrealized			Defined Benefit			Total
		Translation			Holding Gains on			Pension Plans
		Adjustments			Securities
	Beginning balance, January 1, 2014	$	(11.5	)	$	163.9		$	(4.6	)	$	147.8
	Other comprehensive income before reclassifications		(0.2	)		1.3			0			1.1
	Amounts reclassified from accumulated other comprehensive income		0			0			0			0
	Net current-period other comprehensive income		(0.2	)		1.3			0			1.1
	Ending balance, March 31, 2014	$	(11.7	)	$	165.2		$	(4.6	)	$	148.9
	(in millions)	Foreign Currency			Unrealized			Defined Benefit			Total
		Translation			Holding Gains on			Pension Plans
		Adjustments			Securities
	Beginning balance, January 1, 2013	$	(8.7	)	$	204.1		$	(5.9	)	$	189.5
	Other comprehensive income before reclassifications		0			13.8			0			13.8
	Amounts reclassified from accumulated other comprehensive income		0			(0.8	)		0			(0.8	)
	Net current-period other comprehensive income		0			13			0			13
	Ending balance, March 31, 2013	$	(8.7	)	$	217.1		$	(5.9	)	$	202.5
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	'
	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:
		For the Three Months
		Ended March 31,
	(in millions)	2014			2013
	Unrealized gains and losses on securities
	Net realized investment gains	$	(0.1	)	$	(0.7	)
	Provision for income taxes		0.1			(0.1	)
	Net of taxes	$	0		$	(0.8	)

Net_Income_Per_Common_Share_Ta

Net Income Per Common Share (Tables)	3 Months Ended
	Mar. 31, 2014
Earnings Per Share [Abstract]	'
Net Income Per Common Share on Basic and Diluted Basis	'
	The following table presents the calculation of net income per common share on a basic and diluted basis:
		For the Three Months
		Ended March 31,
	(in millions, except number of shares and per share amounts)	2014		2013
	Net income	$	40.2	$	32.7
	Weighted average common shares outstanding - basic		26,514,481		27,247,009
	Effect of dilutive securities
	Equity compensation awards		451,980		910,064
	Weighted average common shares outstanding - diluted		26,966,461		28,157,073
	Net income per common share - basic	$	1.52	$	1.2
	Net income per common share - diluted	$	1.49	$	1.16

Supplemental_Cash_Flow_Informa1

Supplemental Cash Flow Information (Tables)	3 Months Ended
	Mar. 31, 2014
Supplemental Cash Flow Elements [Abstract]	'
Schedule of Interest Paid	'
	Interest paid. Interest paid for the three months ended March 31, was as follows:
		For the Three Months
		Ended March 31,
	(in millions)	2014		2013
	Senior unsecured fixed rate notes	$	2.3	$	2.3
	Junior subordinated debentures		2		1.9
	Other indebtedness		0.9		0.9
	Total interest paid	$	5.2	$	5.1

Sharebased_Compensation_Tables

Share-based Compensation (Tables)	3 Months Ended
	Mar. 31, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]	'
Fair Value Assumptions	'
	The expected dividend yield is based on our history and expected dividend payouts. The following table summarizes the assumptions we used for the three months ended March 31, 2014 and 2013:
		2014			2013
	Risk-free rate of return		1.74	%		0.8	%
	Expected dividend yields		1.49	%		1.64	%
	Expected award life (years)		4.85			5.07
	Expected volatility		24.91	%		31.9	%
Summary of Restricted Share Units Activity Under 2007 Plan	'
	A summary of restricted share activity under the 2007 Plan as of March 31, 2014 and changes during the three months then ended is as follows:
					Weighted-Average
					Grant Date
		Shares			Fair Value
	Outstanding at January 1, 2014		257,975		$	32.97
	Granted		4,373		$	40.08
	Vested and issued		(57,205	)	$	31.58
	Expired or forfeited		(1,036	)	$	40.08
	Outstanding at March 31, 2014		204,107		$	33.48
Summary of Stock-Settled SARs Activity Under 2007 Plan	'
	A summary of stock-settled SARs activity under the 2007 Plan as of March 31, 2014 and changes during the three months then ended is as follows:
					Weighted-Average
		Shares			Exercise Price
	Outstanding at January 1, 2014		1,288,698		$	29.47
	Granted		28,069		$	36.44
	Exercised		(58,093	)	$	28.01
	Expired or forfeited		(11,237	)	$	36.44
	Outstanding at March 31, 2014		1,247,437		$	29.63
Summary of Cash-Settled SARs Activity Under 2007 Plan	'
	A summary of cash-settled SARs activity under the 2007 Plan as of March 31, 2014 and changes during the three months then ended is as follows:
					Weighted-Average
		Shares			Exercise Price
	Outstanding at January 1, 2014		1,743,758		$	33
	Granted		3,574		$	39.75
	Exercised		(81,093	)	$	30.15
	Expired or forfeited		(19,906	)	$	35.9
	Outstanding at March 31, 2014		1,646,333		$	33.09

Underwriting_Acquisition_and_I1

Underwriting, Acquisition and Insurance Expenses (Tables)	3 Months Ended
	Mar. 31, 2014
Text Block [Abstract]	'
Underwriting, Acquisition and Insurance Expenses	'
	Underwriting, acquisition and insurance expenses were as follows:
		For the Three Months
		Ended March 31,
	(in millions)	2014			2013
	Commissions	$	54.9		$	51.9
	General expenses		68.8			68.7
	Premium taxes, boards and bureaus		6.6			6.8
			130.3			127.4
	Net deferral of policy acquisition costs		(1.6	)		(0.7	)
	Total underwriting, acquisition and insurance expenses	$	128.7		$	126.7

Income_Taxes_Tables

Income Taxes (Tables)	3 Months Ended
	Mar. 31, 2014
Income Tax Disclosure [Abstract]	'
Income Tax Provision (Benefit)	'
	Our income tax provision includes the following components:
			For the Three Months
			Ended March 31,
	(in millions)		2014			2013
	Current tax provision		$	1.4		$	1.8
	Deferred tax provision related to:
	Future tax deductions			30.2			2.4
	Valuation allowance change			(29.1	)		0.6
	Income tax provision		$	2.5		$	4.8
Schedule of Pre-Tax Income (Loss) and Effective Income Tax Rates	'
	For the three months ended March 31, 2014 and 2013, pre-tax income (loss) attributable to our operations and the operations’ effective tax rates were as follows:
			For the Three Months Ended March 31,
	(in millions)		2014						2013
			Pre-tax			Effective Tax			Pre-tax			Effective Tax
			income (loss)			Rate			income (loss)			Rate
	Bermuda		$	21.9			0	%	$	16.1			0	%
	United States			14.8			20.6	%		17.5			18.3	%
	United Kingdom			7.3			-7.7	%		6			25.5	%
	Belgium			0	(1)		18.1	%		0	(1)		-67.6	%
	Brazil			0			0	%		(1.8	)		0	%
	Dubai			(0.9	)		0	%		0			0	%
	Malta			(0.4	)		0	%		(0.3	)		0	%
	Switzerland			0	(1)		-19.8	%		0	(1)		24.5	%
	Pre-tax income (loss)		$	42.7					$	37.5
	(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 is as follows:
			For the Three Months
			Ended March 31,
	(in millions)		2014			2013
	Income tax provision at expected rate		$	4.7		$	6.7
	Tax effect of:
	Tax-exempt interest			(1.2	)		(1.4	)
	Dividends received deduction			(0.6	)		(0.6	)
	Valuation allowance change			(29.1	)		0.6
	Other permanent adjustments, net			0.1			(0.9	)
	Adjustment for annualized rate			0.8			0.4
	United States state tax benefit			0.1			(0.2	)
	PXRE Reinsurance capital loss carryforward			29.8			0
	Foreign exchange adjustments			(2.2	)		0.2
	Foreign withholding taxes			0.1			0
	Income tax provision		$	2.5		$	4.8
	Income tax (benefit) provision - Foreign		$	(0.6	)	$	1.6
	Income tax provision - United States Federal			2.8			3.5
	Income tax provision (benefit) - United States State			0.2			(0.3	)
	Foreign withholding tax - United States			0.1			0
	Income tax provision		$	2.5		$	4.8

Segment_Information_Tables

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2014
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 months ended March 31, 2014 and 2013 were as follows:
		For the Three Months
		Ended March 31,
	(in millions)	2014			2013
	Revenue:
	Earned premiums
	Excess and Surplus Lines	$	117.4		$	105.1
	Commercial Specialty		71.7			75.2
	International Specialty		37			32.7
	Syndicate 1200		99.3			90.9
	Run-off Lines		0.3			0.3
	Total earned premiums		325.7			304.2
	Net investment income
	Excess and Surplus Lines		9.3			10.9
	Commercial Specialty		4.7			5.9
	International Specialty		1.8			2.3
	Syndicate 1200		3.7			2.8
	Run-off Lines		2.5			2.8
	Corporate and Other		1.3			3.2
	Total net investment income		23.3			27.9
	Net realized investment gains and other		11.1			9.5
	Total revenue	$	360.1		$	341.6
		For the Three Months
		Ended March 31,
	(in millions)	2014			2013
	Income (loss) before income taxes
	Excess and Surplus Lines	$	20.1		$	13.9
	Commercial Specialty		1.7			5.7
	International Specialty		6.8			4.8
	Syndicate 1200		15.8			8.4
	Run-off Lines		(5.9	)		(0.4	)
	Total segment income before taxes		38.5			32.4
	Corporate and Other		(6.9	)		(4.4	)
	Net realized investment gains and other		11.1			9.5
	Total income before income taxes	$	42.7		$	37.5
Schedule of Earned Premiums by Geographic Location	'
	The table below presents earned premiums by geographic location for the three months ended March 31, 2014 and 2013. 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
		Ended March 31,
	(in millions)	2014			2013
	Bermuda	$	24.4		$	25.8
	Brazil		12.6			10
	Malta		0.5			0.4
	United Kingdom		98.9			87
	United States		189.3			181
	Total earned premiums	$	325.7		$	304.2
Identifiable Assets	'
	The following table presents our identifiable assets.
		March 31,			December 31,
	(in millions)	2014			2013
	Excess and Surplus Lines	$	2,310.10		$	2,252.20
	Commercial Specialty		1,332.60			1,333.40
	International Specialty		781.2			743.6
	Syndicate 1200		1,390.90			1,638.80
	Run-off Lines		556.1			555.9
	Corporate and Other		60.5			67.1
	Total	$	6,431.40		$	6,591.00

Information_Provided_in_Connec1

Information Provided in Connection with Outstanding Debt of Subsidiaries (Tables)	3 Months Ended
	Mar. 31, 2014
Text Block [Abstract]	'
Condensed Consolidating Balance Sheet	'
	CONDENSED CONSOLIDATING BALANCE SHEET
	March 31, 2014
	(in millions)
			Argo Group			Argo Group US, Inc.			Other Subsidiaries and			Consolidating			Total
			International			and Subsidiaries			Eliminations (1)			Adjustments (2)
			Holdings, Ltd (Parent			(Subsidiary Issuer)
			Guarantor)
	Assets
	Investments		$	4.6		$	2,797.90		$	1,316.00		$	0		$	4,118.50
	Cash			0			118.4			31.1			0			149.5
	Accrued investment income			0			18.6			6.4			0			25
	Premiums receivable			0			153.5			217.6			0			371.1
	Reinsurance recoverables			0			1,165.90			(187.8	)		0			978.1
	Goodwill and other intangible assets, net			0			136.9			101.6			0			238.5
	Current income taxes receivable, net			0			5.6			4.8			0			10.4
	Deferred acquisition costs, net			0			54.4			63.1			0			117.5
	Ceded unearned premiums			0			106.8			145.1			0			251.9
	Other assets			6.4			107.4			57.1			0			170.9
	Due from affiliates			0.2			4.7			(4.7	)		(0.2	)		0
	Intercompany note receivable			0			88			(88.0	)		0			0
	Investments in subsidiaries			1,666.10			0			0			(1,666.1	)		0
	Total assets		$	1,677.30		$	4,758.10		$	1,662.30		$	(1,666.3	)	$	6,431.40
	Liabilities and Shareholders’ Equity
	Reserves for losses and loss adjustment expenses		$	0		$	2,174.10		$	868.3		$	0		$	3,042.40
	Unearned premiums			0			421.8			364.5			0			786.3
	Funds held and ceded reinsurance payable, net			0			682.2			(290.1	)		0			392.1
	Long-term debt			49			288.9			65.7			0			403.6
	Deferred tax liabilities, net			0			27.6			6.6			0			34.2
	Accrued underwriting expenses and other liabilities			14.2			97			66.7			0			177.9
	Intercompany note payable			19.2			0			0			(19.2	)		0
	Total liabilities			82.4			3,691.60			1,081.70			(19.2	)		4,836.50
	Total shareholders’ equity			1,594.90			1,066.50			580.6			(1,647.1	)		1,594.90
	Total liabilities and shareholders’ equity		$	1,677.30		$	4,758.10		$	1,662.30		$	(1,666.3	)	$	6,431.40
	CONDENSED CONSOLIDATING BALANCE SHEET
	December 31, 2013
	(in millions)
			Argo Group			Argo Group US, Inc.			Other Subsidiaries and			Consolidating			Total
			International			and Subsidiaries			Eliminations (1)			Adjustments (2)
			Holdings, Ltd (Parent			(Subsidiary Issuer)
			Guarantor)
	Assets
	Investments		$	1.1		$	2,785.10		$	1,293.00		$	0		$	4,079.20
	Cash			0			132.1			25.3			0			157.4
	Accrued investment income			0			19.7			6			0			25.7
	Premiums receivable			0			150.8			197.6			0			348.4
	Reinsurance recoverables			0			1,145.90			117.6			0			1,263.50
	Goodwill and other intangible assets, net			0			137.4			102.4			0			239.8
	Current income taxes receivable, net			0			0.5			(0.5	)		0			0
	Deferred acquisition costs, net			0			55.6			58.3			0			113.9
	Ceded unearned premiums			0			88.2			108.1			0			196.3
	Other assets			4.6			97.8			64.4			0			166.8
	Due from affiliates			3.1			1.5			(1.5	)		(3.1	)		0
	Intercompany note receivable			0			87.4			(87.4	)		0			0
	Investments in subsidiaries			1,625.20			0			0			(1,625.2	)		0
	Total assets		$	1,634.00		$	4,702.00		$	1,883.30		$	(1,628.3	)	$	6,591.00
	Liabilities and Shareholders’ Equity
	Reserves for losses and loss adjustment expenses		$	0		$	2,157.00		$	1,073.30		$	0		$	3,230.30
	Unearned premiums			0			425.5			353.6			0			779.1
	Funds held and ceded reinsurance payable, net			0			642.4			(244.0	)		0			398.4
	Long-term debt			49			288.9			65.5			0			403.4
	Current income taxes payable, net			0			8.9			(3.7	)		0			5.2
	Deferred tax liabilities, net			0			22.8			5.9			0			28.7
	Accrued underwriting expenses and other liabilities			15.4			98.2			69.3			0			182.9
Intercompany note payable			6.6			0			0			(6.6	)		0
Total liabilities			71			3,643.70			1,319.90			(6.6	)		5,028.00
Total shareholders’ equity			1,563.00			1,058.30			563.4			(1,621.7	)		1,563.00
Total liabilities and shareholders’ equity		$	1,634.00		$	4,702.00		$	1,883.30		$	(1,628.3	)	$	6,591.00
(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 MARCH 31, 2014
	(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		$	112		$	213.7		$	0		$	325.7
	Net investment income			(0.1	)		15			8.4			0			23.3
	Net realized investment gains and other			0			11.5			(0.4	)		0			11.1
	Total revenue			(0.1	)		138.5			221.7			0			360.1
	Expenses:
	Losses and loss adjustment expenses			0			72.5			110			0			182.5
	Underwriting, acquisition and insurance expenses			6			46.3			76.4			0			128.7
	Interest expense			0.6			3.8			0.7			(0.1	)		5
	Fee expense, net			0			1.3			0.1			0			1.4
	Foreign currency exchange gain			0			(0.1	)		(0.1	)		0			(0.2	)
	Total expenses			6.6			123.8			187.1			(0.1	)		317.4
	Income (loss) before income taxes			(6.7	)		14.7			34.6			0.1			42.7
	Provision for income taxes			0			3			(0.5	)		0			2.5
	Net (loss) income before equity in earnings of subsidiaries			(6.7	)		11.7			35.1			0.1			40.2
	Equity in undistributed earnings of subsidiaries			46.9			0			0			(46.9	)		0
	Net income		$	40.2		$	11.7		$	35.1		$	(46.8	)	$	40.2
	(1)	Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations
	(2)	Includes all Argo Group parent company eliminations
	CONDENSED CONSOLIDATING STATEMENT OF INCOME
	FOR THE THREE MONTHS ENDED MARCH 31, 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		$	107.7		$	196.5		$	0		$	304.2
	Net investment income			0			20.9			7.3			(0.3	)		27.9
	Fee (expense) income, net			0			(0.3	)		0.3			0			0
	Net realized investment gains and other			0			7.6			1.9			0			9.5
	Total revenue			0			135.9			206			(0.3	)		341.6
	Expenses:
	Losses and loss adjustment expenses			0			64.3			106.2			0			170.5
	Other reinsurance-related expenses			0			1.5			3.6			0			5.1
	Underwriting, acquisition and insurance expenses			6.2			48.7			71.8			0			126.7
	Interest expense			0.8			3.7			0.7			(0.3	)		4.9
	Foreign currency exchange (gain) loss			(0.1	)		0.2			(3.2	)		0			(3.1	)
	Total expenses			6.9			118.4			179.1			(0.3	)		304.1
	Income (loss) before income taxes			(6.9	)		17.5			26.9			0			37.5
	Provision for income taxes			0			3.2			1.6			0			4.8
	Net income (loss) before equity in earnings of subsidiaries			(6.9	)		14.3			25.3			0			32.7
	Equity in undistributed earnings of subsidiaries			39.6			0			0			(39.6	)		0
	Net income		$	32.7		$	14.3		$	25.3		$	(39.6	)	$	32.7
	(1)	Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations
	(2)	Includes all Argo Group parent company eliminations
Condensed Consolidating Statement of Cash Flows	'
	CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS
	FOR THE THREE MONTHS ENDED MARCH 31, 2014
	(in millions)
			Argo Group			Argo Group US, Inc.			Other Subsidiaries			Consolidating			Total
			International Holdings,			and Subsidiaries			and Eliminations (1)			Adjustments (2)
			Ltd (Parent Guarantor)			(Subsidiary Issuer)
	Net cash flows from operating activities		$	(9.4	)	$	(11.2	)		64.2		$	0		$	43.6
	Cash flows from investing activities:
	Proceeds from sales of investments			0			163.3			100			0			263.3
	Proceeds from maturities and mandatory calls of investments			0			51.3			27.1			0			78.4
	Purchases of investments			0			(178.6	)		(128.7	)		0			(307.3	)
	Change in short-term investments and foreign regulatory deposits			(0.9	)		(29.8	)		(23.2	)		0			(53.9	)
	Settlements of foreign currency exchange forward contracts			0.9			0			(2.3	)		0			(1.4	)
	Issuance of intercompany note, net			0			0			(12.5	)		12.5			0
	Other, net			0.1			(5.1	)		(19.1	)		0			(24.1	)
	Cash provided (used) by investing activities			0.1			1.1			(58.7	)		12.5			(45.0	)
	Cash flows from financing activities:
	Borrowings under intercompany note, net			12.5			0			0			(12.5	)		0
	Activity under stock incentive plans			0.8			0			0			0			0.8
	Repurchase of Company’s common shares			0			(3.7	)		0			0			(3.7	)
	Excess tax expense from share-based payment arrangements			0			0.1			0			0			0.1
	Payment of cash dividends to common shareholders			(4.0	)		0			0			0			(4.0	)
	Cash provided (used) by financing activities			9.3			(3.6	)		0			(12.5	)		(6.8	)
	Effect of exchange rate changes on cash			0			0			0.3			0			0.3
	Change in cash			0			(13.7	)		5.8			0			(7.9	)
	Cash, beginning of period			0			132.1			25.3			0			157.4
	Cash, end of period		$	0		$	118.4		$	31.1		$	0		$	149.5
	(1)	Includes all other subsidiaries of Argo Group International Holdings, Ltd and all intercompany eliminations
	(2)	Includes all Argo Group parent company eliminations
	CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS
	FOR THE THREE MONTHS ENDED MARCH 31, 2013
	(in millions)
			Argo Group			Argo Group US, Inc.			Other Subsidiaries			Consolidating			Total
			International Holdings,			and Subsidiaries			and Eliminations (1)			Adjustments (2)
			Ltd (Parent Guarantor)			(Subsidiary Issuer)
	Net cash flows from operating activities		$	(0.5	)	$	2.7		$	(86.2	)	$	0		$	(84.0	)
	Cash flows from investing activities:
	Proceeds from sales of investments			0			199			237.2			0			436.2
	Proceeds from maturities and mandatory calls of investments			0			86.7			28.4			0			115.1
	Purchases of investments			0			(282.9	)		(159.9	)		0			(442.8	)
	Change in short-term investments and foreign regulatory deposits			(2.2	)		28.6			(32.8	)		0			(6.4	)
	Settlements of foreign currency exchange forward contracts			0.2			0			(2.1	)		0			(1.9	)
	Issuance of intercompany note, net			0			(10.0	)		5.1			4.9			0
	Other, net			0			(6.2	)		0.7			0			(5.5	)
	Cash (used) provided by investing activities			(2.0	)		15.2			76.6			4.9			94.7
	Cash flows from financing activities:
	Borrowings under intercompany note, net			4.9			0			0			(4.9	)		0
	Proceeds from issuance of senior unsecured fixed rate notes			1.3			0			(1.3	)		0			0
	Activity under stock incentive plans			0			0			1.3			0			1.3
	Repurchase of Company’s common shares			0			(12.9	)		0			0			(12.9	)
	Payment of cash dividends to common shareholders			(3.7	)		0			0			0			(3.7	)
	Cash provided (used) by financing activities			2.5			(12.9	)		0			(4.9	)		(15.3	)
	Effect of exchange rate changes on cash			0			0			0			0			0
	Change in cash			0			5			(9.6	)		0			(4.6	)
	Cash, beginning of period			0			73.9			21.9			0			95.8
	Cash, end of period		$	0		$	78.9		$	12.3		$	0		$	91.2
	(1)	Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations
	(2)	Includes all Argo Group parent company eliminations

Basis_of_Presentation_Addition

Basis of Presentation - Additional Information (Detail)	3 Months Ended		6 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Jun. 30, 2013	7-May-13
Organization Consolidation And Presentation Of Financial Statements [Abstract]	'	'	'	'
Stock dividend declared	'	'	'	10.00%
Additional stock issued as dividend	'	'	2,447,839	'
Dividends declaration date	'	'	7-May-13	'
Dividends payable date	'	'	17-Jun-13	'
Dividends payable date of record	3-Mar-14	1-Mar-13	3-Jun-13	'

Investments_Schedule_of_Amorti

Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	$3,867.30	$3,836
Gross Unrealized Gains	279.7	274.8
Gross Unrealized Losses	28.5	31.6
Fair Value	4,118.50	4,079.20
Other investments [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	399.7	377.4
Gross Unrealized Gains	4	2.8
Gross Unrealized Losses	0	1.3
Fair Value	403.7	378.9
Fixed maturities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	2,739	2,760.10
Gross Unrealized Gains	89.4	83.4
Gross Unrealized Losses	26.6	29.1
Fair Value	2,801.80	2,814.40
Fixed maturities [Member] | U.S. Governments [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	223.6	273.4
Gross Unrealized Gains	2.1	2.4
Gross Unrealized Losses	0.4	0.5
Fair Value	225.3	275.3
Fixed maturities [Member] | Non-U.S. Governments [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	70.4	60.9
Gross Unrealized Gains	0.7	0.3
Gross Unrealized Losses	0.7	1
Fair Value	70.4	60.2
Fixed maturities [Member] | Obligations of states and political subdivisions [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	504.2	563.7
Gross Unrealized Gains	23.2	22.9
Gross Unrealized Losses	4.3	8.8
Fair Value	523.1	577.8
Fixed maturities [Member] | Credit-Financial [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	455.9	426.3
Gross Unrealized Gains	15.8	15.4
Gross Unrealized Losses	1.3	1.8
Fair Value	470.4	439.9
Fixed maturities [Member] | Credit-Industrial [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	418	385.5
Gross Unrealized Gains	14.3	13.8
Gross Unrealized Losses	1.2	2.2
Fair Value	431.1	397.1
Fixed maturities [Member] | Credit-Utility [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	146.1	162
Gross Unrealized Gains	5.6	5.4
Gross Unrealized Losses	0.3	0.8
Fair Value	151.4	166.6
Fixed maturities [Member] | CMO/MBS-agency [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	200.1	219.4
Gross Unrealized Gains	8.9	9.2
Gross Unrealized Losses	1.1	1.7
Fair Value	207.9	226.9
Fixed maturities [Member] | CMO/MBS-non agency [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	18.4	19.4
Gross Unrealized Gains	1	0.8
Gross Unrealized Losses	0	0
Fair Value	19.4	20.2
Fixed maturities [Member] | CMBS [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	164.5	162.7
Gross Unrealized Gains	3.3	3.1
Gross Unrealized Losses	0.5	1
Fair Value	167.3	164.8
Fixed maturities [Member] | ABS-residential [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	6.3	6.8
Gross Unrealized Gains	0.3	0.3
Gross Unrealized Losses	0.4	0.5
Fair Value	6.2	6.6
Fixed maturities [Member] | ABS-non residential [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	147.3	116.8
Gross Unrealized Gains	0.6	0.5
Gross Unrealized Losses	0.1	0.1
Fair Value	147.8	117.2
Fixed maturities [Member] | Foreign denominated: Governments [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	187.8	207.7
Gross Unrealized Gains	8.4	4.9
Gross Unrealized Losses	8.4	5.9
Fair Value	187.8	206.7
Fixed maturities [Member] | Foreign Denominated: Credit [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	196.4	155.5
Gross Unrealized Gains	5.2	4.4
Gross Unrealized Losses	7.9	4.8
Fair Value	193.7	155.1
Equity securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	345.9	346.9
Gross Unrealized Gains	186.3	188.6
Gross Unrealized Losses	1.9	1.2
Fair Value	530.3	534.3
Short-Term Investments [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	382.7	351.6
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$382.70	$351.60

Investments_Additional_Informa

Investments - Additional Information (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013	Dec. 28, 2011
	Security
Schedule of Investments [Line Items]	'	'	'	'
Fair value of investments, assets managed on behalf of the trade capital providers	$69	'	$87.30	'
Number of securities in an unrealized loss position, total	5,914	'	'	'
Number of securities in an unrealized loss position for less than one year	1,215	'	'	'
Number of securities in an unrealized loss position for a period of one year or greater	52	'	'	'
Other-than-temporarily losses on our fixed maturities portfolio	0	0	'	'
Other-than-temporarily losses equity portfolio	0	1.4	'	'
Total notional amount of the Company's holdings in non-U.S. Dollar denominated higher yielding fixed maturity investments	'	1.7	'	'
Notional amount of the currency forward contracts	'	'	'	100
Fair value of put option	1.2	0.6	'	'
Realized gains from currency forward contracts	2.3	1.6	'	'
Realized losses from currency forward contracts	1.3	2.3	'	'
Amortized Cost	3,867.30	'	3,836	'
Fair Value	4,118.50	'	4,079.20	'
Reported loss incurred	51.3	'	60.2	'
Reported loss expense reserve	51.3	'	60.2	'
Securities Pledged As Collateral [Member]	'	'	'	'
Schedule of Investments [Line Items]	'	'	'	'
Pledged assets separately reported, securities pledged under letters of credit facilities, at amortized cost	79.4	'	96.6	'
Pledged assets separately reported, securities pledged under letters of credit facilities, at fair value	80.4	'	97.5	'
Currency forward contracts [Member]	'	'	'	'
Schedule of Investments [Line Items]	'	'	'	'
Notional amount of the currency forward contracts	46.6	73.7	'	'
Deposit with Various U.S. Insurance and Other Regulatory Agencies [Member]	'	'	'	'
Schedule of Investments [Line Items]	'	'	'	'
Amortized Cost	197.9	'	196.7	'
Fair Value	205.2	'	203.4	'
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	'	$1.50	'	'

Investments_Schedule_of_Amorti1

Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) (USD $)	Mar. 31, 2014
In Millions, unless otherwise specified
Investments Debt And Equity Securities [Abstract]	'
Amortized Cost, Due in one year or less	$228.70
Amortized Cost, Due after one year through five years	1,267.40
Amortized Cost, Due after five years through ten years	533
Amortized Cost, Thereafter	173.3
Amortized Cost, Structured securities	536.6
Amortized Cost, Total	2,739
Fair Value, Due in one year or less	229.8
Fair Value, Due after one year through five years	1,286.20
Fair Value, Due after five years through ten years	561.7
Fair Value, Thereafter	175.5
Fair Value, Structured securities	548.6
Fair Value, Total	$2,801.80

Investments_Schedule_of_Aging_

Investments - Schedule of Aging of Unrealized Losses on Company's Investments in Fixed Maturities, Equity Securities and Other Investments (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	$837.40	$907.90
Unrealized Losses, Less Than One Year	27.3	30.2
Fair Value, One Year or Greater	22.8	22.6
Unrealized Losses, One Year or Greater	1.2	1.4
Fair Value, Total	860.2	930.5
Unrealized Losses, Total	28.5	31.6
Other investments [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	'	-1.4
Unrealized Losses, Less Than One Year	'	1.3
Fair Value, One Year or Greater	'	0
Unrealized Losses, One Year or Greater	'	0
Fair Value, Total	'	-1.4
Unrealized Losses, Total	0	1.3
Fixed maturities [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	805.4	879.3
Unrealized Losses, Less Than One Year	25.4	27.7
Fair Value, One Year or Greater	22.8	22.6
Unrealized Losses, One Year or Greater	1.2	1.4
Fair Value, Total	828.2	901.9
Unrealized Losses, Total	26.6	29.1
Fixed maturities [Member] | U.S. Governments [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	34.6	55.3
Unrealized Losses, Less Than One Year	0.4	0.5
Fair Value, One Year or Greater	0	0
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	34.6	55.3
Unrealized Losses, Total	0.4	0.5
Fixed maturities [Member] | Non-US Governments [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	31.7	36.3
Unrealized Losses, Less Than One Year	0.6	1
Fair Value, One Year or Greater	0.7	0
Unrealized Losses, One Year or Greater	0.1	0
Fair Value, Total	32.4	36.3
Unrealized Losses, Total	0.7	1
Fixed maturities [Member] | Obligations of states and political subdivisions [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	142.8	154.6
Unrealized Losses, Less Than One Year	4.3	8.8
Fair Value, One Year or Greater	3.8	3.1
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	146.6	157.7
Unrealized Losses, Total	4.3	8.8
Fixed maturities [Member] | Credit-Financial [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	84	88.7
Unrealized Losses, Less Than One Year	1.3	1.7
Fair Value, One Year or Greater	3	2.4
Unrealized Losses, One Year or Greater	0	0.1
Fair Value, Total	87	91.1
Unrealized Losses, Total	1.3	1.8
Fixed maturities [Member] | Credit-Industrial [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	90.8	85.7
Unrealized Losses, Less Than One Year	1.2	2
Fair Value, One Year or Greater	2.1	2.9
Unrealized Losses, One Year or Greater	0	0.2
Fair Value, Total	92.9	88.6
Unrealized Losses, Total	1.2	2.2
Fixed maturities [Member] | Credit-Utility [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	18.9	21.2
Unrealized Losses, Less Than One Year	0.2	0.7
Fair Value, One Year or Greater	1.3	1.9
Unrealized Losses, One Year or Greater	0.1	0.1
Fair Value, Total	20.2	23.1
Unrealized Losses, Total	0.3	0.8
Fixed maturities [Member] | CMO/MBS-agency [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	39.5	58.4
Unrealized Losses, Less Than One Year	0.6	1.2
Fair Value, One Year or Greater	5.5	5.6
Unrealized Losses, One Year or Greater	0.5	0.5
Fair Value, Total	45	64
Unrealized Losses, Total	1.1	1.7
Fixed maturities [Member] | CMBS [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	15.6	24.2
Unrealized Losses, Less Than One Year	0.4	1
Fair Value, One Year or Greater	1.8	1.1
Unrealized Losses, One Year or Greater	0.1	0
Fair Value, Total	17.4	25.3
Unrealized Losses, Total	0.5	1
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	3.6	3.6
Unrealized Losses, One Year or Greater	0.4	0.5
Fair Value, Total	3.6	3.6
Unrealized Losses, Total	0.4	0.5
Fixed maturities [Member] | ABS-non residential [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	59.2	59.1
Unrealized Losses, Less Than One Year	0.1	0.1
Fair Value, One Year or Greater	1	2
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	60.2	61.1
Unrealized Losses, Total	0.1	0.1
Fixed maturities [Member] | Foreign denominated: Governments [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	155.9	170.5
Unrealized Losses, Less Than One Year	8.4	5.9
Fair Value, One Year or Greater	0	0
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	155.9	170.5
Unrealized Losses, Total	8.4	5.9
Fixed maturities [Member] | Foreign Denominated: Credit [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	132.4	125.3
Unrealized Losses, Less Than One Year	7.9	4.8
Fair Value, One Year or Greater	0	0
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	132.4	125.3
Unrealized Losses, Total	7.9	4.8
Equity securities [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	29.4	25.5
Unrealized Losses, Less Than One Year	1.9	1.2
Fair Value, One Year or Greater	0	0
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	29.4	25.5
Unrealized Losses, Total	1.9	1.2
Short-Term Investments [Member]	'	'
Schedule of Investments [Line Items]	'	'
Fair Value, Less Than One Year	2.6	4.5
Unrealized Losses, Less Than One Year	0	0
Fair Value, One Year or Greater	0	0
Unrealized Losses, One Year or Greater	0	0
Fair Value, Total	2.6	4.5
Unrealized Losses, Total	$0	$0

Investments_Schedule_of_Aging_1

Investments - Schedule of Aging of Unrealized Losses on Company's Investments in Fixed Maturities, Equity Securities and Other Investments (Parenthetical) (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
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
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Schedule of Investments [Line Items]	'	'
Gross realized investment gains	$23.80	$23.20
Gross realized investment and other losses	-12.7	-13.7
Net realized investment gains and other	11.1	9.5
Other investments [Member]	'	'
Schedule of Investments [Line Items]	'	'
Gross realized investment gains	12.7	11.1
Gross realized investment and other losses	-4.9	-5.7
Other Assets [Member]	'	'
Schedule of Investments [Line Items]	'	'
Gross realized investment and other losses	-4	0
Fixed maturities [Member]	'	'
Schedule of Investments [Line Items]	'	'
Gross realized investment gains	4.6	11.5
Gross realized investment and other losses	-3.5	-6.5
Equity securities [Member]	'	'
Schedule of Investments [Line Items]	'	'
Gross realized investment gains	6.5	0.5
Gross realized investment and other losses	-0.1	0
Short-Term Investments [Member]	'	'
Schedule of Investments [Line Items]	'	'
Gross realized investment gains	0	0.1
Gross realized investment and other losses	-0.2	-0.1
Other-than-temporary impairment losses on equity securities [Member]	'	'
Schedule of Investments [Line Items]	'	'
Gross realized investment and other losses	$0	($1.40)

Investments_Securities_and_Let

Investments - Securities and Letters of Credit Pledged to Lloyd's as Company's Corporate Member's Capital (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Investments Debt And Equity Securities [Abstract]	'	'
Fixed maturities, at fair value	$251.30	$250.70
Short-term investments, at fair value	0	0.1
Total securities pledged to Lloyd's	$251.30	$250.80

Investments_Financial_Assets_M

Investments - Financial Assets Measured at Fair Value on Recurring Basis (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	$4,118.50	$4,079.20
Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	3,842	3,820.70
Other investments [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	403.7	378.9
Other investments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	127.2	117.8
Other Assets [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	'	2.6
Fixed maturities [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	2,801.80	2,814.40
Fixed maturities [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	2,801.80	2,814.40
Fixed maturities [Member] | U.S. Governments [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	225.3	275.3
Fixed maturities [Member] | U.S. Governments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	225.3	275.3
Fixed maturities [Member] | Non-U.S. Governments [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	70.4	60.2
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	70.4	60.2
Fixed maturities [Member] | Obligations of states and political subdivisions [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	523.1	577.8
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	523.1	577.8
Fixed maturities [Member] | Credit-Financial [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	470.4	439.9
Fixed maturities [Member] | Credit-Financial [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	470.4	439.9
Fixed maturities [Member] | Credit-Industrial [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	431.1	397.1
Fixed maturities [Member] | Credit-Industrial [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	431.1	397.1
Fixed maturities [Member] | Credit-Utility [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	151.4	166.6
Fixed maturities [Member] | Credit-Utility [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	151.4	166.6
Fixed maturities [Member] | CMO/MBS-agency [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	207.9	226.9
Fixed maturities [Member] | CMO/MBS-agency [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	207.9	226.9
Fixed maturities [Member] | CMO/MBS-non agency [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	19.4	20.2
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	19.4	20.2
Fixed maturities [Member] | CMBS [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	167.3	164.8
Fixed maturities [Member] | CMBS [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	167.3	164.8
Fixed maturities [Member] | ABS-residential [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	6.2	6.6
Fixed maturities [Member] | ABS-residential [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	6.2	6.6
Fixed maturities [Member] | ABS-non residential [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	147.8	117.2
Fixed maturities [Member] | ABS-non residential [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	147.8	117.2
Fixed maturities [Member] | Foreign denominated: Governments [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	187.8	206.7
Fixed maturities [Member] | Foreign denominated: Governments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	187.8	206.7
Fixed maturities [Member] | Foreign Denominated: Credit [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	193.7	155.1
Fixed maturities [Member] | Foreign Denominated: Credit [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	193.7	155.1
Equity securities [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	530.3	534.3
Equity securities [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	530.3	534.3
Short-Term Investments [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	382.7	351.6
Short-Term Investments [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	382.7	351.6
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	957	967.5
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] | Other Assets [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	'	0
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	105.5	143.8
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	105.5	143.8
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	472.6	476.7
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	378.9	347
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	2,883.80	2,849.30
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	127.2	117.8
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
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	2,696.30	2,670.60
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	119.8	131.5
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	70.4	60.2
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	523.1	577.8
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	470.4	439.9
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	431.1	397.1
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	151.4	166.6
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	207.9	226.9
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	19.4	20.2
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	167.3	164.8
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	6.2	6.6
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	147.8	117.2
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	187.8	206.7
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	193.7	155.1
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	56.5	56.3
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	3.8	4.6
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	1.2	3.9
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] | Other Assets [Member] | Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Fair Value	'	2.6
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	1.2	1.3
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

Investments_Schedule_of_Reconc

Investments - Schedule of Reconciliation of Beginning and Ending Balances for Investments Categorized as Level 3 (Detail) (USD $)	3 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]	'	'
Beginning balance	$3.90	$8.70
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Included in net income (loss)	0	0.1
Included in other comprehensive income (loss)	0	0
Purchases	0	0
Issuances	0	0
Sales	-0.1	-0.6
Settlements	-2.6	-4.3
Ending balance	1.2	3.9
Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at end of period	0	0
Other Assets [Member]	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]	'	'
Beginning balance	2.6	6.9
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Included in net income (loss)	0	0
Included in other comprehensive income (loss)	0	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	-2.6	-4.3
Ending balance	0	2.6
Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at end of period	0	0
Equity securities [Member]	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]	'	'
Beginning balance	1.3	1.8
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Included in net income (loss)	0	0.1
Included in other comprehensive income (loss)	0	0
Purchases	0	0
Issuances	0	0
Sales	-0.1	-0.6
Settlements	0	0
Ending balance	1.2	1.3
Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at end of period	$0	$0

Derivative_Instruments_Additio

Derivative Instruments - Additional Information (Detail) (USD $)	0 Months Ended	3 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Dec. 28, 2011	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2011
				Contract
Offsetting [Abstract]	'	'	'	'
Number of reinsurance contracts	'	'	'	1
Coverage amount	$100	'	'	'
Coverage period, months	'24 months	'	'	'
Other reinsurance-related expenses	'	0	5.1	'
Other assets	'	$0	$2.60	'

Reserves_for_Losses_and_Loss_A2

Reserves for Losses and Loss Adjustment Expenses - Reinsurance Recoverables (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Liability For Claims And Claims Adjustment Expense [Abstract]	'	'
Net reserves beginning of the year	$2,107.60	$2,110.90
Losses and LAE incurred during current calendar year, net of reinsurance:	'	'
Current accident year	191.4	175
Prior accident years	-8.9	-4.5
Losses and LAE incurred during calendar year, net of reinsurance	182.5	170.5
Losses and LAE payments made during current calendar year, net of reinsurance:	'	'
Current accident year	27.9	21.9
Prior accident years	150.1	159.7
Losses and LAE payments made during current calendar year, net of reinsurance	178	181.6
Change in participation interest	24.7	10.4
Foreign exchange adjustments	1.7	-6.5
Net reserves - end of period	2,138.50	2,103.70
Reinsurance recoverables on unpaid losses and LAE, end of period	903.9	1,101.30
Gross reserves - end of period	$3,042.40	$3,205

Reserves_for_Losses_and_Loss_A3

Reserves for Losses and Loss Adjustment Expenses - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2012
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]	'	'	'
Net reserves ceded - whole account quota share reinsurance transaction	'	'	$192.20
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	8.9	4.5	'
Commercial Automobile [Member]	'	'	'
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]	'	'	'
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	9.4	'	'
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	2	1.1	'
Unfavorable Development in 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	21.6	2.2	'
Unfavorable Development In Property Lines [Member]	'	'	'
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]	'	'	'
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	4.2	'	'
Commercial Multi Peril [Member]	'	'	'
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]	'	'	'
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	'	3.3	'
Rockwood [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	'
Excess and Surplus Lines [Member]	'	'	'
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]	'	'	'
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	8	5.2	'
Short-Tail Lines [Member]	'	'	'
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]	'	'	'
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	1.1	'	'
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	0.4	0.9	'
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	'	0.5	'
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	8.8	2.2	'
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	5.5	0.9	'
Medical Malpractice Liability [Member]	'	'	'
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]	'	'	'
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	1	'	'
Catastrophe [Member]	'	'	'
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]	'	'	'
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	'	1.3	'
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	21.6	5.9	'
Workers Compensation [Member]	'	'	'
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]	'	'	'
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	4.3	'	'
Unfavorable Development [Member]	'	'	'
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]	'	'	'
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	4.5	'	'
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	'	$4	'

Other_Indebtedness_Additional_

Other Indebtedness - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended		3 Months Ended				0 Months Ended	3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 07, 2014	Mar. 31, 2014	Mar. 31, 2014	Mar. 31, 2014	Mar. 31, 2014	Mar. 07, 2014	Mar. 31, 2014	Dec. 31, 2013
				ABR Borrowings [Member]	ABR Borrowings [Member]	Eurocurrency Borrowings [Member]	Eurocurrency Borrowings [Member]	New Credit Agreement [Member]	New Credit Agreement [Member]	New Credit Agreement [Member]
				Minimum [Member]	Maximum [Member]	Minimum [Member]	Maximum [Member]
Debt Instrument [Line Items]	'	'	'	'	'	'	'	'	'	'
Credit agreement with major money center banks, amount	'	'	$150,000,000	'	'	'	'	$175,000,000	'	'
Revolving credit facility maturity date	'	'	'	'	'	'	'	'	7-Mar-18	'
Line of credit facility	'(a) the Prime Rate, (b) the Federal Funds Effective Rate plus 1b 2 of 1% and (c) the Adjusted Eurocurrency Rate for a one month interest period plus 1%, plus an applicable margin of 0.375% to 1.25%. bEurocurrency Borrowingsb will bear interest at a rate per annum equal to LIBOR plus an applicable margin of 1.375% to 2.25%.	'	'	'	'	'	'	'	'	'
LIBOR plus margin rate	'	'	'	0.38%	1.25%	1.38%	2.25%	'	'	'
Amount drawn under revolving letter of credit	'	'	'	'	'	'	'	200,000	'	'
Revolving letter of credit amount outstanding	'	'	'	'	'	'	'	'	17,500,000	'
Borrowed amount	'	'	'	'	'	'	'	'	0	0
Maximum borrowing capacity under revolving credit facility before amendment	'	'	150,000,000	'	'	'	'	'	'	'
Note payable as part of ARIS acquisition	800,000	800,000	'	'	'	'	'	'	'	'
Note payable on recourse basis	$200,000	$200,000	'	'	'	'	'	'	'	'
Variable interest rate of note payable, above 30-day LIBOR	2.00%	2.00%	'	'	'	'	'	'	'	'
Interest rate at the end of the year	'	2.21%	'	'	'	'	'	'	'	'
Debt Instrument, expiry date	16-Dec-13	'	'	'	'	'	'	'	'	'

Disclosures_about_Fair_Value_o2

Disclosures about Fair Value of Financial Instruments - Additional Information (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Financial Instruments Owned and Pledged as Collateral [Line Items]	'	'
Premiums receivable	$371.10	$348.40	[1]
Reinsurance recoverable	978.1	1,263.50	[1]
Over 90 Days [Member]	'	'
Financial Instruments Owned and Pledged as Collateral [Line Items]	'	'
Premiums receivable	19.7	9.8
Reinsurance recoverable	15.1	22.2
Premiums receivable, allowance for doubtful accounts	4.6	4.5
Reinsurance recoverables, allowance for doubtful accounts	2.2	1.6
Premiums receivables, secured by collateral	0.5	0.4
Reinsurance recoverables, secured by collateral	$0.40	$0.30
[1]	Derived from audited consolidated financial statements.

Disclosures_about_Fair_Value_o3

Disclosures about Fair Value of Financial Instruments - Summary of Company's Financial Instruments Whose Carrying Amount Did Not Equal Fair Value (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Carrying Amount	$403.60	$403.40
Fair Value	341.7	325
Junior Subordinated Debentures [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Carrying Amount	193.3	193.3
Fair Value	155.8	155.5
Senior Unsecured Fixed Rate Notes [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Carrying Amount	143.8	143.8
Fair Value	132.3	116.2
Other Indebtedness [Member] | Floating Rate Loan Stock [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Carrying Amount	65.7	65.5
Fair Value	53	52.7
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

Shareholders_Equity_Additional

Shareholders Equity - Additional Information (Detail) (USD $)	0 Months Ended			3 Months Ended		6 Months Ended	0 Months Ended		3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 17, 2014	Feb. 18, 2014	Nov. 13, 2007	Mar. 31, 2014	Mar. 31, 2013	Jun. 30, 2013	Feb. 18, 2011	Nov. 05, 2013	Mar. 31, 2014	Mar. 31, 2014	Mar. 31, 2013
							2011 Repurchase Authorization [Member]	2013 Repurchase Authorization [Member]	2013 Repurchase Authorization [Member]	Open Market [Member]	10% Stock Dividend [Member]
Equity [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Dividend declared per common share	'	$0.15	'	$0.15	$0.14	'	'	'	'	'	$0.14
Dividend payable, date of record	'	'	'	3-Mar-14	1-Mar-13	3-Jun-13	'	'	'	'	'
Cash dividends paid - common shares, total	$4	'	'	'	$3.70	'	'	'	'	'	'
Common stock repurchase authorization, maximum	'	'	150	'	'	'	150	'	'	'	'
Shares purchased during period, Value	'	'	'	'	'	'	'	150	136.1	'	'
Stock repurchase program, number of shares authorized to be repurchased	'	'	'	0	'	'	'	'	'	'	'
Common shares repurchased	'	'	'	165,184	'	'	'	'	'	0	'
Common shares repurchased, total cost	'	'	'	$7.50	'	'	'	'	'	'	'

Shareholders_Equity_Schedule_o

Shareholders Equity - Schedule of Repurchase of Shares (Detail) (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2014
Schedule Of Share Repurchase Programs [Line Items]	'
Number of Shares Repurchased	165,184
Total Cost	$7.50
12/17/2013 [Member]	'
Schedule Of Share Repurchase Programs [Line Items]	'
Date Trading Plan Initiated	17-Dec-13
2014 Purchase Period	'02/04/14-02/12/14
Trading Plans Purchase Period Date Range Start	'02/04/14
Trading Plans Purchase Period Date Range End	'02/12/14
Number of Shares Repurchased	15,789
Average Price of Shares Repurchased	$43.28
Total Cost	0.7
Repurchase Authorization Year	'2013
03/18/2014 [Member]	'
Schedule Of Share Repurchase Programs [Line Items]	'
Date Trading Plan Initiated	18-Mar-14
2014 Purchase Period	'03/18/14-04/28/14
Trading Plans Purchase Period Date Range Start	'03/18/14
Trading Plans Purchase Period Date Range End	'04/28/14
Number of Shares Repurchased	436,867
Average Price of Shares Repurchased	$45.81
Total Cost	$20
Repurchase Authorization Year	'2013

Accumulated_Other_Comprehensiv2

Accumulated Other Comprehensive Income - Changes in Accumulated Other Comprehensive Income (Loss) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014		Mar. 31, 2013
Accumulated Other Comprehensive Income Loss Net Of Tax [Abstract]	'		'
Beginning balance, Foreign currency translation adjustments	($11.50)		($8.70)
Other comprehensive income before reclassifications, Foreign currency translation adjustments	-0.2		0
Amounts reclassified from accumulated other comprehensive income, Foreign currency translation adjustments	0		0
Net current-period other comprehensive income, Foreign currency translation adjustments	-0.2		0
Ending balance, Foreign currency translation adjustments	-11.7		-8.7
Beginning balance, Unrealized holding gains on securities	163.9		204.1
Other comprehensive income before reclassifications, Unrealized holding gains on securities	1.3		13.8
Amounts reclassified from accumulated other comprehensive income, Unrealized holding gains on securities	0		-0.8
Net current-period other comprehensive income, Unrealized holding gains on securities	1.3		13
Ending balance, Unrealized holding gains on securities	165.2		217.1
Beginning balance, Defined benefit pension plans	-4.6		-5.9
Other comprehensive income before reclassifications, Defined benefit pension plans	0		0
Amounts reclassified from accumulated other comprehensive income, Defined benefit pension plans	0		0
Net current-period other comprehensive income, Defined benefit pension plans	0		0
Ending balance, Defined benefit pension plans	-4.6		-5.9
Beginning balance	147.8	[1]	189.5
Other comprehensive income before reclassifications	1.1		13.8
Amounts reclassified from accumulated other comprehensive income	0		-0.8
Other comprehensive income, net of tax	1.1		13
Ending balance	$148.90		$202.50
[1]	Derived from audited consolidated financial statements.

Accumulated_Other_Comprehensiv3

Accumulated Other Comprehensive Income - Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Unrealized gains and losses on securities	'	'
Net realized investment gains	($0.10)	($0.70)
Provision for income taxes	0.1	-0.1
Net of taxes	$0	($0.80)

Net_Income_Per_Common_Share_Ne

Net Income Per Common Share - Net Income Per Common Share on Basic and Diluted Basis (Detail) (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Earnings Per Share [Abstract]	'	'
Net income	$40.20	$32.70
Weighted average common shares outstanding - basic	26,514,481	27,247,009
Effect of dilutive securities	'	'
Equity compensation awards	451,980	910,064
Weighted average common shares outstanding - diluted	26,966,461	28,157,073
Net income per common share - basic	$1.52	$1.20
Net income per common share - diluted	$1.49	$1.16

Net_Income_Per_Common_Share_Ad

Net Income Per Common Share - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Earnings Per Share [Abstract]	'	'	'
Treasury shares	7,723,529	6,785,438	7,558,345	[1]
Antidilutive securities excluded from computation of diluted net income per share, shares	3,300	214,453	'
[1]	Derived from audited consolidated financial statements.

Supplemental_Cash_Flow_Informa2

Supplemental Cash Flow Information - Additional Information (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Income Taxes Paid Net [Abstract]	'	'
Income taxes paid	$15.60	$2.50

Supplemental_Cash_Flow_Informa3

Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Interest Paid [Abstract]	'	'
Senior unsecured fixed rate notes	$2.30	$2.30
Junior subordinated debentures	2	1.9
Other indebtedness	0.9	0.9
Total interest paid	$5.20	$5.10

ShareBased_Compensation_Additi

Share-Based Compensation - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]	'	'
Share-based compensation arrangement by share-based payment award, award vesting period, maximum, years	'5 years	'5 years

Sharebased_Compensation_Fair_V

Share-based Compensation - Fair Value Assumptions (Detail)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]	'	'
Risk-free rate of return	1.74%	0.80%
Expected dividend yields	1.49%	1.64%
Expected award life (years)	'4 years 10 months 6 days	'5 years 26 days
Expected volatility	24.91%	31.90%

ShareBased_Compensation_2007_L

Share-Based Compensation - 2007 Long-Term Incentive Plan - Additional Information (Detail) (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013	Nov. 30, 2007
Schedule Of Share Repurchase Programs [Line Items]	'	'	'
Share-based compensation arrangement by share-based payment award, award vesting period, years	'5 years	'5 years	'
2007 Plan [Member]	'	'	'
Schedule Of Share Repurchase Programs [Line Items]	'	'	'
Share-based compensation arrangement by share-based payment award, number of shares authorized	'	'	4,500,000
Share-based compensation arrangement by share-based payment award, 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.	'	'
Count of restricted shares, settle in common shares	2.75	'	'
Share-based compensation arrangement by share-based payment award, compensation expense	$0	$0	'
2007 Plan [Member] | Maximum [Member]	'	'	'
Schedule Of Share Repurchase Programs [Line Items]	'	'	'
Term of shares options, (in years)	'Seven years	'	'
Share-based compensation arrangement by share-based payment award, award vesting period, years	'5 years	'	'
2007 Plan [Member] | Minimum [Member]	'	'	'
Schedule Of Share Repurchase Programs [Line Items]	'	'	'
Share-based compensation arrangement by share-based payment award, award vesting period, years	'1 year	'	'
Restricted Share Units [Member]	'	'	'
Schedule Of Share Repurchase Programs [Line Items]	'	'	'
Share-based compensation arrangement by share-based payment award, compensation expense	0.7	1	'
Employee service share-based compensation, unrecognized compensation costs on nonvested awards	4.8	'	'
Restricted Share Units [Member] | Maximum [Member]	'	'	'
Schedule Of Share Repurchase Programs [Line Items]	'	'	'
Share-based compensation arrangement by share-based payment award, award vesting period, years	'4 years	'	'
Restricted Share Units [Member] | Minimum [Member]	'	'	'
Schedule Of Share Repurchase Programs [Line Items]	'	'	'
Share-based compensation arrangement by share-based payment award, award vesting period, years	'2 years	'	'
Stock-Settled SARs Activity [Member]	'	'	'
Schedule Of Share Repurchase Programs [Line Items]	'	'	'
Share-based compensation arrangement by share-based payment award, compensation expense	0.6	0.7	'
Employee service share-based compensation, unrecognized compensation costs on nonvested awards	3.3	'	'
Stock-Settled SARs Activity [Member] | Maximum [Member]	'	'	'
Schedule Of Share Repurchase Programs [Line Items]	'	'	'
Share-based compensation arrangement by share-based payment award, award vesting period, years	'4 years	'	'
Stock-Settled SARs Activity [Member] | Minimum [Member]	'	'	'
Schedule Of Share Repurchase Programs [Line Items]	'	'	'
Share-based compensation arrangement by share-based payment award, award vesting period, years	'1 year	'	'
Cash-Settled SARs Activity [Member]	'	'	'
Schedule Of Share Repurchase Programs [Line Items]	'	'	'
Employee service share-based compensation, unrecognized compensation costs on nonvested awards	7.2	'	'
Deferred compensation arrangement with individual, compensation expense	$2.70	$6.50	'
Cash-Settled SARs Activity [Member] | Maximum [Member]	'	'	'
Schedule Of Share Repurchase Programs [Line Items]	'	'	'
Share-based compensation arrangement by share-based payment award, award vesting period, years	'4 years	'	'
Cash-Settled SARs Activity [Member] | Minimum [Member]	'	'	'
Schedule Of Share Repurchase Programs [Line Items]	'	'	'
Share-based compensation arrangement by share-based payment award, award vesting period, years	'1 year	'	'

Sharebased_Compensation_Summar

Share-based Compensation - Summary of Restricted Share Units Activity Under 2007 Plan (Detail) (Restricted Share Units [Member], USD $)	3 Months Ended
	Mar. 31, 2014
Restricted Share Units [Member]	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'
Shares, Outstanding at January 1, 2014	257,975
Granted, Shares	4,373
Vested and issued, Shares	-57,205
Expired or forfeited, Shares	-1,036
Shares, Outstanding at March 31, 2014	204,107
Weighted-Average Grant Date Fair Value, Outstanding at January 1, 2014	$32.97
Granted, Weighted-Average Grant Date Fair Value	$40.08
Vested and issued, Weighted-Average Grant Date Fair Value	$31.58
Expired or forfeited, Weighted-Average Grant Date Fair Value	$40.08
Weighted-Average Grant Date Fair Value, March 31, 2014	$33.48

Sharebased_Compensation_Summar1

Share-based Compensation - Summary of Stock-Settled SARs Activity Under 2007 Plan (Detail) (Stock-Settled SARs Activity [Member], USD $)	3 Months Ended
	Mar. 31, 2014
Stock-Settled SARs Activity [Member]	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'
Shares, Outstanding at January 1, 2014	1,288,698
Granted, Shares	28,069
Exercised, shares	-58,093
Expired or forfeited, Shares	-11,237
Shares, Outstanding at March 31, 2014	1,247,437
Weighted-Average Exercise Price, Outstanding at January 1, 2014	$29.47
Granted, Weighted-Average Exercise Price	$36.44
Exercised, Weighted-Average Exercise Price	$28.01
Expired or forfeited, Weighted-Average Exercise Price	$36.44
Weighted-Average Exercise Price, March 31, 2014	$29.63

Sharebased_Compensation_Summar2

Share-based Compensation - Summary of Cash-Settled SARs Activity Under 2007 Plan (Detail) (Cash-Settled SARs Activity [Member], USD $)	3 Months Ended
	Mar. 31, 2014
Cash-Settled SARs Activity [Member]	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'
Shares, Outstanding at January 1, 2014	1,743,758
Granted, Shares	3,574
Exercised, shares	-81,093
Expired or forfeited, Shares	-19,906
Shares, Outstanding at March 31, 2014	1,646,333
Weighted-Average Exercise Price, Outstanding at January 1, 2014	$33
Granted, Weighted-Average Exercise Price	$39.75
Exercised, Weighted-Average Exercise Price	$30.15
Expired or forfeited, Weighted-Average Exercise Price	$35.90
Weighted-Average Exercise Price, March 31, 2014	$33.09

ShareBased_Compensation_Deferr

Share-Based Compensation - Deferred Compensation Plan for Non-Employee Directors - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Directors Plan [Member]	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'
Deferred compensation arrangement with individual, description	'Under the Directors Plan, non-employee directors can elect each year to defer payment of 0%, 50% or 100% of their cash compensation payable during the next calendar year.	'	'
Deferred compensation arrangement with individual, employer contribution percentage	75.00%	'	'
Deferred compensation arrangement with individual, interest rate	'Two percent above the prime commercial lending rate	'	'
Share-based compensation arrangement by share-based payment award, discount from market price	0.00%	'	'
Deferred compensation arrangement with individual, distributions period, months	'6 months	'	'
Deferred compensation arrangement with individual, shares authorized for issuance	1,650	'	'
Directors Plan [Member] | Minimum [Member]	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'
Share-based compensation arrangement by share-based payment award, discount from market price	50.00%	'	'
Directors Plan [Member] | Maximum [Member]	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'
Share-based compensation arrangement by share-based payment award, discount from market price	100.00%	'	'
Deferred Compensation Plan [Member]	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'
Employee service share-based compensation, cash paid to exercise of stock options	'	'	$1.90
Deferred compensation arrangement with individual, compensation expense	$0	$0.60	'

ShareBased_Compensation_Amende

Share-Based Compensation - Amended and Restated Stock Incentive Plan - Additional Information (Detail) (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Schedule Of Share Repurchase Programs [Line Items]	'	'
Share-based compensation arrangement by share-based payment award, award vesting period, years	'5 years	'5 years
Amended Plan [Member]	'	'
Schedule Of Share Repurchase Programs [Line Items]	'	'
Share-based compensation arrangement by share-based payment award, number of shares authorized	6,250,000	'
Share-based compensation arrangement by share-based payment award, compensation expense	0	0
Amended Plan [Member] | Minimum [Member]	'	'
Schedule Of Share Repurchase Programs [Line Items]	'	'
Share-based compensation arrangement by share-based payment award, award vesting period, years	'7 years	'
Amended Plan [Member] | Maximum [Member]	'	'
Schedule Of Share Repurchase Programs [Line Items]	'	'
Share-based compensation arrangement by share-based payment award, award vesting period, years	'11 years	'

Underwriting_Acquisition_and_I2

Underwriting, Acquisition and Insurance Expenses - Underwriting, Acquisition and Insurance Expenses (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Other Income And Expenses [Abstract]	'	'
Commissions	$54.90	$51.90
General expenses	68.8	68.7
Premium taxes, boards and bureaus	6.6	6.8
Underwriting Acquisition and Insurance Expenses, Total	130.3	127.4
Net deferral of policy acquisition costs	-1.6	-0.7
Total underwriting, acquisition and insurance expenses	$128.70	$126.70

Underwriting_Acquisition_and_I3

Underwriting, Acquisition and Insurance Expenses - Additional Information (Detail) (Cash-Settled SARs Activity [Member], USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Cash-Settled SARs Activity [Member]	'	'
Underwriting Acquisition and Insurance Expense [Line Items]	'	'
Expense associated with cash settled stock appreciation rights	$4.30	$8.60

Income_Taxes_Income_Tax_Provis

Income Taxes - Income Tax Provision (Benefit) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Income Tax Disclosure [Abstract]	'	'
Current tax provision	$1.40	$1.80
Future tax deductions	30.2	2.4
Valuation allowance change	-29.1	0.6
Income tax provision	$2.50	$4.80

Income_Taxes_Schedule_of_PreTa

Income Taxes - Schedule of Pre-Tax Income (Loss) and Effective Income Tax Rates (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Income Tax Examination [Line Items]	'	'
Pre-tax income (loss)	$42.70	$37.50
Bermuda [Member]	'	'
Income Tax Examination [Line Items]	'	'
Pre-tax income (loss)	21.9	16.1
Effective Tax Rate	0.00%	0.00%
United States [Member]	'	'
Income Tax Examination [Line Items]	'	'
Pre-tax income (loss)	14.8	17.5
Effective Tax Rate	20.60%	18.30%
United Kingdom [Member]	'	'
Income Tax Examination [Line Items]	'	'
Pre-tax income (loss)	7.3	6
Effective Tax Rate	-7.70%	25.50%
Belgium [Member]	'	'
Income Tax Examination [Line Items]	'	'
Pre-tax income (loss)	0	0
Effective Tax Rate	18.10%	-67.60%
Brazil [Member]	'	'
Income Tax Examination [Line Items]	'	'
Pre-tax income (loss)	0	-1.8
Effective Tax Rate	0.00%	0.00%
Dubai [Member]	'	'
Income Tax Examination [Line Items]	'	'
Pre-tax income (loss)	-0.9	0
Effective Tax Rate	0.00%	0.00%
Malta [Member]	'	'
Income Tax Examination [Line Items]	'	'
Pre-tax income (loss)	-0.4	-0.3
Effective Tax Rate	0.00%	0.00%
Switzerland [Member]	'	'
Income Tax Examination [Line Items]	'	'
Pre-tax income (loss)	$0	$0
Effective Tax Rate	-19.80%	24.50%

Income_Taxes_Schedule_of_PreTa1

Income Taxes - Schedule of Pre-Tax Income (Loss) and Effective Income Tax Rates (Parenthetical) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Income Tax Examination [Line Items]	'	'
Pre-tax income (loss)	$42.70	$37.50
Belgium [Member]	'	'
Income Tax Examination [Line Items]	'	'
Pre-tax income (loss)	0	0
Belgium [Member] | Maximum [Member]	'	'
Income Tax Examination [Line Items]	'	'
Pre-tax income (loss)	'	0.1
Switzerland [Member]	'	'
Income Tax Examination [Line Items]	'	'
Pre-tax income (loss)	0	0
Switzerland [Member] | Maximum [Member]	'	'
Income Tax Examination [Line Items]	'	'
Pre-tax income (loss)	$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
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Income Tax Disclosure [Abstract]	'	'
Income tax provision at expected rate	$4.70	$6.70
Tax-exempt interest	-1.2	-1.4
Dividends received deduction	-0.6	-0.6
Valuation allowance change	-29.1	0.6
Other permanent adjustments, net	0.1	-0.9
Adjustment for annualized rate	0.8	0.4
United States state tax benefit	0.1	-0.2
PXRE Reinsurance capital loss carryforward	29.8	0
Foreign exchange adjustments	-2.2	0.2
Foreign withholding taxes	0.1	0
Income tax provision	2.5	4.8
Income tax (benefit) provision - Foreign	-0.6	1.6
Income tax provision - United States Federal	2.8	3.5
Income tax provision (benefit) - United States State	0.2	-0.3
Foreign withholding tax - United States	$0.10	$0

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Income Tax Contingency [Line Items]	'	'
Total net deferred tax liabilities	$7,000,000	'
Net operating loss carryforward	18,500,000	'
Valuation allowance	27,200,000	'
Loss carryforwards per year	2,800,000	'
Capital loss carryforward	29,800,000	0
Unrecognized tax benefits	0	0
Malta [Member]	'	'
Income Tax Contingency [Line Items]	'	'
Valuation allowance	1,800,000	'
Increased valuation allowance	100,000	'
Brazil [Member]	'	'
Income Tax Contingency [Line Items]	'	'
Valuation allowance	8,800,000	'
Increased valuation allowance	800,000	'
December 31, 2025 [Member]	'	'
Income Tax Contingency [Line Items]	'	'
Loss carryforwards, amounts expiring if not utilized	17,000,000	'
December 31, 2027 [Member]	'	'
Income Tax Contingency [Line Items]	'	'
Loss carryforwards, amounts expiring if not utilized	1,500,000	'
PXRE Corporation [Member]	'	'
Income Tax Contingency [Line Items]	'	'
Valuation allowance	16,600,000	'
Capital loss carryforward	200,000	'
PXRE Reinsurance Company [Member]	'	'
Income Tax Contingency [Line Items]	'	'
Reduced valuation allowance	29,800,000	'
ARIS [Member]	'	'
Income Tax Contingency [Line Items]	'	'
Net operating loss carryforward	1,000,000	'
ARIS [Member] | December 31, 2027 [Member]	'	'
Income Tax Contingency [Line Items]	'	'
Loss carryforwards, amounts expiring if not utilized	200,000	'
ARIS [Member] | December 31, 2028 [Member]	'	'
Income Tax Contingency [Line Items]	'	'
Loss carryforwards, amounts expiring if not utilized	400,000	'
ARIS [Member] | December 31, 2029 [Member]	'	'
Income Tax Contingency [Line Items]	'	'
Loss carryforwards, amounts expiring if not utilized	$400,000	'

Commitments_and_Contingencies_

Commitments and Contingencies - Additional Information (Detail) (USD $)	Mar. 31, 2014
In Millions, unless otherwise specified
Commitments And Contingencies Disclosure [Abstract]	'
Contractual commitments related to its limited partnership investments	$26.30

Segment_Information_Revenue_an

Segment Information - Revenue and (Loss) Income Before Income Taxes for Each Segment (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Segment Reporting Information [Line Items]	'	'
Earned premiums	$325.70	$304.20
Net investment income	23.3	27.9
Total segment income (loss) before taxes	38.5	32.4
Net realized investment and other gains	11.1	9.5
Total revenue	360.1	341.6
Net realized investment gains and other	11.1	9.5
Total (loss) income before income taxes	42.7	37.5
Excess and Surplus Lines [Member]	'	'
Segment Reporting Information [Line Items]	'	'
Earned premiums	117.4	105.1
Net investment income	9.3	10.9
Total segment income (loss) before taxes	20.1	13.9
Commercial Specialty [Member]	'	'
Segment Reporting Information [Line Items]	'	'
Earned premiums	71.7	75.2
Net investment income	4.7	5.9
Total segment income (loss) before taxes	1.7	5.7
International Specialty [Member]	'	'
Segment Reporting Information [Line Items]	'	'
Earned premiums	37	32.7
Net investment income	1.8	2.3
Total segment income (loss) before taxes	6.8	4.8
Syndicate 1200 [Member]	'	'
Segment Reporting Information [Line Items]	'	'
Earned premiums	99.3	90.9
Net investment income	3.7	2.8
Total segment income (loss) before taxes	15.8	8.4
Run-Off Lines [Member]	'	'
Segment Reporting Information [Line Items]	'	'
Earned premiums	0.3	0.3
Net investment income	2.5	2.8
Total segment income (loss) before taxes	-5.9	-0.4
Corporate and Other [Member]	'	'
Segment Reporting Information [Line Items]	'	'
Net investment income	1.3	3.2
Net realized investment and other gains	-6.9	-4.4
Net realized investment gains and other	($6.90)	($4.40)

Segment_Information_Schedule_o

Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Revenues from External Customers and Long-Lived Assets [Line Items]	'	'
Total earned premiums	$325.70	$304.20
Bermuda [Member]	'	'
Revenues from External Customers and Long-Lived Assets [Line Items]	'	'
Total earned premiums	24.4	25.8
Brazil [Member]	'	'
Revenues from External Customers and Long-Lived Assets [Line Items]	'	'
Total earned premiums	12.6	10
Malta [Member]	'	'
Revenues from External Customers and Long-Lived Assets [Line Items]	'	'
Total earned premiums	0.5	0.4
United Kingdom [Member]	'	'
Revenues from External Customers and Long-Lived Assets [Line Items]	'	'
Total earned premiums	98.9	87
United States [Member]	'	'
Revenues from External Customers and Long-Lived Assets [Line Items]	'	'
Total earned premiums	$189.30	$181

Segment_Information_Identifiab

Segment Information - Identifiable Assets (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Identifiable Assets [Line Items]	'	'
Identifiable assets	$6,431.40	$6,591	[1]
Excess and Surplus Lines [Member]	'	'
Identifiable Assets [Line Items]	'	'
Identifiable assets	2,310.10	2,252.20
Commercial Specialty [Member]	'	'
Identifiable Assets [Line Items]	'	'
Identifiable assets	1,332.60	1,333.40
International Specialty [Member]	'	'
Identifiable Assets [Line Items]	'	'
Identifiable assets	781.2	743.6
Syndicate 1200 [Member]	'	'
Identifiable Assets [Line Items]	'	'
Identifiable assets	1,390.90	1,638.80
Run-Off Lines [Member]	'	'
Identifiable Assets [Line Items]	'	'
Identifiable assets	556.1	555.9
Corporate and Other [Member]	'	'
Identifiable Assets [Line Items]	'	'
Identifiable assets	$60.50	$67.10
[1]	Derived from audited consolidated financial statements.

Segment_Information_Additional

Segment Information - Additional Information (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Segment Reporting [Abstract]	'	'
Assets associated with trade capital providers	$246.40	$386.80

Information_Provided_in_Connec2

Information Provided in Connection With Outstanding Debt of Subsidiaries - Additional Information (Detail)	Mar. 31, 2014
Debt Disclosure [Abstract]	'
Ownership Percentage	100.00%

Information_Provided_in_Connec3

Information Provided in Connection with Outstanding Debt of Subsidiaries - Condensed Consolidating Balance Sheet (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013		Mar. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Assets	'	'		'	'
Investments	$4,118.50	$4,079.20	[1]	'	'
Cash	149.5	157.4	[1]	91.2	95.8
Accrued investment income	25	25.7	[1]	'	'
Premiums receivable	371.1	348.4	[1]	'	'
Reinsurance recoverables	978.1	1,263.50	[1]	'	'
Goodwill and other intangible assets, net	238.5	239.8		'	'
Current income taxes receivable, net	10.4	0	[1]	'	'
Deferred acquisition costs, net	117.5	113.9	[1]	'	'
Ceded unearned premiums	251.9	196.3	[1]	'	'
Other assets	170.9	166.8	[1]	'	'
Due from affiliates	0	0		'	'
Intercompany note receivable	0	0		'	'
Investments in subsidiaries	0	0		'	'
Total assets	6,431.40	6,591	[1]	'	'
Liabilities and Shareholders' Equity	'	'		'	'
Reserves for losses and loss adjustment expenses	3,042.40	3,230.30	[1]	'	'
Unearned premiums	786.3	779.1	[1]	'	'
Funds held and ceded reinsurance payable, net	392.1	398.4		'	'
Long-term debt	403.6	403.4		'	'
Current income taxes payable, net	0	5.2	[1]	'	'
Deferred tax liabilities, net	34.2	28.7	[1]	'	'
Accrued underwriting expenses and other liabilities	177.9	182.9		'	'
Intercompany note payable	0	0		'	'
Total liabilities	4,836.50	5,028	[1]	'	'
Total shareholders' equity	1,594.90	1,563	[1]	'	'
Total liabilities and shareholders' equity	6,431.40	6,591	[1]	'	'
Consolidating Adjustments [Member]	'	'		'	'
Assets	'	'		'	'
Investments	0	0		'	'
Cash	0	0		0	0
Accrued investment income	0	0		'	'
Premiums receivable	0	0		'	'
Reinsurance recoverables	0	0		'	'
Goodwill and other intangible assets, net	0	0		'	'
Current income taxes receivable, net	0	0		'	'
Deferred acquisition costs, net	0	0		'	'
Ceded unearned premiums	0	0		'	'
Other assets	0	0		'	'
Due from affiliates	-0.2	-3.1		'	'
Intercompany note receivable	0	0		'	'
Investments in subsidiaries	-1,666.10	-1,625.20		'	'
Total assets	-1,666.30	-1,628.30		'	'
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		'	'
Deferred tax liabilities, net	0	0		'	'
Accrued underwriting expenses and other liabilities	0	0		'	'
Intercompany note payable	-19.2	-6.6		'	'
Total liabilities	-19.2	-6.6		'	'
Total shareholders' equity	-1,647.10	-1,621.70		'	'
Total liabilities and shareholders' equity	-1,666.30	-1,628.30		'	'
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]	'	'		'	'
Assets	'	'		'	'
Investments	4.6	1.1		'	'
Cash	0	0		0	0
Accrued investment income	0	0		'	'
Premiums receivable	0	0		'	'
Reinsurance recoverables	0	0		'	'
Goodwill and other intangible assets, net	0	0		'	'
Current income taxes receivable, net	0	0		'	'
Deferred acquisition costs, net	0	0		'	'
Ceded unearned premiums	0	0		'	'
Other assets	6.4	4.6		'	'
Due from affiliates	0.2	3.1		'	'
Intercompany note receivable	0	0		'	'
Investments in subsidiaries	1,666.10	1,625.20		'	'
Total assets	1,677.30	1,634		'	'
Liabilities and Shareholders' Equity	'	'		'	'
Reserves for losses and loss adjustment expenses	0	0		'	'
Unearned premiums	0	0		'	'
Funds held and ceded reinsurance payable, net	0	0		'	'
Long-term debt	49	49		'	'
Current income taxes payable, net	'	0		'	'
Deferred tax liabilities, net	0	0		'	'
Accrued underwriting expenses and other liabilities	14.2	15.4		'	'
Intercompany note payable	19.2	6.6		'	'
Total liabilities	82.4	71		'	'
Total shareholders' equity	1,594.90	1,563		'	'
Total liabilities and shareholders' equity	1,677.30	1,634		'	'
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]	'	'		'	'
Assets	'	'		'	'
Investments	2,797.90	2,785.10		'	'
Cash	118.4	132.1		78.9	73.9
Accrued investment income	18.6	19.7		'	'
Premiums receivable	153.5	150.8		'	'
Reinsurance recoverables	1,165.90	1,145.90		'	'
Goodwill and other intangible assets, net	136.9	137.4		'	'
Current income taxes receivable, net	5.6	0.5		'	'
Deferred acquisition costs, net	54.4	55.6		'	'
Ceded unearned premiums	106.8	88.2		'	'
Other assets	107.4	97.8		'	'
Due from affiliates	4.7	1.5		'	'
Intercompany note receivable	88	87.4		'	'
Investments in subsidiaries	0	0		'	'
Total assets	4,758.10	4,702		'	'
Liabilities and Shareholders' Equity	'	'		'	'
Reserves for losses and loss adjustment expenses	2,174.10	2,157		'	'
Unearned premiums	421.8	425.5		'	'
Funds held and ceded reinsurance payable, net	682.2	642.4		'	'
Long-term debt	288.9	288.9		'	'
Current income taxes payable, net	'	8.9		'	'
Deferred tax liabilities, net	27.6	22.8		'	'
Accrued underwriting expenses and other liabilities	97	98.2		'	'
Intercompany note payable	0	0		'	'
Total liabilities	3,691.60	3,643.70		'	'
Total shareholders' equity	1,066.50	1,058.30		'	'
Total liabilities and shareholders' equity	4,758.10	4,702		'	'
Other Subsidiaries and Eliminations [Member]	'	'		'	'
Assets	'	'		'	'
Investments	1,316	1,293		'	'
Cash	31.1	25.3		12.3	21.9
Accrued investment income	6.4	6		'	'
Premiums receivable	217.6	197.6		'	'
Reinsurance recoverables	-187.8	117.6		'	'
Goodwill and other intangible assets, net	101.6	102.4		'	'
Current income taxes receivable, net	4.8	-0.5		'	'
Deferred acquisition costs, net	63.1	58.3		'	'
Ceded unearned premiums	145.1	108.1		'	'
Other assets	57.1	64.4		'	'
Due from affiliates	-4.7	-1.5		'	'
Intercompany note receivable	-88	-87.4		'	'
Investments in subsidiaries	0	0		'	'
Total assets	1,662.30	1,883.30		'	'
Liabilities and Shareholders' Equity	'	'		'	'
Reserves for losses and loss adjustment expenses	868.3	1,073.30		'	'
Unearned premiums	364.5	353.6		'	'
Funds held and ceded reinsurance payable, net	-290.1	-244		'	'
Long-term debt	65.7	65.5		'	'
Current income taxes payable, net	'	-3.7		'	'
Deferred tax liabilities, net	6.6	5.9		'	'
Accrued underwriting expenses and other liabilities	66.7	69.3		'	'
Intercompany note payable	0	0		'	'
Total liabilities	1,081.70	1,319.90		'	'
Total shareholders' equity	580.6	563.4		'	'
Total liabilities and shareholders' equity	$1,662.30	$1,883.30		'	'
[1]	Derived from audited consolidated financial statements.

Information_Provided_in_Connec4

Information Provided in Connection with Outstanding Debt of Subsidiaries - Condensed Consolidating Statement of Income (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Premiums and other revenue:	'	'
Earned premiums	$325.70	$304.20
Net investment income	23.3	27.9
Fee (expense) income, net	-1.4	0
Net realized investment gains and other	11.1	9.5
Total revenue	360.1	341.6
Expenses:	'	'
Losses and loss adjustment expenses	182.5	170.5
Other reinsurance-related expenses	0	5.1
Underwriting, acquisition and insurance expenses	128.7	126.7
Interest expense	5	4.9
Fee expense, net	1.4	0
Foreign currency exchange (gain) loss	-0.2	-3.1
Total expenses	317.4	304.1
Income (loss) before income taxes	42.7	37.5
Provision for income taxes	2.5	4.8
Net (loss) income before equity in earnings of subsidiaries	40.2	32.7
Equity in undistributed earnings of subsidiaries	0	0
Net income	40.2	32.7
Consolidating Adjustments [Member]	'	'
Premiums and other revenue:	'	'
Earned premiums	0	0
Net investment income	0	-0.3
Fee (expense) income, net	0	0
Net realized investment gains and other	0	0
Total revenue	0	-0.3
Expenses:	'	'
Losses and loss adjustment expenses	0	0
Other reinsurance-related expenses	'	0
Underwriting, acquisition and insurance expenses	0	0
Interest expense	-0.1	-0.3
Fee expense, net	0	0
Foreign currency exchange (gain) loss	0	0
Total expenses	-0.1	-0.3
Income (loss) before income taxes	0.1	0
Provision for income taxes	0	0
Net (loss) income before equity in earnings of subsidiaries	0.1	0
Equity in undistributed earnings of subsidiaries	-46.9	-39.6
Net income	-46.8	-39.6
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]	'	'
Premiums and other revenue:	'	'
Earned premiums	0	0
Net investment income	-0.1	0
Fee (expense) income, net	0	0
Net realized investment gains and other	0	0
Total revenue	-0.1	0
Expenses:	'	'
Losses and loss adjustment expenses	0	0
Other reinsurance-related expenses	'	0
Underwriting, acquisition and insurance expenses	6	6.2
Interest expense	0.6	0.8
Fee expense, net	0	0
Foreign currency exchange (gain) loss	0	-0.1
Total expenses	6.6	6.9
Income (loss) before income taxes	-6.7	-6.9
Provision for income taxes	0	0
Net (loss) income before equity in earnings of subsidiaries	-6.7	-6.9
Equity in undistributed earnings of subsidiaries	46.9	39.6
Net income	40.2	32.7
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]	'	'
Premiums and other revenue:	'	'
Earned premiums	112	107.7
Net investment income	15	20.9
Fee (expense) income, net	-1.3	-0.3
Net realized investment gains and other	11.5	7.6
Total revenue	138.5	135.9
Expenses:	'	'
Losses and loss adjustment expenses	72.5	64.3
Other reinsurance-related expenses	'	1.5
Underwriting, acquisition and insurance expenses	46.3	48.7
Interest expense	3.8	3.7
Fee expense, net	1.3	0.3
Foreign currency exchange (gain) loss	-0.1	0.2
Total expenses	123.8	118.4
Income (loss) before income taxes	14.7	17.5
Provision for income taxes	3	3.2
Net (loss) income before equity in earnings of subsidiaries	11.7	14.3
Equity in undistributed earnings of subsidiaries	0	0
Net income	11.7	14.3
Other Subsidiaries and Eliminations [Member]	'	'
Premiums and other revenue:	'	'
Earned premiums	213.7	196.5
Net investment income	8.4	7.3
Fee (expense) income, net	-0.1	0.3
Net realized investment gains and other	-0.4	1.9
Total revenue	221.7	206
Expenses:	'	'
Losses and loss adjustment expenses	110	106.2
Other reinsurance-related expenses	'	3.6
Underwriting, acquisition and insurance expenses	76.4	71.8
Interest expense	0.7	0.7
Fee expense, net	0.1	-0.3
Foreign currency exchange (gain) loss	-0.1	-3.2
Total expenses	187.1	179.1
Income (loss) before income taxes	34.6	26.9
Provision for income taxes	-0.5	1.6
Net (loss) income before equity in earnings of subsidiaries	35.1	25.3
Equity in undistributed earnings of subsidiaries	0	0
Net income	$35.10	$25.30

Information_Provided_in_Connec5

Information Provided in Connection with Outstanding Debt of Subsidiaries - Condensed Consolidating Statement of Cash Flows (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014		Mar. 31, 2013
Condensed Financial Statements, Captions [Line Items]	'		'
Net cash flows from operating activities	$43.60		($84)
Cash flows from investing activities:	'		'
Proceeds from sales of investments	263.3		436.2
Proceeds from maturities and mandatory calls of investments	78.4		115.1
Purchases of investments	-307.3		-442.8
Change in short-term investments and foreign regulatory deposits	-53.9		-6.4
Settlements of foreign currency exchange forward contracts	-1.4		-1.9
Issuance of intercompany note, net	0		0
Other, net	-18.1		-1.3
Cash (used) provided by investing activities	-45		94.7
Cash flows from financing activities:	'		'
Borrowings under intercompany note, net	0		0
Proceeds from issuance of senior unsecured fixed rate notes	'		0
Activity under stock incentive plans	0.8		1.3
Repurchase of Company's common shares	-3.7		-12.9
Excess tax expense from share-based payment arrangements	0.1		0
Payment of cash dividends to common shareholders	-4		-3.7
Cash used by financing activities	-6.8		-15.3
Effect of exchange rate changes on cash	0.3		0
Change in cash	-7.9		-4.6
Cash, beginning of period	157.4	[1]	95.8
Cash, end of period	149.5		91.2
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
Issuance of intercompany note, net	12.5		4.9
Other, net	0		0
Cash (used) provided by investing activities	12.5		4.9
Cash flows from financing activities:	'		'
Borrowings under intercompany note, net	-12.5		-4.9
Proceeds from issuance of senior unsecured fixed rate notes	'		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		'
Payment of cash dividends to common shareholders	0		0
Cash used by financing activities	-12.5		-4.9
Effect of exchange rate changes on cash	0		0
Change in cash	0		0
Cash, beginning of period	0		0
Cash, end of period	0		0
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]	'		'
Condensed Financial Statements, Captions [Line Items]	'		'
Net cash flows from operating activities	-9.4		-0.5
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	-0.9		-2.2
Settlements of foreign currency exchange forward contracts	0.9		0.2
Issuance of intercompany note, net	0		0
Other, net	0.1		0
Cash (used) provided by investing activities	0.1		-2
Cash flows from financing activities:	'		'
Borrowings under intercompany note, net	12.5		4.9
Proceeds from issuance of senior unsecured fixed rate notes	'		1.3
Activity under stock incentive plans	0.8		0
Repurchase of Company's common shares	0		0
Excess tax expense from share-based payment arrangements	0		'
Payment of cash dividends to common shareholders	-4		-3.7
Cash used by financing activities	9.3		2.5
Effect of exchange rate changes on cash	0		0
Change in cash	0		0
Cash, beginning of period	0		0
Cash, end of period	0		0
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]	'		'
Condensed Financial Statements, Captions [Line Items]	'		'
Net cash flows from operating activities	-11.2		2.7
Cash flows from investing activities:	'		'
Proceeds from sales of investments	163.3		199
Proceeds from maturities and mandatory calls of investments	51.3		86.7
Purchases of investments	-178.6		-282.9
Change in short-term investments and foreign regulatory deposits	-29.8		28.6
Settlements of foreign currency exchange forward contracts	0		0
Issuance of intercompany note, net	0		-10
Other, net	-5.1		-6.2
Cash (used) provided by investing activities	1.1		15.2
Cash flows from financing activities:	'		'
Borrowings under intercompany note, net	0		0
Proceeds from issuance of senior unsecured fixed rate notes	'		0
Activity under stock incentive plans	0		0
Repurchase of Company's common shares	-3.7		-12.9
Excess tax expense from share-based payment arrangements	0.1		'
Payment of cash dividends to common shareholders	0		0
Cash used by financing activities	-3.6		-12.9
Effect of exchange rate changes on cash	0		0
Change in cash	-13.7		5
Cash, beginning of period	132.1		73.9
Cash, end of period	118.4		78.9
Other Subsidiaries and Eliminations [Member]	'		'
Condensed Financial Statements, Captions [Line Items]	'		'
Net cash flows from operating activities	64.2		-86.2
Cash flows from investing activities:	'		'
Proceeds from sales of investments	100		237.2
Proceeds from maturities and mandatory calls of investments	27.1		28.4
Purchases of investments	-128.7		-159.9
Change in short-term investments and foreign regulatory deposits	-23.2		-32.8
Settlements of foreign currency exchange forward contracts	-2.3		-2.1
Issuance of intercompany note, net	-12.5		5.1
Other, net	-19.1		0.7
Cash (used) provided by investing activities	-58.7		76.6
Cash flows from financing activities:	'		'
Borrowings under intercompany note, net	0		0
Proceeds from issuance of senior unsecured fixed rate notes	'		-1.3
Activity under stock incentive plans	0		1.3
Repurchase of Company's common shares	0		0
Excess tax expense from share-based payment arrangements	0		'
Payment of cash dividends to common shareholders	0		0
Cash used by financing activities	0		0
Effect of exchange rate changes on cash	0.3		0
Change in cash	5.8		-9.6
Cash, beginning of period	25.3		21.9
Cash, end of period	$31.10		$12.30
[1]	Derived from audited consolidated financial statements.