Consolidating Financial Information (Consolidating Balance Sheet) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
Investments, Debt and Equity Securities [Abstract]
Fixed maturity investments, at fair value	$ 1,743.6		$ 1,700.9
Short-term investments, at amortized cost (which approximates fair value)	152.7		157.0
Common equity securities, at fair value	347.1		336.9
Convertible fixed maturity investments, at fair value	29.3		30.5
Other investments	143.0		139.6
Total investments	2,415.7		2,364.9
Cash	138.6		168.1	99.4	43.9
Reinsurance recoverables	83.1	[1]	89.9
Premiums receivable	249.2		228.2
Deferred acquisition costs	105.9		103.7
Net deferred tax asset	79.8		90.6
Investment income accrued	9.1		10.1
Accounts receivable on unsettled investment sales	6.7		3.3
Investments in subsidiaries	0		0
Other assets	289.9		272.7
Assets held for sale	1,803.1		1,880.1
Total assets	5,181.1		5,211.6
Liabilities
Unpaid loss and loss adjustment expense reserves	1,069.8		1,054.3	1,010.3	1,000.0
Unearned premiums	582.2		544.9
Debt	274.7		274.7
Accounts payable on unsettled investment purchases	25.6		11.6
Other liabilities	292.6		338.6
Liabilities held for sale	1,803.1		1,880.1
Total liabilities	4,048.0		4,104.2
Equity [Abstract]
OneBeacon's common shareholders' equity	1,130.3		1,104.3
Total noncontrolling interests	2.8		3.1
Total OneBeacon's common shareholders' equity and noncontrolling interests	1,133.1		1,107.4	1,066.3	1,017.3
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	5,181.1		5,211.6
The Company (guarantor)
Investments, Debt and Equity Securities [Abstract]
Fixed maturity investments, at fair value	0		0
Short-term investments, at amortized cost (which approximates fair value)	2.0		13.0
Common equity securities, at fair value	0		0
Convertible fixed maturity investments, at fair value	0		0
Other investments	0		0
Total investments	2.0		13.0
Cash	0		0	0	0
Reinsurance recoverables	0		0
Premiums receivable	0		0
Deferred acquisition costs	0		0
Net deferred tax asset	0		0
Investment income accrued	0		0
Accounts receivable on unsettled investment sales	0		0
Investments in subsidiaries	1,130.5		1,092.0
Other assets	(2.3)		(0.8)
Assets held for sale	0		0
Total assets	1,130.2		1,104.2
Liabilities
Unpaid loss and loss adjustment expense reserves	0		0
Unearned premiums	0		0
Debt	0		0
Accounts payable on unsettled investment purchases	0		0
Other liabilities	(0.1)		(0.1)
Liabilities held for sale	0		0
Total liabilities	(0.1)		(0.1)
Equity [Abstract]
OneBeacon's common shareholders' equity	1,130.3		1,104.3
Total noncontrolling interests	0		0
Total OneBeacon's common shareholders' equity and noncontrolling interests	1,130.3		1,104.3
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	1,130.2		1,104.2
Non-guarantor subsidiaries
Investments, Debt and Equity Securities [Abstract]
Fixed maturity investments, at fair value	1,765.7		1,725.4
Short-term investments, at amortized cost (which approximates fair value)	143.3		140.0
Common equity securities, at fair value	347.1		336.9
Convertible fixed maturity investments, at fair value	29.3		30.5
Other investments	143.0		139.6
Total investments	2,428.4		2,372.4
Cash	138.6		166.6	99.3	43.9
Reinsurance recoverables	83.1		89.9
Premiums receivable	249.2		228.2
Deferred acquisition costs	105.9		103.7
Net deferred tax asset	82.1		93.1
Investment income accrued	9.6		10.3
Accounts receivable on unsettled investment sales	6.7		3.3
Investments in subsidiaries	0		0
Other assets	289.5		272.4
Assets held for sale	1,803.1		1,880.1
Total assets	5,196.2		5,220.0
Liabilities
Unpaid loss and loss adjustment expense reserves	1,069.8		1,054.3
Unearned premiums	582.2		544.9
Debt	0		0
Accounts payable on unsettled investment purchases	25.6		11.6
Other liabilities	285.9		335.0
Liabilities held for sale	1,803.1		1,880.1
Total liabilities	3,766.6		3,825.9
Equity [Abstract]
OneBeacon's common shareholders' equity	1,426.8		1,391.0
Total noncontrolling interests	2.8		3.1
Total OneBeacon's common shareholders' equity and noncontrolling interests	1,429.6		1,394.1
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	5,196.2		5,220.0
OBH (issuer)
Investments, Debt and Equity Securities [Abstract]
Fixed maturity investments, at fair value	2.9		0
Short-term investments, at amortized cost (which approximates fair value)	7.4		4.0
Common equity securities, at fair value	0		0
Convertible fixed maturity investments, at fair value	0		0
Other investments	0		0
Total investments	10.3		4.0
Cash	0		1.5	0.1	0
Reinsurance recoverables	0		0
Premiums receivable	0		0
Deferred acquisition costs	0		0
Net deferred tax asset	(2.4)		(2.4)
Investment income accrued	0		0
Accounts receivable on unsettled investment sales	0		0
Investments in subsidiaries	1,071.0		1,034.9
Other assets	2.7		1.1
Assets held for sale	0		0
Total assets	1,081.6		1,039.1
Liabilities
Unpaid loss and loss adjustment expense reserves	0		0
Unearned premiums	0		0
Debt	299.7		299.7
Accounts payable on unsettled investment purchases	0		0
Other liabilities	7.3		3.9
Liabilities held for sale	0		0
Total liabilities	307.0		303.6
Equity [Abstract]
OneBeacon's common shareholders' equity	774.6		735.5
Total noncontrolling interests	0		0
Total OneBeacon's common shareholders' equity and noncontrolling interests	774.6		735.5
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	1,081.6		1,039.1
Consolidating adjustments
Investments, Debt and Equity Securities [Abstract]
Fixed maturity investments, at fair value	(25.0)		(24.5)
Short-term investments, at amortized cost (which approximates fair value)	0		0
Common equity securities, at fair value	0		0
Convertible fixed maturity investments, at fair value	0		0
Other investments	0		0
Total investments	(25.0)		(24.5)
Cash	0		0	0	0
Reinsurance recoverables	0		0
Premiums receivable	0		0
Deferred acquisition costs	0		0
Net deferred tax asset	0.1		(0.1)
Investment income accrued	(0.5)		(0.2)
Accounts receivable on unsettled investment sales	0		0
Investments in subsidiaries	(2,201.5)		(2,126.9)
Other assets	0		0
Assets held for sale	0		0
Total assets	(2,226.9)		(2,151.7)
Liabilities
Unpaid loss and loss adjustment expense reserves	0		0
Unearned premiums	0		0
Debt	(25.0)		(25.0)
Accounts payable on unsettled investment purchases	0		0
Other liabilities	(0.5)		(0.2)
Liabilities held for sale	0		0
Total liabilities	(25.5)		(25.2)
Equity [Abstract]
OneBeacon's common shareholders' equity	(2,201.4)		(2,126.5)
Total noncontrolling interests	0		0
Total OneBeacon's common shareholders' equity and noncontrolling interests	(2,201.4)		(2,126.5)
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	$ (2,226.9)		$ (2,151.7)

[1] Standard & Poor's ratings as detailed above are "AA" (Very strong), "A" (Strong) and "BBB+" (Adequate).