Earnings per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended		63 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012
Common Shares Issued
Shares issued to directors (in shares)	20,055	21,980
Common Shares Repurchased And Retired
Common shares repurchased and retired (in shares)					5,600,000
Earnings attributable to OneBeacon's common shareholders - basic and diluted:
Net (loss) income attributable to OneBeacon's common shareholders	$ 44.6	$ (12.2)	$ 109.9	$ 44.2
Allocation of loss (income) for participating restricted common shares	(0.4)	0.1	(1.0)	(0.1)
Dividends paid on participating restricted common shares	(0.2)	(0.1)	(0.6)	(1.0)
Total allocation to restricted common shares	(0.6)		(1.6)	(1.1)
Net (loss) income attributable to OneBeacon's common shareholders, net of restricted common share amounts	44.0	(12.2)	108.3	43.1
Distributed current period net earnings:
Net income attributable to OneBeacon's common shareholders, net of restricted common share amounts	44.0	(12.2)	108.3	43.1
Dividends paid, net of restricted common shares	(19.8)	(19.8)	(59.5)	(153.8)
Total (overdistributed) undistributed current period net earnings, net of restricted common share amounts	$ 24.2	$ (32.0)	$ 48.8	$ (110.7)
Earnings per share denominator - basic and diluted:
Total weighted average common shares outstanding	95,400,000	95,100,000	95,300,000	94,700,000
Weighted average unvested restricted common shares	(900,000)	(700,000)	(800,000)	(300,000)
Basic earnings per share denominator	94,500,000	94,400,000	94,500,000	94,400,000
Earnings per share attributable to OneBeacon's common shareholders - basic and diluted:
Net (loss) income attributable to OneBeacon's common shareholders per share	$ 0.47	$ (0.13)	$ 1.15	$ 0.46
Dividends declared and paid per share	$ (0.21)	$ (0.21)	$ (0.63)	$ (1.63)
(Overdistributed) undistributed current period earnings per share	$ 0.26	$ (0.34)	$ 0.52	$ (1.17)