Discontinued Operations (Details 4) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Loss attributable to OneBeacon's common shareholders - basic and diluted:
Allocation of income for participating restricted common shares	$ (0.4)	$ 0.1	$ (1.0)	$ (0.1)
Net (loss) income attributable to OneBeacon's common shareholders	(62.2)	(32.9)	(5.9)	24.5
Loss per share denominator - basic and diluted:
Total weighted average common shares outstanding	95.4	95.1	95.3	94.7
Weighted average unvested restricted common shares	(0.9)	(0.7)	(0.8)	(0.3)
Basic earnings per share denominator	94.5	94.4	94.5	94.4
Loss per share attributable to OneBeacon's common shareholders - basic and diluted:
Net (loss) income attributable to OneBeacon's common shareholders per share (in dollars per share)	$ (0.65)	$ (0.35)	$ (0.07)	$ 0.25
Runoff and AutoOne business
Loss attributable to OneBeacon's common shareholders - basic and diluted:
Net loss attributable to OneBeacon's shareholders	(106.8)	(20.7)	(115.8)	(19.7)
Allocation of income for participating restricted common shares	1.0	0.1	1.0	0.1
Net (loss) income attributable to OneBeacon's common shareholders	$ (105.8)	$ (20.6)	$ (114.8)	$ (19.6)
Loss per share denominator - basic and diluted:
Total weighted average common shares outstanding	95.4	95.1	95.3	94.7
Weighted average unvested restricted common shares	(0.9)	(0.6)	(0.9)	(0.3)
Basic earnings per share denominator	94.5	94.5	94.4	94.4
Loss per share attributable to OneBeacon's common shareholders - basic and diluted:
Net (loss) income attributable to OneBeacon's common shareholders per share (in dollars per share)	$ (1.12)	$ (0.22)	$ (1.22)	$ (0.21)