Discontinued Operations (Loss per Share Related to Discontinued Operations) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Income (loss) attributable to OneBeacon's common shareholders—basic and diluted (in millions):
Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders	$ (0.5)		$ 0.5
Allocation of loss for participating unvested restricted common shares	(0.4)		(0.7)
Net income attributable to OneBeacon's common shareholders	47.0		68.4
Income (loss) per share denominator—basic and diluted (in millions):
Total weighted average common shares outstanding	95.4		95.4
Weighted average unvested restricted common shares	(0.8)	[1]	(0.9)	[1]
Basic and diluted earnings (loss) per share denominator	94.6		94.5
Income (loss) per share attributable to OneBeacon's common shareholders—basic and diluted (in dollars):
Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders per share	$ 0.49		$ 0.71
Segment, Discontinued Operations
Income (loss) attributable to OneBeacon's common shareholders—basic and diluted (in millions):
Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders	(0.5)		0.5
Allocation of loss for participating unvested restricted common shares	0		0
Income (loss) from discontinued operations, net of tax	$ (0.5)		$ 0.5
Income (loss) per share denominator—basic and diluted (in millions):
Total weighted average common shares outstanding	95.4		95.4
Weighted average unvested restricted common shares	(0.8)		(0.9)
Basic and diluted earnings (loss) per share denominator	94.6		94.5
Income (loss) per share attributable to OneBeacon's common shareholders—basic and diluted (in dollars):
Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders per share	$ (0.01)		$ 0.00

[1] Restricted shares outstanding vest in equal installments upon a stated date or upon the occurrence of a specified event.