Earnings per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Earnings attributable to OneBeacon's common shareholders—basic and diluted (in millions):
Net income from continuing operations attributable to OneBeacon's common shareholders	$ 47.5		$ 67.9
Allocation of loss (income) for participating unvested restricted common shares	(0.4)		(0.7)
Dividends paid on participating restricted common shares	(0.1)		(0.2)
Total allocation to restricted common shares	(0.5)		(0.9)
Net income from continuing operations attributable to OneBeacon's common shareholders, net of restricted common share amounts	47.0		67.0
Undistributed Earnings, Basic [Abstract]
Net income from continuing operations attributable to OneBeacon's common shareholders, net of restricted common share amounts	47.0		67.0
Dividends paid, net of restricted common share amounts	(19.9)		(19.8)
Total undistributed net earnings, net of restricted common share amounts	$ 27.1		$ 47.2
Weighted Average Number of Shares Outstanding, Basic [Abstract]
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
Earnings Per Share, Basic and Diluted, Other Disclosures [Abstract]
Net income from continuing operations attributable to OneBeacon's common shareholders	$ 0.50		$ 0.71
Dividends declared and paid	$ (0.21)		$ (0.21)
Undistributed earnings	$ 0.29		$ 0.50

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