Common Shareholders' Equity (Details) (USD $)	3 Months Ended		0 Months Ended	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Aug. 22, 2007 Class A common	Mar. 31, 2014 Class A common	Mar. 31, 2014 Common Stock	Mar. 31, 2013 Common Stock	Mar. 31, 2014 Total OneBeacon common shareholders' equity	Mar. 31, 2013 Total OneBeacon common shareholders' equity	Mar. 31, 2014 Accum. other comprehensive income (loss)	Mar. 31, 2013 Accum. other comprehensive income (loss)
Equity, Class of Treasury Stock [Line Items]
Stock Repurchase Program, Authorized Amount			$ 200,000,000	$ 87,700,000
Repurchase and retirement of common shares	1,800,000				106,366	3,300	1,800,000		0
Dividends	20,900,000	20,800,000					20,000,000	20,000,000	0	0
Dividends, per share, cash paid	$ 0.21	$ 0.21					$ 0.21	$ 0.21
Pre-tax changes to accumulated other comprehensive income	100,000	200,000
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	$ 0	$ (100,000)					$ 0	$ (100,000)	$ 0	$ (100,000)