Common Shareholders' Equity (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	1 Months Ended	3 Months Ended			9 Months Ended
	Jan. 31, 2012	Sep. 30, 2012	Mar. 31, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Additional Paid in Capital
Gain on sale of OneBeacon Holdings (Luxembourg) S.a r.l.			$ 14.0		$ (14.0)
Proceeds from sale of OneBeacon Holdings (Luxembourg) S.a r.l.	24.7
Gain on sale of subsidiary, net of transaction costs		13.6			13.6
Dividends on Common Shares
Dividends declared and paid per OneBeacon's common share (in dollars per share)		$ 0.21		$ 0.21	$ 0.63	$ 1.63
Cash dividends declared and paid					60.0	154.8
Cash dividends declared and paid, regular						59.7
Regular dividends declared and paid per share (in dollars per share)						$ 0.63
Cash dividends declared and paid, special						95.1
Recognized loss due to settlement and re-measurement through other comprehensive income		$ 3.7			$ 3.7