Income Taxes (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended		9 Months Ended
	Jan. 31, 2012	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 M	Sep. 30, 2011	Jan. 24, 2012
Income Taxes
Effective tax rates (as a percent)		24.00%	(48.10%)	20.20%	4.00%
U.S. statutory rate (as a percent)		35.00%	35.00%	35.00%	35.00%
Effective tax rates non-U.S. (as a percent)		0.70%	4.10%	0.70%	2.90%
Effective tax rates U.S. (as a percent)		37.40%	(30.90%)	34.00%	(6.50%)
Foreign tax credit carryforwards		$ 20.6		$ 20.6
Reasonably possible period during which resolution could occur (in months)				12
Expected decrease in unrecognized tax benefits upon completion of the Runoff Transaction				11.5
Proceeds from sale of OneBeacon Holdings (Luxembourg) S.a r.l.	24.7
Net operating loss carryforwards						$ 1,031.7