Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended			9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Jul. 28, 2011 Tax Assessment Year 2007 to 2009 Internal Revenue Service (IRS)	Sep. 30, 2014 OneBeacon
Income Tax Contingency [Line Items]
Effective tax rates (as a percent)	22.70%	14.80%	24.30%	23.40%
U.S. statutory rate (as a percent)	35.00%	35.00%	35.00%	35.00%
Income Tax Expense (Benefit)	$ 7.8	$ 8.2	$ 62.0	$ 49.2		$ (5.0)
Total tax assessment					72.0
Tax assessments related to disallowed deductions					$ 60.2