Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Jul. 28, 2011 Tax Assessment Year 2007 to 2009 Internal Revenue Service (IRS)	Mar. 31, 2014 OneBeacon
Income Tax Contingency [Line Items]
Effective tax rates (as a percent)	27.00%	27.40%
U.S. statutory rate (as a percent)	35.00%	35.00%
Income Tax Expense (Benefit)	$ 30.3	$ 41.6		$ 5.0
Total tax assessment			69.3
Tax assessments related to disallowed deductions			$ 60.2