Municipal Bond Guarantee (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended	1 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended	6 Months Ended						3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jul. 31, 2012 White Mountains	Sep. 30, 2012 White Mountains	Dec. 31, 2012 White Mountains	Jun. 30, 2013 HG Global	Jun. 30, 2013 HG Global	Dec. 31, 2012 HG Global	Jun. 30, 2013 HG Global Preferred Stock	Dec. 31, 2012 HG Global Preferred Stock	Jun. 30, 2013 HG Global Common Stock	Dec. 31, 2012 HG Global Common Stock	Jun. 30, 2013 Build America Mutual	Jun. 30, 2013 Build America Mutual	Dec. 31, 2012 Noncontrolling interest	Dec. 31, 2012 HG Global
Guarantor Obligations [Line Items]
Municipal bond contract	318	3
Insured Financial Obligations with Credit Deterioration, Remaining Weighted Average Contract Period	14 years 2 months 12 days	10 years 4 months 24 days
Municipal bond outstanding principal	$ 2,125.7	$ 25.8
Municipal bond outstanding interest	1,260.3	8.9
Funds to Capitalize HG Global				600.0	594.5										14.5
Funds to Capitalized HG Global			594.5
Ownership interest (as a percent)									97.30%	97.30%	88.70%	88.70%
Surplus Notes								503.0
Statutory Accounting Practices, Statutory Capital and Surplus, Balance																421.7
Percentage of par value of policy reinsured	15.00%										15.00%
Income (Loss) from Continuing Operations before Income Taxes, Foreign						7.8	17.7						26.7	45.1
Interest income (expense), surplus notes						10.0	20.1						(10.0)	(20.1)
Financial Guarantee Insurance Contracts, Unearned Premium Revenue	$ 5.3	$ 0.1