Municipal Bond Guarantee (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended	1 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended						3 Months Ended	12 Months Ended	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Jul. 31, 2012 White Mountains	Sep. 30, 2012 White Mountains	Dec. 31, 2012 White Mountains	Mar. 31, 2013 HG Global	Dec. 31, 2012 HG Global	Mar. 31, 2013 HG Global Preferred Stock	Dec. 31, 2012 HG Global Preferred Stock	Mar. 31, 2013 HG Global Common Stock	Dec. 31, 2012 HG Global Common Stock	Mar. 31, 2013 Build America Mutual	Dec. 31, 2012 Non-controlling interest	Mar. 31, 2013 BAM	Dec. 31, 2012 HG Global
Guarantor Obligations [Line Items]
Municipal bond contract	105	3
Insured Financial Obligations with Credit Deterioration, Remaining Weighted Average Contract Period	13 years 1 month 6 days	10 years 4 months 24 days
Municipal bond outstanding principal	$ 698.6	$ 25.8
Municipal bond outstanding interest	497.6	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															417.7
Percentage of par value of policy reinsured	15.00%									15.00%
Income (Loss) from Continuing Operations before Income Taxes, Foreign						9.9						18.4
Interest income (expense), surplus notes						10.1								10.1
Financial Guarantee Insurance Contracts, Unearned Premium Revenue	$ 2.0	$ 0.1