Municipal Bond Guarantee Insurance (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Percentage of par value of policy reinsured					15.00%						15.00%
Pre-tax loss	$ 134.2	$ 55.3	$ 3.6	$ 151.8	$ (50.8)	$ 135.1	$ 37.6	$ 140.9	$ 344.9		$ 262.8		$ 98.0
Net investment income (loss) - surplus note interest									0		0
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Statutory capital and surplus	469.0								469.0
Municipal bond contract	218				3				218		3
Insured Financial Obligations with Credit Deterioration, Remaining Weighted Average Contract Period									13 years 9 months 21 days		10 years 4 months 24 days
Municipal bond outstanding principal	4,703.7				25.8				4,703.7		25.8
Municipal bond outstanding interest	3,264.4				8.9				3,264.4		8.9
Financial Guarantee Insurance Contracts, Unearned Premium Revenue	13.2				0.1				13.2		0.1
White Mountains
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Capital contributions											594.5
HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Capital contributions						600.0
Ownership interest (as a percent)	97.30%								97.30%
Surplus notes	503.0				503.0				503.0		503.0
Percentage of par value of policy reinsured	15.00%								15.00%
BAM Management
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Capital contributions											14.5
Preferred stocks | HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ownership interest (as a percent)	97.30%				97.30%				97.30%		97.30%
Common Stock | HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ownership interest (as a percent)	88.70%				88.70%				88.70%		88.70%
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes	503.0								503.0
Interest income (expense), surplus notes									(40.2)	[1]	(18.4)	[1]
HG Re
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Statutory capital and surplus	436.9				412.0				436.9		412.0
HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Percentage of par value of policy reinsured	15.00%								15.00%
Pre-tax loss									38.1		14.2
Net investment income (loss) - surplus note interest									40.2		18.4
Non-controlling Interests | BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Pre-tax loss									(78.6)	[1]	(36.3)	[1]
Surplus Note | HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Interest Receivable	$ 58.6								$ 58.6

[1] BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes surplus notes and is not reduced by accruals of interest expense on the surplus notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the New York Department of Financial Services.