Municipal Bond Guarantee Insurance (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Percentage of par value of policy reinsured	15.00%								15.00%
Pre-tax loss	$ 46.2	$ 34.4	$ 109.0	$ 112.1	$ 134.2	$ 55.3	$ 3.6	$ 151.8	$ 301.7	$ 344.9	$ 262.8
Net investment income (loss) - surplus note interest									0	0	0
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Municipal bond contract	1,750				701				1,750	701
Insured Financial Obligations with Credit Deterioration, Remaining Weighted Average Contract Period									12 years 10 months 0 days	13 years 9 months 21 days
Municipal bond outstanding principal	12,362.5				4,703.7				12,362.5	4,703.7
Municipal bond outstanding interest	7,086.9				3,264.4				7,086.9	3,264.4
Financial Guarantee Insurance Contracts, Unearned Premium Revenue	27.6				13.2				27.6	13.2
White Mountains
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Capital contributions										594.5	594.5
HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
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	14.5
Preferred stocks | HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ownership interest (as a percent)	96.90%				97.30%				96.90%	97.30%
Common Stock | HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ownership interest (as a percent)	88.40%				88.70%				88.40%	88.70%
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes	503.0								503.0
Pre-tax loss									(40.5)	(78.6)	(36.3)
Net investment income (loss) - surplus note interest									(15.7)	(40.2)	(18.4)
HG Re
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Statutory capital and surplus	449.1				436.9				449.1	436.9
Collateral Held in Supplement Trust	120,000,000				105,400,000				120,000,000	105,400,000
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									18.3	38.1	14.2
Net investment income (loss) - surplus note interest									15.7	40.2	18.4
Surplus Note | HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Interest Receivable	$ 74.4								$ 74.4