Municipal Bond Guarantee Insurance - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) yr	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2018
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Percentage of discounted future cash flows	10.00%
Percentage of par value of policy reinsured	15.00%
Percentage of risk premium ceded	60.00%
Repayments of debt, principal	$ 365.0	$ 404.6	$ 76.1
Interest paid	1.4	$ 2.1	$ 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]
Surplus notes				503.0
Assets Held-in-trust	$ 603.0
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Year of U.S. Treasury Rate | yr	1
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)	96.90%	96.90%
Common Stock | HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ownership interest (as a percent)	88.40%	88.40%
HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Percentage of par value of policy reinsured		15.00%
Collateral held in supplement trust	$ 715.1	$ 465.4
Interest receivable	126.0
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes	499.0	300.0
BAM | HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes	$ 499.0	$ 503.0
Surplus Note | BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Interest rate (as a percent)	3.78%	3.54%
Debt instrument, basis spread on variable rate	30000.00%
Surplus Note | BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Repayments of debt, principal	$ 4.0
Interest paid	$ 1.0
Combination One | Surplus Note | BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of years of change in interest rate | yr	5
Combination Two | Surplus Note | BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of years of change in interest rate | yr	3
Fixed interest rate | Surplus Note | BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Interest rate (as a percent)	8.00%
Series A BAM Surplus Note | HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes	$ 203.0
Series B BAM Surplus Note | HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes	$ 300.0
Forecast | Surplus Note | BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Interest rate (as a percent)					4.60%