Municipal Bond Guarantee Insurance - Schedule of BAM's Insured Obligations (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) Contract	Dec. 31, 2016 USD ($) Contract
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Gross unearned insurance premiums	$ 136.8	$ 82.9
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Contracts outstanding | Contract	6,371	4,807
Remaining weighted average contract period (in years)	10 years 11 months	10 years 10 months
Principal	$ 42,090.6	$ 33,057.3
Interest	21,057.1	16,396.6
Total debt service outstanding	63,147.7	$ 49,453.9
Gross unearned insurance premiums	$ 136.8