Municipal Bond Guarantee Insurance - Schedule of Municipal Bond Guarantee Insured Obligations (Details) - BAM $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) Contract	Dec. 31, 2019 USD ($) Contract	Jun. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)
Guarantor Obligations [Line Items]
Contracts outstanding | Contract	10,997	8,987
Remaining weighted average contract period (in years)	10 years 8 months 12 days	10 years 8 months 12 days
Principal	$ 75,287.7	$ 62,250.5	$ 36.9	$ 1,100.0
Interest and capital appreciation	36,448.8	31,799.7
Total debt service outstanding	111,736.5	94,050.2
Gross unearned insurance premiums	$ 237.5	$ 198.4