Municipal Bond Guarantee - Schedule of Municipal Bond Guarantee Insured Obligations (Details) - BAM $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($) Contract	Dec. 31, 2019 USD ($) Contract	Sep. 30, 2019 USD ($)
Guarantor Obligations [Line Items]
Contracts outstanding | Contract	10,495	8,987
Remaining weighted average contract period outstanding (in years)	10 years 8 months 12 days	10 years 8 months 12 days
Municipal bond outstanding principal	$ 70,954.1	$ 62,250.5	$ 1,100.0
Municipal bond outstanding interest	34,842.8	31,799.7
Municipal bond debt service outstanding	105,796.9	94,050.2
Total gross unearned insurance premiums	$ 226.9	$ 198.4