Municipal Bond Guarantee Insurance - Schedule of Municipal Bond Guarantee Insured Obligations (Details) - BAM $ in Millions	12 Months Ended
	Dec. 31, 2022 USD ($) Contract	Dec. 31, 2021 USD ($) Contract	Sep. 30, 2022 USD ($)	Mar. 31, 2021 USD ($)	Jun. 30, 2020 USD ($)
Guarantor Obligations [Line Items]
Contracts outstanding | Contract	13,382	12,350
Remaining weighted average contract period (in years)	10 years 9 months 18 days	10 years 9 months 18 days
Principal	$ 99,996.9	$ 89,196.5	$ 42.5	$ 805.5	$ 36.9
Interest	48,880.6	41,486.5
Total debt service outstanding	148,877.5	130,683.0
Gross unearned insurance premiums (in millions)	$ 298.3	$ 266.3