Municipal Bond Guarantee Insurance - Schedule of Municipal Bond Guarantee Insured Obligations (Details) - BAM $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2023 USD ($) contract	Dec. 31, 2022 USD ($) contract
Guarantor Obligations [Line Items]
Contracts outstanding | contract	14,084	13,382
Remaining weighted average contract period (in years)	11 years	10 years 9 months 18 days
Principal	$ 105,918.1	$ 99,996.9
Interest	52,628.8	48,880.6
Total debt service outstanding	158,546.9	148,877.5
Total gross unearned insurance premiums	$ 312.3	$ 298.3