Municipal Bond Guarantee Insurance - Schedule of Municipal Bond Guarantee Insured Obligations (Details) - BAM $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024 USD ($) contract	Dec. 31, 2023 USD ($) contract
Guarantor Obligations [Line Items]
Contracts outstanding | contract	14,725	14,485
Remaining weighted average contract period (in years)	10 years 6 months	11 years 1 month 6 days
Principal	$ 112,216.7	$ 109,673.8
Interest	57,545.8	56,012.2
Total debt service outstanding	169,762.5	165,686.0
Total gross unearned insurance premiums	$ 328.5	$ 325.8