Municipal Bond Guarantee Insurance - Schedule of Municipal Bond Guarantee Insured Obligations (Details) - Build America Mutual Assurance Company $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2023	Dec. 31, 2023 USD ($) contract	Dec. 31, 2022 USD ($) contract
Guarantor Obligations [Line Items]
Contracts outstanding | contract		14,485	13,382
Remaining weighted average contract period (in years)	10 years 9 months 18 days	11 years 1 month 6 days
Principal		$ 109,673.8	$ 99,996.9
Interest		56,012.2	48,880.6
Total debt service outstanding		165,686.0	148,877.5
Gross unearned insurance premiums (in millions)		$ 325.8	$ 298.3