Outstanding Exposure - BIG Net Par Outstanding (Details) $ in Millions	Sep. 30, 2024 USD ($) risk	Dec. 31, 2023 USD ($) risk
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, credit derivative	$ 4,282	$ 3,608
Net	258,196	249,153
BIG
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	10,617	5,383
Net par outstanding, credit derivative	33	54
Net	$ 10,650	$ 5,437
Number of risks, financial guaranty insurance | risk	214	217
Number of risks, credit derivative | risk	7	11
Number of risks | risk	221	228
BIG | BIG 1
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	$ 7,456	$ 2,394
Net par outstanding, credit derivative	29	16
Net	$ 7,485	$ 2,410
Number of risks, financial guaranty insurance | risk	102	95
Number of risks, credit derivative | risk	2	2
Number of risks | risk	104	97
BIG | BIG 2
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	$ 1,546	$ 979
Net par outstanding, credit derivative	4	10
Net	$ 1,550	$ 989
Number of risks, financial guaranty insurance | risk	12	13
Number of risks, credit derivative | risk	1	2
Number of risks | risk	13	15
BIG | BIG 3
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	$ 1,615	$ 2,010
Net par outstanding, credit derivative	0	28
Net	$ 1,615	$ 2,038
Number of risks, financial guaranty insurance | risk	100	109
Number of risks, credit derivative | risk	4	7
Number of risks | risk	104	116