Outstanding Exposure - BIG Net Par Outstanding (Details) $ in Millions	Mar. 31, 2025 USD ($) risk	Dec. 31, 2024 USD ($) risk
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, credit derivative	$ 3,761	$ 4,212
Net	263,591	261,552
BIG
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	9,511	10,150
Net par outstanding, credit derivative	31	32
Net	$ 9,542	$ 10,182
Number of risks, financial guaranty insurance | risk	200	207
Number of risks, credit derivative | risk	7	7
Number of risks | risk	207	214
BIG | BIG 1
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	$ 7,052	$ 8,074
Net par outstanding, credit derivative	27	28
Net	$ 7,079	$ 8,102
Number of risks, financial guaranty insurance | risk	91	98
Number of risks, credit derivative | risk	3	3
Number of risks | risk	94	101
BIG | BIG 2
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	$ 1,095	$ 702
Net par outstanding, credit derivative	4	4
Net	$ 1,099	$ 706
Number of risks, financial guaranty insurance | risk	13	12
Number of risks, credit derivative | risk	1	1
Number of risks | risk	14	13
BIG | BIG 3
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	$ 1,364	$ 1,374
Net par outstanding, credit derivative	0	0
Net	$ 1,364	$ 1,374
Number of risks, financial guaranty insurance | risk	96	97
Number of risks, credit derivative | risk	3	3
Number of risks | risk	99	100