Outstanding Exposure - BIG Net Par Outstanding (Details) $ in Millions	Sep. 30, 2023 USD ($) risk	Aug. 31, 2023 USD ($)	Dec. 31, 2022 USD ($) risk
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, credit derivative	$ 3,489		$ 3,203
Net	241,833	$ 108	233,258
BIG
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	5,119		5,835
Net par outstanding, credit derivative	55		57
Net	$ 5,174		$ 5,892
Number of risks, financial guaranty insurance | risk	224		247
Number of risks, credit derivative | risk	11		13
Number of risks | risk	235		260
BIG | BIG 1
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	$ 2,115		$ 3,357
Net par outstanding, credit derivative	16		6
Net	$ 2,131		$ 3,363
Number of risks, financial guaranty insurance | risk	102		122
Number of risks, credit derivative | risk	2		1
Number of risks | risk	104		123
BIG | BIG 2
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	$ 980		$ 171
Net par outstanding, credit derivative	10		10
Net	$ 990		$ 181
Number of risks, financial guaranty insurance | risk	13		14
Number of risks, credit derivative | risk	2		2
Number of risks | risk	15		16
BIG | BIG 3
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	$ 2,024		$ 2,307
Net par outstanding, credit derivative	29		41
Net	$ 2,053		$ 2,348
Number of risks, financial guaranty insurance | risk	109		111
Number of risks, credit derivative | risk	7		10
Number of risks | risk	116		121