Outstanding Exposure - BIG Net Par Outstanding (Details) $ in Millions	Sep. 30, 2021 USD ($) risk rISK	Dec. 31, 2020 USD ($) risk rISK
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, credit derivative	$ 4,286	$ 5,371
Total	235,668	234,153
BIG
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	7,284	7,855
Net par outstanding, credit derivative	62	120
Total	$ 7,346	$ 7,975
Number of risks, financial guaranty insurance | risk	257	270
Number of risks, credit derivative | rISK	11	14
Number of risks | rISK	268	284
BIG | BIG 1
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	$ 2,381	$ 2,781
Net par outstanding, credit derivative	16	70
Total	$ 2,397	$ 2,851
Number of risks, financial guaranty insurance | risk	112	125
Number of risks, credit derivative | rISK	3	6
Number of risks | rISK	115	131
BIG | BIG 2
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	$ 180	$ 130
Net par outstanding, credit derivative	4	4
Total	$ 184	$ 134
Number of risks, financial guaranty insurance | risk	17	19
Number of risks, credit derivative | rISK	1	1
Number of risks | rISK	18	20
BIG | BIG 3
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	$ 4,723	$ 4,944
Net par outstanding, credit derivative	42	46
Total	$ 4,765	$ 4,990
Number of risks, financial guaranty insurance | risk	128	126
Number of risks, credit derivative | rISK	7	7
Number of risks | rISK	135	133