Outstanding Exposure - Components of BIG Net Par Outstanding (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 234,270	$ 234,153
Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	225,040	224,625
Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	6,380	6,538
Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	9,230	9,528
BIG
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	7,509	7,975
BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	5,913	6,334
BIG | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	154	161
BIG | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,596	1,641
BIG | BIG 1
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	2,361	2,851
BIG | BIG 1 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	2,203	2,623
BIG | BIG 1 | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	24	28
BIG | BIG 1 | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	158	228
BIG | BIG 2
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	132	134
BIG | BIG 2 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	58	57
BIG | BIG 2 | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	49	51
BIG | BIG 2 | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	74	77
BIG | BIG 3
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	5,016	4,990
BIG | BIG 3 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	3,652	3,654
BIG | BIG 3 | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	81	82
BIG | BIG 3 | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,364	1,336
United States | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	172,941	171,597
United States | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	2,850	2,990
United States | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	8,678	8,952
United States | BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	5,398	5,439
United States | BIG | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,442	1,480
United States | BIG | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,596	1,641
United States | BIG | BIG 1 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,735	1,777
United States | BIG | BIG 1 | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	134	200
United States | BIG | BIG 2 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	58	57
United States | BIG | BIG 2 | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	25	26
United States | BIG | BIG 3 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	3,605	3,605
United States | BIG | BIG 3 | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,283	1,254
Non United States | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	52,099	53,028
Non United States | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	552	576
Non United States | BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	515	895
Non United States | BIG | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	0	0
Non United States | BIG | BIG 1 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	468	846
Non United States | BIG | BIG 2 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	0	0
Non United States | BIG | BIG 3 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 47	$ 49