Outstanding Exposure - Financial Guaranty Portfolio by Asset Class (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	$ 244,191	$ 269,386
Net Par Outstanding	241,802	264,952
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	230,665	252,314
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	11,137	12,638
United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	196,506
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	187,919	211,441
Net Par Outstanding	186,562	209,392
United States [Member] | Public Finance [Member] | General obligation [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	78,800	90,705
United States [Member] | Public Finance [Member] | Tax backed [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	40,616	44,350
United States [Member] | Public Finance [Member] | Municipal utilities [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	28,462	32,357
United States [Member] | Public Finance [Member] | Transportation [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	15,197	17,030
United States [Member] | Public Finance [Member] | Health Care [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	6,750	8,763
United States [Member] | Public Finance [Member] | Higher education [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	6,643	8,195
United States [Member] | Public Finance [Member] | Infrastructure finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	5,489	4,216
United States [Member] | Public Finance [Member] | Housing [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,435	1,319
United States [Member] | Public Finance [Member] | Investor-owned utilities [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,001	523
United States [Member] | Public Finance [Member] | Other public/structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	2,169	1,934
United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	10,352	11,652
Net Par Outstanding	9,944	11,224
United States [Member] | Structured Finance [Member] | Other public/structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	675	602
United States [Member] | Structured Finance [Member] | Pooled corporate obligations [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,215	1,347
United States [Member] | Structured Finance [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	4,270	4,818
United States [Member] | Structured Finance [Member] | Consumer receivables [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,255	1,590
United States [Member] | Structured Finance [Member] | Insurance Securitizations [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,435	1,449
United States [Member] | Structured Finance [Member] | Financial product [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,094	1,418
Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	45,296
Non United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	44,714	44,860
Net Par Outstanding	44,103	42,922
Non United States [Member] | Public Finance [Member] | Infrastructure finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	17,216	18,234
Non United States [Member] | Public Finance [Member] | Regulated utilities [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	18,325	16,689
Non United States [Member] | Public Finance [Member] | Pooled infrastructure [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,373	1,561
Non United States [Member] | Public Finance [Member] | Other public/structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	7,189	6,438
Non United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	1,206	1,433
Net Par Outstanding	1,193	1,414
Non United States [Member] | Structured Finance [Member] | Other public/structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	491	620
Non United States [Member] | Structured Finance [Member] | Pooled corporate obligations [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	126	157
Non United States [Member] | Structured Finance [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 576	$ 637