Outstanding Insurance Exposure - Financial Guaranty Portfolio by Asset Class (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	$ 238,156	$ 244,191
Net par amount outstanding	236,807	241,802
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	226,746	230,665
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	10,061	11,137
United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	184,787
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	176,047	187,919
Net par amount outstanding	175,487	186,562
United States [Member] | Public Finance [Member] | General obligation [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	73,467	78,800
United States [Member] | Public Finance [Member] | Tax backed [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	37,047	40,616
United States [Member] | Public Finance [Member] | Municipal utilities [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	26,195	28,402
United States [Member] | Public Finance [Member] | Transportation [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	16,209	15,197
United States [Member] | Public Finance [Member] | Health Care [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	7,148	6,750
United States [Member] | Public Finance [Member] | Higher education [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	5,916	6,643
United States [Member] | Public Finance [Member] | Infrastructure finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	5,429	5,489
United States [Member] | Public Finance [Member] | Housing [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,321	1,435
United States [Member] | Public Finance [Member] | Investor-owned utilities [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	655	846
United States [Member] | Public Finance [Member] | Renewable Energy [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	210	215
United States [Member] | Public Finance [Member] | Other public/structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,890	2,169
United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	9,800	10,352
Net par amount outstanding	9,300	9,944
United States [Member] | Structured Finance [Member] | Other public/structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	596	675
United States [Member] | Structured Finance [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	3,546	4,270
United States [Member] | Structured Finance [Member] | Life Insurance Transactions [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,776	1,435
United States [Member] | Structured Finance [Member] | Financial product [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,019	1,094
United States [Member] | Structured Finance [Member] | Consumer receivables [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	962	1,255
United States [Member] | Structured Finance [Member] | Pooled corporate obligations [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,401	1,215
Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	52,020
Non United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	51,538	44,714
Net par amount outstanding	51,259	44,103
Non United States [Member] | Public Finance [Member] | Infrastructure finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	17,952	17,166
Non United States [Member] | Public Finance [Member] | Renewable Energy [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,555	1,346
Non United States [Member] | Public Finance [Member] | Regulated utilities [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	18,995	18,124
Non United States [Member] | Public Finance [Member] | Sovereign and Sub Sovereign [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	11,341	6,094
Non United States [Member] | Public Finance [Member] | Pooled Infrastructure Transactions [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,416	1,373
Non United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	771	1,206
Net par amount outstanding	761	1,193
Non United States [Member] | Structured Finance [Member] | Other public/structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	279	491
Non United States [Member] | Structured Finance [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	427	576
Non United States [Member] | Structured Finance [Member] | Pooled corporate obligations [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 55	$ 126