Outstanding Insurance Exposure - Geographic Distribution of Net Par Outstanding (Details) $ in Millions	Dec. 31, 2019 USD ($) risk	Dec. 31, 2018 USD ($)
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	8,849
Net Par Outstanding	$ 236,807	$ 241,802
Percent of Total Net Par Outstanding	100.00%
United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	8,490
Net Par Outstanding	$ 184,787
Percent of Total Net Par Outstanding	78.10%
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 4,270	4,767
Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	359
Net Par Outstanding	$ 52,020
Percent of Total Net Par Outstanding	21.90%
United Kingdom [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	288
Net Par Outstanding	$ 38,450
Percent of Total Net Par Outstanding	16.20%
France [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	7
Net Par Outstanding	$ 3,130
Percent of Total Net Par Outstanding	1.30%
Canada [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	8
Net Par Outstanding	$ 2,495
Percent of Total Net Par Outstanding	1.10%
Australia [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	11
Net Par Outstanding	$ 2,112
Percent of Total Net Par Outstanding	0.90%
Austria [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	3
Net Par Outstanding	$ 1,250
Percent of Total Net Par Outstanding	0.50%
Other [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	42
Net Par Outstanding	$ 4,583
Percent of Total Net Par Outstanding	1.90%
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 226,746	230,665
Public Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	8,040
Net Par Outstanding	$ 175,487	186,562
Percent of Total Net Par Outstanding	74.20%
Public Finance [Member] | California [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	1,318
Net Par Outstanding	$ 33,368
Percent of Total Net Par Outstanding	14.10%
Public Finance [Member] | Pennsylvania [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	665
Net Par Outstanding	$ 15,895
Percent of Total Net Par Outstanding	6.70%
Public Finance [Member] | Texas [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	1,090
Net Par Outstanding	$ 14,860
Percent of Total Net Par Outstanding	6.30%
Public Finance [Member] | New York [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	749
Net Par Outstanding	$ 14,682
Percent of Total Net Par Outstanding	6.20%
Public Finance [Member] | Illinois [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	602
Net Par Outstanding	$ 13,977
Percent of Total Net Par Outstanding	5.90%
Public Finance [Member] | New Jersey [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	337
Net Par Outstanding	$ 10,504
Percent of Total Net Par Outstanding	4.40%
Public Finance [Member] | Florida [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	266
Net Par Outstanding	$ 7,107
Percent of Total Net Par Outstanding	3.00%
Public Finance [Member] | Michigan [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	305
Net Par Outstanding	$ 5,345
Percent of Total Net Par Outstanding	2.30%
Public Finance [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	17
Net Par Outstanding	$ 4,270
Percent of Total Net Par Outstanding	1.80%
Public Finance [Member] | Louisiana [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	162
Net Par Outstanding	$ 4,167
Percent of Total Net Par Outstanding	1.80%
Public Finance [Member] | Other [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	2,529
Net Par Outstanding	$ 51,312
Percent of Total Net Par Outstanding	21.70%
Public Finance [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 51,259	44,103
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 10,061	11,137
Structured Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	450
Net Par Outstanding	$ 9,300	9,944
Percent of Total Net Par Outstanding	3.90%
Structured Finance [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 761	$ 1,193