Outstanding Exposure - Geographic Distribution of Net Par Outstanding (Details) $ in Millions	Dec. 31, 2018 USD ($) risk	Dec. 31, 2017 USD ($)
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	9,413
Net Par Outstanding	$ 241,802	$ 264,952
Percent of Total Net Par Outstanding	100.00%
United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	9,200
Net Par Outstanding	$ 196,506
Percent of Total Net Par Outstanding	81.20%
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 4,767	4,966
Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	213
Net Par Outstanding	$ 45,296
Percent of Total Net Par Outstanding	18.80%
United Kingdom [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	130
Net Par Outstanding	$ 31,128
Percent of Total Net Par Outstanding	12.90%
France [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	10
Net Par Outstanding	$ 3,189
Percent of Total Net Par Outstanding	1.30%
Canada [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	9
Net Par Outstanding	$ 2,659
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	$ 2,103
Percent of Total Net Par Outstanding	0.90%
Italy [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	8
Net Par Outstanding	$ 1,176
Percent of Total Net Par Outstanding	0.50%
Other Countries [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	45
Net Par Outstanding	$ 5,041
Percent of Total Net Par Outstanding	2.10%
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 230,665	252,314
Public Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	8,715
Net Par Outstanding	$ 186,562	209,392
Percent of Total Net Par Outstanding	77.10%
Public Finance [Member] | California [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	1,361
Net Par Outstanding	$ 33,847
Percent of Total Net Par Outstanding	14.00%
Public Finance [Member] | Texas [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	1,154
Net Par Outstanding	$ 16,915
Percent of Total Net Par Outstanding	7.00%
Public Finance [Member] | Pennsylvania [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	704
Net Par Outstanding	$ 16,866
Percent of Total Net Par Outstanding	7.00%
Public Finance [Member] | New York [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	829
Net Par Outstanding	$ 15,077
Percent of Total Net Par Outstanding	6.20%
Public Finance [Member] | Illinois [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	642
Net Par Outstanding	$ 14,914
Percent of Total Net Par Outstanding	6.20%
Public Finance [Member] | New Jersey [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	370
Net Par Outstanding	$ 10,998
Percent of Total Net Par Outstanding	4.50%
Public Finance [Member] | Florida [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	273
Net Par Outstanding	$ 8,518
Percent of Total Net Par Outstanding	3.50%
Public Finance [Member] | Michigan [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	349
Net Par Outstanding	$ 5,635
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	18
Net Par Outstanding	$ 4,767
Percent of Total Net Par Outstanding	2.00%
Public Finance [Member] | Alabama [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	289
Net Par Outstanding	$ 4,230
Percent of Total Net Par Outstanding	1.70%
Public Finance [Member] | Other States and U.S. Territories [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	2,726
Net Par Outstanding	$ 54,795
Percent of Total Net Par Outstanding	22.70%
Public Finance [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 44,103	42,922
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 11,137	12,638
Structured Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	485
Net Par Outstanding	$ 9,944	11,224
Percent of Total Net Par Outstanding	4.10%
Structured Finance [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 1,193	$ 1,414