Outstanding Exposure - Geographic Distribution of Net Par Outstanding (Details) $ in Millions	Dec. 31, 2017 USD ($) risk	Dec. 31, 2016 USD ($)
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	10,239
Net Par Outstanding	$ 264,952	$ 296,318
Percent of Total Net Par Outstanding	100.00%
United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	10,029
Net Par Outstanding	$ 220,616
Percent of Total Net Par Outstanding	83.30%
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 4,966	4,786
Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	210
Net Par Outstanding	$ 44,336
Percent of Total Net Par Outstanding	16.70%
United Kingdom [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	126
Net Par Outstanding	$ 30,062
Percent of Total Net Par Outstanding	11.30%
France [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	10
Net Par Outstanding	$ 3,167
Percent of Total Net Par Outstanding	1.20%
Canada [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	9
Net Par Outstanding	$ 2,690
Percent of Total Net Par Outstanding	1.00%
Australia [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	12
Net Par Outstanding	$ 2,309
Percent of Total Net Par Outstanding	0.90%
Italy [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	9
Net Par Outstanding	$ 1,497
Percent of Total Net Par Outstanding	0.60%
Other Countries [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	44
Net Par Outstanding	$ 4,611
Percent of Total Net Par Outstanding	1.70%
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 252,314	271,179
Public Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	9,517
Net Par Outstanding	$ 209,392	244,798
Percent of Total Net Par Outstanding	79.10%
Public Finance [Member] | California [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	1,368
Net Par Outstanding	$ 36,507
Percent of Total Net Par Outstanding	13.80%
Public Finance [Member] | Texas [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	1,229
Net Par Outstanding	$ 19,027
Percent of Total Net Par Outstanding	7.20%
Public Finance [Member] | Pennsylvania [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	744
Net Par Outstanding	$ 18,061
Percent of Total Net Par Outstanding	6.80%
Public Finance [Member] | Illinois [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	702
Net Par Outstanding	$ 17,044
Percent of Total Net Par Outstanding	6.40%
Public Finance [Member] | New York [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	871
Net Par Outstanding	$ 15,672
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	444
Net Par Outstanding	$ 12,441
Percent of Total Net Par Outstanding	4.70%
Public Finance [Member] | Florida [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	294
Net Par Outstanding	$ 10,272
Percent of Total Net Par Outstanding	3.90%
Public Finance [Member] | Michigan [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	439
Net Par Outstanding	$ 6,353
Percent of Total Net Par Outstanding	2.40%
Public Finance [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	18
Net Par Outstanding	$ 4,968
Percent of Total Net Par Outstanding	1.90%
Public Finance [Member] | Alabama [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	296
Net Par Outstanding	$ 4,808
Percent of Total Net Par Outstanding	1.80%
Public Finance [Member] | Other States and U.S. Territories [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	3,112
Net Par Outstanding	$ 64,239
Percent of Total Net Par Outstanding	24.30%
Public Finance [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 42,922	26,381
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 12,638	25,139
Structured Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	512
Net Par Outstanding	$ 11,224	22,057
Percent of Total Net Par Outstanding	4.20%
Structured Finance [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 1,414	$ 3,082