Outstanding Exposure - Components of BIG Net Par Outstanding (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 298,243	$ 296,318
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	277,393	271,179
Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,057	2,057
Trust Preferred Securities (TruPS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,616	1,892
Student Loan [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,400
Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	11,820	15,553
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	20,850	25,139
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	13,228	13,074
BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	9,272	8,722
BIG [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	126	126
BIG [Member] | Trust Preferred Securities (TruPS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	254	430
BIG [Member] | Student Loan [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	118
BIG [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	567	645
BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	3,956	4,352
BIG [Member] | BIG 1 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	4,914	4,495
BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	4,234	3,690
BIG [Member] | BIG 1 [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	0	0
BIG [Member] | BIG 1 [Member] | Trust Preferred Securities (TruPS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	192	304
BIG [Member] | BIG 1 [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	298	304
BIG [Member] | BIG 1 [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	680	805
BIG [Member] | BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	3,831	4,059
BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	3,164	3,177
BIG [Member] | BIG 2 [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	0	0
BIG [Member] | BIG 2 [Member] | Trust Preferred Securities (TruPS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	62	126
BIG [Member] | BIG 2 [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	193	263
BIG [Member] | BIG 2 [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	667	882
BIG [Member] | BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	4,483	4,520
BIG [Member] | BIG 3 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,874	1,855
BIG [Member] | BIG 3 [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	126	126
BIG [Member] | BIG 3 [Member] | Trust Preferred Securities (TruPS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	0	0
BIG [Member] | BIG 3 [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	76	78
BIG [Member] | BIG 3 [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,609	2,665
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	238,050	244,798
United States [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	5,357	5,637
United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	18,446	22,057
United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	7,199	7,380
United States [Member] | BIG [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	3,009	3,151
United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	3,707	4,059
United States [Member] | BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,424	2,402
United States [Member] | BIG [Member] | BIG 1 [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	190	197
United States [Member] | BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,901	3,123
United States [Member] | BIG [Member] | BIG 2 [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	412	493
United States [Member] | BIG [Member] | BIG 3 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,874	1,855
United States [Member] | BIG [Member] | BIG 3 [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,407	2,461
Non United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	39,343	26,381
Non United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,404	3,082
Non United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,073	1,342
Non United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	249	293
Non United States [Member] | BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,810	1,288
Non United States [Member] | BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	263	54
Non United States [Member] | BIG [Member] | BIG 3 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 0	$ 0