Outstanding Exposure - Components of BIG Net Par Outstanding (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 264,952	$ 296,318
Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,199	2,057
Trust preferred securities (TruPS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,349	1,892
Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	5,272	15,553
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	252,314	271,179
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	12,638	25,139
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	12,238	13,074
BIG [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	85	126
BIG [Member] | Trust preferred securities (TruPS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	161	430
BIG [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	360	645
BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	8,871	8,722
BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	3,367	4,352
BIG [Member] | BIG 1 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	4,528	4,495
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	161	304
BIG [Member] | BIG 1 [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	170	304
BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	3,823	3,690
BIG [Member] | BIG 1 [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	705	805
BIG [Member] | BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,361	4,059
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	0	126
BIG [Member] | BIG 2 [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	118	263
BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	939	3,177
BIG [Member] | BIG 2 [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	422	882
BIG [Member] | BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	6,349	4,520
BIG [Member] | BIG 3 [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	85	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	72	78
BIG [Member] | BIG 3 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	4,109	1,855
BIG [Member] | BIG 3 [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,240	2,665
United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	220,616
United States [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	4,818	5,637
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	209,392	244,798
United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	11,224	22,057
United States [Member] | BIG [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,761	3,151
United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	7,140	7,380
United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	3,261	4,059
United States [Member] | BIG [Member] | BIG 1 [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	374	197
United States [Member] | BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,368	2,402
United States [Member] | BIG [Member] | BIG 2 [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	304	493
United States [Member] | BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	663	3,123
United States [Member] | BIG [Member] | BIG 3 [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,083	2,461
United States [Member] | BIG [Member] | BIG 3 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	4,109	1,855
Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	44,336
Non United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	42,922	26,381
Non United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,414	3,082
Non United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,731	1,342
Non United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	106	293
Non United States [Member] | BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,455	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	276	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