Outstanding Exposure - Components of BIG Net Par Outstanding (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 257,804	$ 264,952
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	245,820	252,314
Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,191	1,199
Trust Preferred Securities (TruPS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,183	1,349
Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	4,847	5,272
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	11,984	12,638
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	10,638	12,238
BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	7,447	8,871
BIG [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	85	85
BIG [Member] | Trust Preferred Securities (TruPS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	97	161
BIG [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	337	360
BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	3,191	3,367
BIG [Member] | BIG 1 [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	2,890	4,528
BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	2,428	3,823
BIG [Member] | BIG 1 [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	0	0
BIG [Member] | BIG 1 [Member] | Trust Preferred Securities (TruPS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	97	161
BIG [Member] | BIG 1 [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	183	170
BIG [Member] | BIG 1 [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	462	705
BIG [Member] | BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,112	1,361
BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	652	939
BIG [Member] | BIG 2 [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	0	0
BIG [Member] | BIG 2 [Member] | Trust Preferred Securities (TruPS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	0	0
BIG [Member] | BIG 2 [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	85	118
BIG [Member] | BIG 2 [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	460	422
BIG [Member] | BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	6,636	6,349
BIG [Member] | BIG 3 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	4,367	4,109
BIG [Member] | BIG 3 [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	85	85
BIG [Member] | BIG 3 [Member] | Trust Preferred Securities (TruPS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	0	0
BIG [Member] | BIG 3 [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	69	72
BIG [Member] | BIG 3 [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	2,269	2,240
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	200,378	209,392
United States [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	4,763	4,818
United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	10,749	11,224
United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	6,314	7,140
United States [Member] | BIG [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	2,672	2,761
United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	3,089	3,261
United States [Member] | BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,557	2,368
United States [Member] | BIG [Member] | BIG 1 [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	182	374
United States [Member] | BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	390	663
United States [Member] | BIG [Member] | BIG 2 [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	375	304
United States [Member] | BIG [Member] | BIG 3 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	4,367	4,109
United States [Member] | BIG [Member] | BIG 3 [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	2,115	2,083
Non United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	45,442	42,922
Non United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,235	1,414
Non United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,133	1,731
Non United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	102	106
Non United States [Member] | BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	871	1,455
Non United States [Member] | BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	262	276
Non United States [Member] | BIG [Member] | BIG 3 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 0	$ 0