Outstanding Exposure - Components of BIG Net Par Outstanding (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 246,940	$ 264,952
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	235,153	252,314
Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,185	1,199
Trust Preferred Securities (TruPS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,034	1,349
Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	5,002	5,272
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	11,787	12,638
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	10,293	12,238
BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	7,242	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	81	161
BIG [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	325	360
BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	3,051	3,367
BIG [Member] | BIG 1 [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	2,902	4,528
BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	2,380	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	81	161
BIG [Member] | BIG 1 [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	173	170
BIG [Member] | BIG 1 [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	522	705
BIG [Member] | BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	976	1,361
BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	640	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	82	118
BIG [Member] | BIG 2 [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	336	422
BIG [Member] | BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	6,415	6,349
BIG [Member] | BIG 3 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	4,222	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	70	72
BIG [Member] | BIG 3 [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	2,193	2,240
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	190,418	209,392
United States [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	4,566	4,818
United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	10,611	11,224
United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	6,171	7,140
United States [Member] | BIG [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	2,560	2,761
United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	2,949	3,261
United States [Member] | BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,559	2,368
United States [Member] | BIG [Member] | BIG 1 [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	268	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	254	304
United States [Member] | BIG [Member] | BIG 3 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	4,222	4,109
United States [Member] | BIG [Member] | BIG 3 [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	2,038	2,083
Non United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	44,735	42,922
Non United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,176	1,414
Non United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,071	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	821	1,455
Non United States [Member] | BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	250	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