Outstanding Exposure - Components of BIG Net Par Outstanding (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 229,375	$ 241,802
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	219,397	230,665
Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,223	1,184
Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	5,068	5,683
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	9,978	11,137
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	8,515	10,160
BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	6,623	7,429
BIG [Member] | Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	40	85
BIG [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	186	259
BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,892	2,731
BIG [Member] | BIG 1 [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	2,612	3,058
BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	2,423	2,563
BIG [Member] | BIG 1 [Member] | Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	0	0
BIG [Member] | BIG 1 [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	71	127
BIG [Member] | BIG 1 [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	189	495
BIG [Member] | BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	551	937
BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	398	644
BIG [Member] | BIG 2 [Member] | Life Insurance Transaction [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	65	79
BIG [Member] | BIG 2 [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	153	293
BIG [Member] | BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	5,352	6,165
BIG [Member] | BIG 3 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	3,802	4,222
BIG [Member] | BIG 3 [Member] | Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	40	85
BIG [Member] | BIG 3 [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	50	53
BIG [Member] | BIG 3 [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,550	1,943
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	176,515	186,562
United States [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	3,687	4,270
United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	9,226	9,944
United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	5,760	6,388
United States [Member] | BIG [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,666	2,387
United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,851	2,632
United States [Member] | BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,604	1,767
United States [Member] | BIG [Member] | BIG 1 [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	118	368
United States [Member] | BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	398	399
United States [Member] | BIG [Member] | BIG 2 [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	88	214
United States [Member] | BIG [Member] | BIG 3 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	3,758	4,222
United States [Member] | BIG [Member] | BIG 3 [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,460	1,805
Non United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	42,882	44,103
Non United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	752
Non United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	863	1,041
Non United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	41
Non United States [Member] | BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	819	796
Non United States [Member] | BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	0	245
Non United States [Member] | BIG [Member] | BIG 3 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 44	$ 0