Outstanding Insurance Exposure - Components of BIG Net Par Outstanding (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 230,898	$ 236,807
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	221,370	226,746
Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,788	1,771
Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	4,347	4,744
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	9,528	10,061
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	8,238	8,506
BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	6,467	6,669
BIG [Member] | Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	40	40
BIG [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	151	179
BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,771	1,837
BIG [Member] | BIG 1 [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	2,469	2,667
BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	2,234	2,436
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	45	69
BIG [Member] | BIG 1 [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	235	231
BIG [Member] | BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	553	566
BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	430	430
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	59	62
BIG [Member] | BIG 2 [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	123	136
BIG [Member] | BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	5,216	5,273
BIG [Member] | BIG 3 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	3,803	3,803
BIG [Member] | BIG 3 [Member] | Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	40	40
BIG [Member] | BIG 3 [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	47	48
BIG [Member] | BIG 3 [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,413	1,470
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	172,795	175,487
United States [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	3,393	3,546
United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	8,806	9,300
United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	5,630	5,771
United States [Member] | BIG [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,580	1,618
United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,730	1,796
United States [Member] | BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,441	1,582
United States [Member] | BIG [Member] | BIG 1 [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	190	162
United States [Member] | BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	430	430
United States [Member] | BIG [Member] | BIG 2 [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	64	74
United States [Member] | BIG [Member] | BIG 3 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	3,759	3,759
United States [Member] | BIG [Member] | BIG 3 [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,326	1,382
Non United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	48,575	51,259
Non United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure		761
Non United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	837	898
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	793	854
Non United States [Member] | BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	0	0
Non United States [Member] | BIG [Member] | BIG 3 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 44	$ 44