Outstanding Insurance Exposure - Financial Guaranty Portfolio by Internal Rating (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 233,075	$ 236,807
% of total net par outstanding	100.00%	100.00%
AAA
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 4,108	$ 4,361
% of total net par outstanding	1.80%	1.80%
AA
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 26,021	$ 29,037
% of total net par outstanding	11.20%	12.30%
A
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 103,575	$ 111,329
% of total net par outstanding	44.40%	47.00%
BBB
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 91,404	$ 83,574
% of total net par outstanding	39.20%	35.30%
BIG
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 7,967	$ 8,506
% of total net par outstanding	3.40%	3.60%
Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 223,812	$ 226,746
Public finance | U.S.
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 172,570	$ 175,487
% of total net par outstanding	100.00%	100.00%
Public finance | Non United States
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 51,242	$ 51,259
% of total net par outstanding	100.00%	100.00%
Public finance | AAA | U.S.
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 360	$ 381
% of total net par outstanding	0.20%	0.20%
Public finance | AAA | Non United States
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 2,529	$ 2,541
% of total net par outstanding	4.90%	5.00%
Public finance | AA | U.S.
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 17,046	$ 19,847
% of total net par outstanding	9.90%	11.30%
Public finance | AA | Non United States
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 5,053	$ 5,142
% of total net par outstanding	9.90%	10.00%
Public finance | A | U.S.
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 91,769	$ 94,488
% of total net par outstanding	53.20%	53.90%
Public finance | A | Non United States
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 10,628	$ 15,627
% of total net par outstanding	20.70%	30.40%
Public finance | BBB | U.S.
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 58,044	$ 55,000
% of total net par outstanding	33.60%	31.30%
Public finance | BBB | Non United States
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 32,172	$ 27,051
% of total net par outstanding	62.80%	52.80%
Public finance | BIG
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 6,211	$ 6,669
Public finance | BIG | U.S.
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 5,351	$ 5,771
% of total net par outstanding	3.10%	3.30%
Public finance | BIG | Non United States
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 860	$ 898
% of total net par outstanding	1.70%	1.80%
Structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 9,263	$ 10,061
Structured finance | U.S.
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 8,581	$ 9,300
% of total net par outstanding	100.00%	100.00%
Structured finance | Non United States
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 682	$ 761
% of total net par outstanding	100.00%	100.00%
Structured finance | AAA | U.S.
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 1,068	$ 1,258
% of total net par outstanding	12.40%	13.50%
Structured finance | AAA | Non United States
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 151	$ 181
% of total net par outstanding	22.10%	23.80%
Structured finance | AA | U.S.
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 3,887	$ 4,010
% of total net par outstanding	45.30%	43.10%
Structured finance | AA | Non United States
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 35	$ 38
% of total net par outstanding	5.10%	5.00%
Structured finance | A | U.S.
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 1,001	$ 1,030
% of total net par outstanding	11.70%	11.10%
Structured finance | A | Non United States
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 177	$ 184
% of total net par outstanding	26.00%	24.20%
Structured finance | BBB | U.S.
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 910	$ 1,206
% of total net par outstanding	10.60%	13.00%
Structured finance | BBB | Non United States
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 278	$ 317
% of total net par outstanding	40.80%	41.60%
Structured finance | BIG
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 1,756	$ 1,837
Structured finance | BIG | U.S.
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 1,715	$ 1,796
% of total net par outstanding	20.00%	19.30%
Structured finance | BIG | Non United States
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 41	$ 41
% of total net par outstanding	6.00%	5.40%