Outstanding Insurance Exposure - Financial Guaranty Portfolio by Internal Rating (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 236,807	$ 241,802
% of total net par outstanding	100.00%	100.00%
United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 184,787
Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	52,020
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 4,361	$ 4,618
% of total net par outstanding	1.80%	1.90%
AA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 29,037	$ 27,021
% of total net par outstanding	12.30%	11.20%
A [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 111,329	$ 119,415
% of total net par outstanding	47.00%	49.40%
BBB [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 83,574	$ 80,588
% of total net par outstanding	35.30%	33.30%
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 8,506	$ 10,160
% of total net par outstanding	3.60%	4.20%
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 226,746	$ 230,665
Public Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 175,487	$ 186,562
% of total net par outstanding	100.00%	100.00%
Public Finance [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 51,259	$ 44,103
% of total net par outstanding	100.00%	100.00%
Public Finance [Member] | AAA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 381	$ 413
% of total net par outstanding	0.20%	0.20%
Public Finance [Member] | AAA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 2,541	$ 2,399
% of total net par outstanding	5.00%	5.40%
Public Finance [Member] | AA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 19,847	$ 21,646
% of total net par outstanding	11.30%	11.60%
Public Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 5,142	$ 1,711
% of total net par outstanding	10.00%	3.90%
Public Finance [Member] | A [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 94,488	$ 105,180
% of total net par outstanding	53.90%	56.40%
Public Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 15,627	$ 13,013
% of total net par outstanding	30.40%	29.50%
Public Finance [Member] | BBB [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 55,000	$ 52,935
% of total net par outstanding	31.30%	28.40%
Public Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 27,051	$ 25,939
% of total net par outstanding	52.80%	58.80%
Public Finance [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 6,669	$ 7,429
Public Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 5,771	$ 6,388
% of total net par outstanding	3.30%	3.40%
Public Finance [Member] | BIG [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 898	$ 1,041
% of total net par outstanding	1.80%	2.40%
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 10,061	$ 11,137
Structured Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 9,300	$ 9,944
% of total net par outstanding	100.00%	100.00%
Structured Finance [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 761	$ 1,193
% of total net par outstanding	100.00%	100.00%
Structured Finance [Member] | AAA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 1,258	$ 1,533
% of total net par outstanding	13.50%	15.40%
Structured Finance [Member] | AAA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 181	$ 273
% of total net par outstanding	23.80%	22.90%
Structured Finance [Member] | AA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 4,010	$ 3,599
% of total net par outstanding	43.10%	36.20%
Structured Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 38	$ 65
% of total net par outstanding	5.00%	5.40%
Structured Finance [Member] | A [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 1,030	$ 1,016
% of total net par outstanding	11.10%	10.20%
Structured Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 184	$ 206
% of total net par outstanding	24.20%	17.30%
Structured Finance [Member] | BBB [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 1,206	$ 1,164
% of total net par outstanding	13.00%	11.70%
Structured Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 317	$ 550
% of total net par outstanding	41.60%	46.10%
Structured Finance [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 1,837	$ 2,731
Structured Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 1,796	$ 2,632
% of total net par outstanding	19.30%	26.50%
Structured Finance [Member] | BIG [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 41	$ 99
% of total net par outstanding	5.40%	8.30%