Outstanding Exposure - Financial Guaranty Portfolio by Internal Rating (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 237,325	$ 241,802
% of total net par outstanding	100.00%	100.00%
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 4,610	$ 4,618
% of total net par outstanding	1.90%	1.90%
AA [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 27,630	$ 27,021
% of total net par outstanding	11.60%	11.20%
A [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 116,035	$ 119,415
% of total net par outstanding	48.90%	49.40%
BBB [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 79,424	$ 80,588
% of total net par outstanding	33.50%	33.30%
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 9,626	$ 10,160
% of total net par outstanding	4.10%	4.20%
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 226,023	$ 230,665
Public Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 181,408	$ 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]
Total net exposure	$ 44,615	$ 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]
Total net exposure	$ 412	$ 413
% of total net par outstanding	0.30%	0.20%
Public Finance [Member] | AAA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 2,442	$ 2,399
% of total net par outstanding	5.50%	5.40%
Public Finance [Member] | AA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 21,666	$ 21,646
% of total net par outstanding	11.90%	11.60%
Public Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 1,710	$ 1,711
% of total net par outstanding	3.80%	3.90%
Public Finance [Member] | A [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 101,576	$ 105,180
% of total net par outstanding	56.00%	56.40%
Public Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 13,305	$ 13,013
% of total net par outstanding	29.80%	29.50%
Public Finance [Member] | BBB [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 51,697	$ 52,935
% of total net par outstanding	28.50%	28.40%
Public Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 26,157	$ 25,939
% of total net par outstanding	58.60%	58.80%
Public Finance [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 7,058	$ 7,429
Public Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 6,057	$ 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]
Total net exposure	$ 1,001	$ 1,041
% of total net par outstanding	2.30%	2.40%
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 11,302	$ 11,137
Structured Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 10,337	$ 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]
Total net exposure	$ 965	$ 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]
Total net exposure	$ 1,510	$ 1,533
% of total net par outstanding	14.60%	15.40%
Structured Finance [Member] | AAA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 246	$ 273
% of total net par outstanding	25.50%	22.90%
Structured Finance [Member] | AA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 4,230	$ 3,599
% of total net par outstanding	40.90%	36.20%
Structured Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 24	$ 65
% of total net par outstanding	2.50%	5.40%
Structured Finance [Member] | A [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 950	$ 1,016
% of total net par outstanding	9.20%	10.20%
Structured Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 204	$ 206
% of total net par outstanding	21.10%	17.30%
Structured Finance [Member] | BBB [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 1,166	$ 1,164
% of total net par outstanding	11.30%	11.70%
Structured Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 404	$ 550
% of total net par outstanding	41.90%	46.10%
Structured Finance [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 2,568	$ 2,731
Structured Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 2,481	$ 2,632
% of total net par outstanding	24.00%	26.50%
Structured Finance [Member] | BIG [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 87	$ 99
% of total net par outstanding	9.00%	8.30%