Outstanding Exposure - Financial Guaranty Portfolio by Internal Rating (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 229,375	$ 241,802
% of total net par outstanding	100.00%	100.00%
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 4,337	$ 4,618
% of total net par outstanding	1.90%	1.90%
AA [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 26,130	$ 27,021
% of total net par outstanding	11.40%	11.20%
A [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 111,216	$ 119,415
% of total net par outstanding	48.50%	49.40%
BBB [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 79,177	$ 80,588
% of total net par outstanding	34.50%	33.30%
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 8,515	$ 10,160
% of total net par outstanding	3.70%	4.20%
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 219,397	$ 230,665
Public Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 176,515	$ 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	$ 42,882	$ 44,103
% of total net par outstanding	100.00%
Public Finance [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure		$ 44,103
% of total net par outstanding		100.00%
Public Finance [Member] | AAA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 384	$ 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]
Total net exposure	$ 2,429
% of total net par outstanding	5.60%
Public Finance [Member] | AAA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure		$ 2,399
% of total net par outstanding		5.40%
Public Finance [Member] | AA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 20,646	$ 21,646
% of total net par outstanding	11.70%	11.60%
Public Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 1,707
% of total net par outstanding	4.00%
Public Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure		$ 1,711
% of total net par outstanding		3.90%
Public Finance [Member] | A [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 97,366	$ 105,180
% of total net par outstanding	55.20%	56.40%
Public Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 12,764
% of total net par outstanding	29.80%
Public Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure		$ 13,013
% of total net par outstanding		29.50%
Public Finance [Member] | BBB [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 52,359	$ 52,935
% of total net par outstanding	29.60%	28.40%
Public Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 25,119
% of total net par outstanding	58.60%
Public Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure		$ 25,939
% of total net par outstanding		58.80%
Public Finance [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 6,623	$ 7,429
Public Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 5,760	$ 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	$ 863	$ 1,041
% of total net par outstanding	2.00%
Public Finance [Member] | BIG [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure		$ 1,041
% of total net par outstanding		2.40%
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 9,978	$ 11,137
Structured Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 9,226	$ 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	$ 752
% of total net par outstanding	100.00%
Structured Finance [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure		$ 1,193
% of total net par outstanding		100.00%
Structured Finance [Member] | AAA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 1,345	$ 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	$ 179
% of total net par outstanding	23.80%
Structured Finance [Member] | AAA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure		$ 273
% of total net par outstanding		22.90%
Structured Finance [Member] | AA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 3,740	$ 3,599
% of total net par outstanding	40.50%	36.20%
Structured Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 37
% of total net par outstanding	4.90%
Structured Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure		$ 65
% of total net par outstanding		5.40%
Structured Finance [Member] | A [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 912	$ 1,016
% of total net par outstanding	9.90%	10.20%
Structured Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 174
% of total net par outstanding	23.10%
Structured Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure		$ 206
% of total net par outstanding		17.30%
Structured Finance [Member] | BBB [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 1,378	$ 1,164
% of total net par outstanding	14.90%	11.70%
Structured Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 321
% of total net par outstanding	42.70%
Structured Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure		$ 550
% of total net par outstanding		46.10%
Structured Finance [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 1,892	$ 2,731
Structured Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 1,851	$ 2,632
% of total net par outstanding	20.10%	26.50%
Structured Finance [Member] | BIG [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 41
% of total net par outstanding	5.50%
Structured Finance [Member] | BIG [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure		$ 99
% of total net par outstanding		8.30%