Outstanding Exposure - Financial Guaranty Portfolio by Internal Rating (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 257,089	$ 264,952
% of total net par outstanding	100.00%	100.00%
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 5,377	$ 5,392
% of total net par outstanding	2.10%	2.10%
AA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 31,320	$ 34,212
% of total net par outstanding	12.20%	12.90%
A [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 131,050	$ 134,396
% of total net par outstanding	51.00%	50.70%
BBB [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 77,743	$ 78,714
% of total net par outstanding	30.20%	29.70%
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 11,599	$ 12,238
% of total net par outstanding	4.50%	4.60%
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 245,084	$ 252,314
Public Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 201,337	$ 209,392
% 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	$ 43,747	$ 42,922
% 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	$ 866	$ 877
% of total net par outstanding	0.40%	0.40%
Public Finance [Member] | AAA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 2,568	$ 2,541
% of total net par outstanding	5.90%	5.90%
Public Finance [Member] | AA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 27,283	$ 30,016
% of total net par outstanding	13.60%	14.30%
Public Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 206	$ 205
% of total net par outstanding	0.50%	0.50%
Public Finance [Member] | A [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 115,123	$ 118,620
% of total net par outstanding	57.20%	56.70%
Public Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 14,309	$ 13,936
% of total net par outstanding	32.70%	32.50%
Public Finance [Member] | BBB [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 51,419	$ 52,739
% of total net par outstanding	25.50%	25.20%
Public Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 24,909	$ 24,509
% of total net par outstanding	56.90%	57.10%
Public Finance [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 8,401	$ 8,871
Public Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 6,646	$ 7,140
% 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	$ 1,755	$ 1,731
% of total net par outstanding	4.00%	4.00%
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 12,005	$ 12,638
Structured Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 10,681	$ 11,224
% 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	$ 1,324	$ 1,414
% 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	$ 1,654	$ 1,655
% of total net par outstanding	15.50%	14.70%
Structured Finance [Member] | AAA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 289	$ 319
% of total net par outstanding	21.80%	22.50%
Structured Finance [Member] | AA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 3,763	$ 3,915
% of total net par outstanding	35.20%	34.90%
Structured Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 68	$ 76
% of total net par outstanding	5.10%	5.40%
Structured Finance [Member] | A [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 1,405	$ 1,630
% of total net par outstanding	13.20%	14.50%
Structured Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 213	$ 210
% of total net par outstanding	16.10%	14.90%
Structured Finance [Member] | BBB [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 768	$ 763
% of total net par outstanding	7.20%	6.80%
Structured Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 647	$ 703
% of total net par outstanding	48.90%	49.70%
Structured Finance [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 3,198	$ 3,367
Structured Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 3,091	$ 3,261
% of total net par outstanding	28.90%	29.10%
Structured Finance [Member] | BIG [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 107	$ 106
% of total net par outstanding	8.10%	7.50%