Outstanding Exposure - Financial Guaranty Portfolio by Internal Rating (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 246,940	$ 264,952
% of total net par outstanding	100.00%	100.00%
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 4,819	$ 5,392
% of total net par outstanding	1.90%	2.10%
AA [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 26,392	$ 34,212
% of total net par outstanding	10.70%	12.90%
A [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 124,706	$ 134,396
% of total net par outstanding	50.50%	50.70%
BBB [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 80,730	$ 78,714
% of total net par outstanding	32.70%	29.70%
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 10,293	$ 12,238
% of total net par outstanding	4.20%	4.60%
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 235,153	$ 252,314
Public Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 190,418	$ 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]
Total net exposure	$ 44,735	$ 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]
Total net exposure	$ 428	$ 877
% of total net par outstanding	0.20%	0.40%
Public Finance [Member] | AAA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 2,422	$ 2,541
% of total net par outstanding	5.40%	5.90%
Public Finance [Member] | AA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 22,715	$ 30,016
% of total net par outstanding	12.00%	14.30%
Public Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 200	$ 205
% of total net par outstanding	0.40%	0.50%
Public Finance [Member] | A [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 108,914	$ 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]
Total net exposure	$ 14,047	$ 13,936
% of total net par outstanding	31.40%	32.50%
Public Finance [Member] | BBB [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 52,190	$ 52,739
% of total net par outstanding	27.40%	25.20%
Public Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 26,995	$ 24,509
% of total net par outstanding	60.40%	57.10%
Public Finance [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 7,242	$ 8,871
Public Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 6,171	$ 7,140
% of total net par outstanding	3.20%	3.40%
Public Finance [Member] | BIG [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 1,071	$ 1,731
% of total net par outstanding	2.40%	4.00%
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 11,787	$ 12,638
Structured Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 10,611	$ 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]
Total net exposure	$ 1,176	$ 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]
Total net exposure	$ 1,691	$ 1,655
% of total net par outstanding	15.90%	14.70%
Structured Finance [Member] | AAA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 278	$ 319
% of total net par outstanding	23.60%	22.50%
Structured Finance [Member] | AA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 3,412	$ 3,915
% of total net par outstanding	32.20%	34.90%
Structured Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 65	$ 76
% of total net par outstanding	5.50%	5.40%
Structured Finance [Member] | A [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 1,532	$ 1,630
% of total net par outstanding	14.40%	14.50%
Structured Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 213	$ 210
% of total net par outstanding	18.10%	14.90%
Structured Finance [Member] | BBB [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 1,027	$ 763
% of total net par outstanding	9.70%	6.80%
Structured Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 518	$ 703
% of total net par outstanding	44.10%	49.70%
Structured Finance [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 3,051	$ 3,367
Structured Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 2,949	$ 3,261
% of total net par outstanding	27.80%	29.10%
Structured Finance [Member] | BIG [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 102	$ 106
% of total net par outstanding	8.70%	7.50%