Outstanding Exposure - Financial Guaranty Portfolio by Internal Rating (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 257,804	$ 264,952
% of total net par outstanding	100.00%	100.00%
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 5,009	$ 5,392
% of total net par outstanding	1.90%	2.10%
AA [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 30,363	$ 34,212
% of total net par outstanding	11.80%	12.90%
A [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 129,679	$ 134,396
% of total net par outstanding	50.30%	50.70%
BBB [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 82,115	$ 78,714
% of total net par outstanding	31.90%	29.70%
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 10,638	$ 12,238
% of total net par outstanding	4.10%	4.60%
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 245,820	$ 252,314
Public Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 200,378	$ 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	$ 45,442	$ 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	$ 520	$ 877
% of total net par outstanding	0.30%	0.40%
Public Finance [Member] | AAA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 2,429	$ 2,541
% of total net par outstanding	5.30%	5.90%
Public Finance [Member] | AA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 26,408	$ 30,016
% of total net par outstanding	13.10%	14.30%
Public Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 201	$ 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	$ 113,915	$ 118,620
% of total net par outstanding	56.80%	56.70%
Public Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 14,260	$ 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	$ 53,221	$ 52,739
% of total net par outstanding	26.60%	25.20%
Public Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 27,419	$ 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	$ 7,447	$ 8,871
Public Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 6,314	$ 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	$ 1,133	$ 1,731
% of total net par outstanding	2.50%	4.00%
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 11,984	$ 12,638
Structured Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 10,749	$ 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	$ 1,235	$ 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	$ 1,778	$ 1,655
% of total net par outstanding	16.50%	14.70%
Structured Finance [Member] | AAA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 282	$ 319
% of total net par outstanding	22.80%	22.50%
Structured Finance [Member] | AA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 3,688	$ 3,915
% of total net par outstanding	34.30%	34.90%
Structured Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 66	$ 76
% of total net par outstanding	5.30%	5.40%
Structured Finance [Member] | A [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 1,306	$ 1,630
% of total net par outstanding	12.10%	14.50%
Structured Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 198	$ 210
% of total net par outstanding	16.00%	14.90%
Structured Finance [Member] | BBB [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 888	$ 763
% of total net par outstanding	8.30%	6.80%
Structured Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 587	$ 703
% of total net par outstanding	47.60%	49.70%
Structured Finance [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 3,191	$ 3,367
Structured Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 3,089	$ 3,261
% of total net par outstanding	28.80%	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	8.30%	7.50%