Outstanding Exposure - Financial Guaranty Portfolio by Internal Rating (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 264,952	$ 296,318
% of total net par outstanding	100.00%	100.00%
United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 220,616
Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	44,336
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 5,392	$ 15,491
% of total net par outstanding	2.10%	5.20%
AA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 34,212	$ 52,490
% of total net par outstanding	12.90%	17.70%
A [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 134,396	$ 142,144
% of total net par outstanding	50.70%	48.00%
BBB [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 78,714	$ 73,119
% of total net par outstanding	29.70%	24.70%
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 12,238	$ 13,074
% of total net par outstanding	4.60%	4.40%
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 252,314	$ 271,179
Public Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 209,392	$ 244,798
% 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	$ 42,922	$ 26,381
% 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	$ 877	$ 2,066
% of total net par outstanding	0.40%	0.80%
Public Finance [Member] | AAA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 2,541	$ 2,221
% of total net par outstanding	5.90%	8.40%
Public Finance [Member] | AA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 30,016	$ 46,420
% of total net par outstanding	14.30%	19.00%
Public Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 205	$ 170
% of total net par outstanding	0.50%	0.60%
Public Finance [Member] | A [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 118,620	$ 133,829
% of total net par outstanding	56.70%	54.70%
Public Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 13,936	$ 6,270
% of total net par outstanding	32.50%	23.80%
Public Finance [Member] | BBB [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 52,739	$ 55,103
% of total net par outstanding	25.20%	22.50%
Public Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 24,509	$ 16,378
% of total net par outstanding	57.10%	62.10%
Public Finance [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 8,871	$ 8,722
Public Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 7,140	$ 7,380
% of total net par outstanding	3.40%	3.00%
Public Finance [Member] | BIG [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 1,731	$ 1,342
% of total net par outstanding	4.00%	5.10%
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 12,638	$ 25,139
Structured Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 11,224	$ 22,057
% 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	$ 1,414	$ 3,082
% 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	$ 1,655	$ 9,757
% of total net par outstanding	14.70%	44.20%
Structured Finance [Member] | AAA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 319	$ 1,447
% of total net par outstanding	22.50%	47.00%
Structured Finance [Member] | AA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 3,915	$ 5,773
% of total net par outstanding	34.90%	26.20%
Structured Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 76	$ 127
% of total net par outstanding	5.40%	4.10%
Structured Finance [Member] | A [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 1,630	$ 1,589
% of total net par outstanding	14.50%	7.20%
Structured Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 210	$ 456
% of total net par outstanding	14.90%	14.80%
Structured Finance [Member] | BBB [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 763	$ 879
% of total net par outstanding	6.80%	4.00%
Structured Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 703	$ 759
% of total net par outstanding	49.70%	24.60%
Structured Finance [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 3,367	$ 4,352
Structured Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 3,261	$ 4,059
% of total net par outstanding	29.10%	18.40%
Structured Finance [Member] | BIG [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 106	$ 293
% of total net par outstanding	7.50%	9.50%