Outstanding Exposure - Financial Guaranty Portfolio by Internal Rating (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 275,767	$ 296,318
% of total net par outstanding	100.00%	100.00%
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 6,190	$ 15,491
% of total net par outstanding	2.20%	5.20%
AA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 38,844	$ 52,490
% of total net par outstanding	14.10%	17.70%
A [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 140,035	$ 142,144
% of total net par outstanding	50.80%	48.00%
BBB [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 77,472	$ 73,119
% of total net par outstanding	28.10%	24.70%
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 13,226	$ 13,074
% of total net par outstanding	4.80%	4.40%
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 260,943	$ 271,179
Public Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 218,216	$ 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	$ 42,727	$ 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	$ 915	$ 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	$ 2,523	$ 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	$ 33,614	$ 46,420
% of total net par outstanding	15.40%	19.00%
Public Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 301	$ 170
% of total net par outstanding	0.70%	0.60%
Public Finance [Member] | A [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 124,332	$ 133,829
% of total net par outstanding	57.00%	54.70%
Public Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 13,657	$ 6,270
% of total net par outstanding	32.00%	23.80%
Public Finance [Member] | BBB [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 52,021	$ 55,103
% of total net par outstanding	23.80%	22.50%
Public Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 23,965	$ 16,378
% of total net par outstanding	56.10%	62.10%
Public Finance [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 9,615	$ 8,722
Public Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 7,334	$ 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	$ 2,281	$ 1,342
% of total net par outstanding	5.30%	5.10%
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 14,824	$ 25,139
Structured Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 13,142	$ 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	$ 1,682	$ 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	$ 2,333	$ 9,757
% of total net par outstanding	17.80%	44.20%
Structured Finance [Member] | AAA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 419	$ 1,447
% of total net par outstanding	25.00%	47.00%
Structured Finance [Member] | AA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 4,853	$ 5,773
% of total net par outstanding	36.90%	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	4.50%	4.10%
Structured Finance [Member] | A [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 1,778	$ 1,589
% of total net par outstanding	13.50%	7.20%
Structured Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 268	$ 456
% of total net par outstanding	15.90%	14.80%
Structured Finance [Member] | BBB [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 724	$ 879
% of total net par outstanding	5.50%	4.00%
Structured Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 762	$ 759
% of total net par outstanding	45.30%	24.60%
Structured Finance [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 3,611	$ 4,352
Structured Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 3,454	$ 4,059
% of total net par outstanding	26.30%	18.40%
Structured Finance [Member] | BIG [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 157	$ 293
% of total net par outstanding	9.30%	9.50%
MBIA UK Insurance Limited [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 13,000