Outstanding Exposure - Financial Guaranty Portfolio by Internal Rating (Details) - USD ($) $ in Millions	Sep. 30, 2016		Dec. 31, 2015
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 315,325	[1],[2]	$ 358,571	[3]
% of total net par outstanding	100.00%	[1],[2]	100.00%	[3]
Loss mitigation bonds	$ 2,100		$ 1,500
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 15,598	[2]	$ 20,837
% of total net par outstanding	5.00%	[2]	5.80%
AA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 61,478	[2]	$ 79,402
% of total net par outstanding	19.50%	[2]	22.10%
A [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 146,149	[2]	$ 167,246
% of total net par outstanding	46.30%	[2]	46.70%
BBB [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 78,187	[2]	$ 75,903
% of total net par outstanding	24.80%	[2]	21.20%
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 13,913	[2]	$ 15,183
% of total net par outstanding	4.40%	[2]	4.20%
Public Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 258,650	[1],[2]	$ 291,866	[3]
% of total net par outstanding	100.00%	[1],[2]	100.00%	[3]
Public Finance [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 28,239	[1],[2]	$ 29,577	[3]
% of total net par outstanding	100.00%	[1],[2]	100.00%	[3]
Public Finance [Member] | AAA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 2,182	[2]	$ 3,053
% of total net par outstanding	0.90%	[2]	1.10%
Public Finance [Member] | AAA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 703	[2]	$ 709
% of total net par outstanding	2.50%	[2]	2.40%
Public Finance [Member] | AA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 53,330	[2]	$ 69,274
% of total net par outstanding	20.60%	[2]	23.70%
Public Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 1,768	[2]	$ 2,017
% of total net par outstanding	6.20%	[2]	6.80%
Public Finance [Member] | A [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 137,270	[2]	$ 157,440
% of total net par outstanding	53.10%	[2]	53.90%
Public Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 6,466	[2]	$ 6,765
% of total net par outstanding	22.90%	[2]	22.90%
Public Finance [Member] | BBB [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 58,274	[2]	$ 54,315
% of total net par outstanding	22.50%	[2]	18.60%
Public Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 17,872	[2]	$ 18,708
% of total net par outstanding	63.30%	[2]	63.20%
Public Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 7,594	[2]	$ 7,784
% of total net par outstanding	2.90%	[2]	2.70%
Public Finance [Member] | BIG [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 1,430	[2]	$ 1,378
% of total net par outstanding	5.10%	[2]	4.70%
Structured Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 24,387	[1],[2]	$ 31,770	[3]
% of total net par outstanding	100.00%	[1],[2]	100.00%	[3]
Structured Finance [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 4,049	[1],[2]	$ 5,358	[3]
% of total net par outstanding	100.00%	[1],[2]	100.00%	[3]
Structured Finance [Member] | AAA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 11,187	[2]	$ 14,366
% of total net par outstanding	45.90%	[2]	45.20%
Structured Finance [Member] | AAA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 1,526	[2]	$ 2,709
% of total net par outstanding	37.70%	[2]	50.60%
Structured Finance [Member] | AA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 6,246	[2]	$ 7,934
% of total net par outstanding	25.60%	[2]	25.00%
Structured Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 134	[2]	$ 177
% of total net par outstanding	3.40%	[2]	3.30%
Structured Finance [Member] | A [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 1,744	[2]	$ 2,486
% of total net par outstanding	7.10%	[2]	7.80%
Structured Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 669	[2]	$ 555
% of total net par outstanding	16.50%	[2]	10.30%
Structured Finance [Member] | BBB [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 869	[2]	$ 1,515
% of total net par outstanding	3.60%	[2]	4.80%
Structured Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 1,172	[2]	$ 1,365
% of total net par outstanding	28.90%	[2]	25.50%
Structured Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 4,341	[2]	$ 5,469
% of total net par outstanding	17.80%	[2]	17.20%
Structured Finance [Member] | BIG [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 548	[2]	$ 552
% of total net par outstanding	13.50%	[2]	10.30%
CIFG Holding Inc. [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 3,700

[1] As of September 30, 2016, excludes $2.1 billion of net par as a result of loss mitigation strategies, including loss mitigation securities held in the investment portfolio, which are primarily BIG.

[2] The September 30, 2016 amounts include $3.7 billion of net par acquired from CIFG.

[3] As of December 31, 2015, excludes $1.5 billion of net par as a result of loss mitigation strategies, including loss mitigation securities held in the investment portfolio, which are primarily BIG.