Outstanding Exposure - Financial Guaranty Portfolio by Internal Rating (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 403,729,000,000	[1]	$ 459,107,000,000	[2]
% of total net par outstanding	100.00%	[1]	100.00%	[2]
United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	363,294,000,000
Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	40,435,000,000
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	30,143,000,000		48,015,000,000
% of total net par outstanding	7.50%		10.50%
AA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	101,965,000,000		117,933,000,000
% of total net par outstanding	25.30%		25.70%
A [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	186,875,000,000		205,616,000,000
% of total net par outstanding	46.30%		44.80%
BBB [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	66,499,000,000		65,005,000,000
% of total net par outstanding	16.40%		14.10%
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	18,247,000,000		22,538,000,000
% of total net par outstanding	4.50%		4.90%
Loss mitigation bonds	1,300,000,000	[2]	1,200,000,000	[2]
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	353,482,000,000		386,179,000,000
Public Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	322,123,000,000	[1]	352,181,000,000	[2]
% of total net par outstanding	100.00%	[1]	100.00%	[2]
Public Finance [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	31,359,000,000	[1]	33,998,000,000	[2]
% of total net par outstanding	100.00%	[1]	100.00%	[2]
Public Finance [Member] | AAA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	4,082,000,000		4,998,000,000
% of total net par outstanding	1.30%		1.40%
Public Finance [Member] | AAA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	615,000,000		1,016,000,000
% of total net par outstanding	2.00%		3.00%
Public Finance [Member] | AA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	90,464,000,000		107,503,000,000
% of total net par outstanding	28.10%		30.50%
Public Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,785,000,000		422,000,000
% of total net par outstanding	8.90%		1.20%
Public Finance [Member] | A [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	176,298,000,000		192,841,000,000
% of total net par outstanding	54.70%		54.80%
Public Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	7,192,000,000		9,453,000,000
% of total net par outstanding	22.90%		27.90%
Public Finance [Member] | BBB [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	43,429,000,000		37,745,000,000
% of total net par outstanding	13.50%		10.70%
Public Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	19,363,000,000		21,499,000,000
% of total net par outstanding	61.70%		63.20%
Public Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	7,850,000,000		9,094,000,000
% of total net par outstanding	2.40%		2.60%
Public Finance [Member] | BIG [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,404,000,000		1,608,000,000
% of total net par outstanding	4.50%		4.70%
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	50,247,000,000		72,928,000,000
Structured Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	41,171,000,000	[1]	58,907,000,000	[2]
% of total net par outstanding	100.00%	[1]	100.00%	[2]
Structured Finance [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	9,076,000,000	[1]	14,021,000,000	[2]
% of total net par outstanding	100.00%	[1]	100.00%	[2]
Structured Finance [Member] | AAA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	20,037,000,000		32,317,000,000
% of total net par outstanding	48.70%		54.90%
Structured Finance [Member] | AAA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	5,409,000,000		9,684,000,000
% of total net par outstanding	59.60%		69.10%
Structured Finance [Member] | AA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	8,213,000,000		9,431,000,000
% of total net par outstanding	19.90%		16.00%
Structured Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	503,000,000		577,000,000
% of total net par outstanding	5.50%		4.10%
Structured Finance [Member] | A [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,940,000,000		2,580,000,000
% of total net par outstanding	7.10%		4.40%
Structured Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	445,000,000		742,000,000
% of total net par outstanding	4.90%		5.30%
Structured Finance [Member] | BBB [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,795,000,000		3,815,000,000
% of total net par outstanding	4.40%		6.40%
Structured Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,912,000,000		1,946,000,000
% of total net par outstanding	21.10%		13.90%
Structured Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	8,186,000,000		10,764,000,000
% of total net par outstanding	19.90%		18.30%
Structured Finance [Member] | BIG [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 807,000,000		$ 1,072,000,000
% of total net par outstanding	8.90%		7.60%

[1] Excludes $1.3 billion of loss mitigation securities insured and held by the Company as of December 31, 2014, which are primarily in the BIG category.

[2] Excludes $1.2 billion of loss mitigation securities insured and held by the Company as of December 31, 2013, which are primarily in the BIG category.