Outstanding Exposure - Financial Guaranty Portfolio by Internal Rating (Details) (USD $)	Mar. 31, 2015		Dec. 31, 2014
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 389,099,000,000	[1]	$ 403,729,000,000	[2]
% of total net par outstanding	100.00%	[1]	100.00%	[2]
Loss mitigation bonds	1,300,000,000		1,300,000,000
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	26,749,000,000		30,143,000,000
% of total net par outstanding	6.90%		7.50%
AA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	97,908,000,000		101,965,000,000
% of total net par outstanding	25.20%		25.30%
A [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	180,992,000,000		186,875,000,000
% of total net par outstanding	46.40%		46.30%
BBB [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	65,590,000,000		66,499,000,000
% of total net par outstanding	16.90%		16.40%
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	17,860,000,000		18,247,000,000
% of total net par outstanding	4.60%		4.50%
Public Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	313,444,000,000	[1]	322,123,000,000	[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	29,619,000,000	[1]	31,359,000,000	[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	3,563,000,000		4,082,000,000
% of total net par outstanding	1.10%		1.30%
Public Finance [Member] | AAA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	613,000,000		615,000,000
% of total net par outstanding	2.10%		2.00%
Public Finance [Member] | AA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	86,521,000,000		90,464,000,000
% of total net par outstanding	27.60%		28.10%
Public Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,650,000,000		2,785,000,000
% of total net par outstanding	9.00%		8.90%
Public Finance [Member] | A [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	171,308,000,000		176,298,000,000
% of total net par outstanding	54.70%		54.70%
Public Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	7,091,000,000		7,192,000,000
% of total net par outstanding	23.90%		22.90%
Public Finance [Member] | BBB [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	44,110,000,000		43,429,000,000
% of total net par outstanding	14.10%		13.50%
Public Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	17,891,000,000		19,363,000,000
% of total net par outstanding	60.40%		61.70%
Public Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	7,942,000,000		7,850,000,000
% of total net par outstanding	2.50%		2.40%
Public Finance [Member] | BIG [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,374,000,000		1,404,000,000
% of total net par outstanding	4.60%		4.50%
Structured Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	38,430,000,000	[1]	41,171,000,000	[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	7,606,000,000	[1]	9,076,000,000	[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	18,176,000,000		20,037,000,000
% of total net par outstanding	47.30%		48.70%
Structured Finance [Member] | AAA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	4,397,000,000		5,409,000,000
% of total net par outstanding	57.80%		59.60%
Structured Finance [Member] | AA [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	8,360,000,000		8,213,000,000
% of total net par outstanding	21.70%		19.90%
Structured Finance [Member] | AA [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	377,000,000		503,000,000
% of total net par outstanding	5.00%		5.50%
Structured Finance [Member] | A [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,228,000,000		2,940,000,000
% of total net par outstanding	5.80%		7.10%
Structured Finance [Member] | A [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	365,000,000		445,000,000
% of total net par outstanding	4.70%		4.90%
Structured Finance [Member] | BBB [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,843,000,000		1,795,000,000
% of total net par outstanding	4.80%		4.40%
Structured Finance [Member] | BBB [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,746,000,000		1,912,000,000
% of total net par outstanding	23.00%		21.10%
Structured Finance [Member] | BIG [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	7,823,000,000		8,186,000,000
% of total net par outstanding	20.40%		19.90%
Structured Finance [Member] | BIG [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 721,000,000		$ 807,000,000
% of total net par outstanding	9.50%		8.90%

[1] Excludes $1.3 billion of loss mitigation securities insured and held by the Company as of March 31, 2015, which are primarily in the BIG category.

[2] 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.