Outstanding Exposure - Components of BIG Net Par Outstanding (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 403,729	[1]	$ 459,107	[2]
TruPS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	4,326		4,970
Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	36,504		54,237
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	353,482		386,179
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	18,247		22,538
BIG [Member] | TruPS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,333		1,722
BIG [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,017		2,397
BIG [Member] | BIG 1 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	11,865		14,751
BIG [Member] | BIG 1 [Member] | TruPS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	997		1,587
BIG [Member] | BIG 1 [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,021		1,367
BIG [Member] | BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,689		3,949
BIG [Member] | BIG 2 [Member] | TruPS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	0		135
BIG [Member] | BIG 2 [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	240		309
BIG [Member] | BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	3,693		3,838
BIG [Member] | BIG 3 [Member] | TruPS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	336		0
BIG [Member] | BIG 3 [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	756		721
United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	363,294
United States [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	9,417		13,721
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	322,123	[1]	352,181	[2]
United States [Member] | BIG [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	5,643		7,717
United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	7,850		9,094
United States [Member] | BIG [Member] | BIG 1 [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,868		2,583
United States [Member] | BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	6,577		8,205
United States [Member] | BIG [Member] | BIG 2 [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,291		2,466
United States [Member] | BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,156		440
United States [Member] | BIG [Member] | BIG 3 [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,484		2,668
United States [Member] | BIG [Member] | BIG 3 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	117		449
Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	40,435
Non United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	31,359	[1]	33,998	[2]
Non United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,404		1,608
Non United States [Member] | BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,402		1,009
Non United States [Member] | BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2		599
Non United States [Member] | BIG [Member] | BIG 3 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	0		0
First Lien [Member] | United States [Member] | Prime [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	471		541
First Lien [Member] | United States [Member] | Alt-A [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,532		3,590
First Lien [Member] | United States [Member] | Option ARM [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	407		937
First Lien [Member] | United States [Member] | Subprime [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	4,051		6,130
First Lien [Member] | United States [Member] | BIG [Member] | Prime [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	353		403
First Lien [Member] | United States [Member] | BIG [Member] | Alt-A [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,841		2,728
First Lien [Member] | United States [Member] | BIG [Member] | Option ARM [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	183		598
First Lien [Member] | United States [Member] | BIG [Member] | Subprime [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,575		1,945
First Lien [Member] | United States [Member] | BIG [Member] | BIG 1 [Member] | Prime [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	68		52
First Lien [Member] | United States [Member] | BIG [Member] | BIG 1 [Member] | Alt-A [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	585		656
First Lien [Member] | United States [Member] | BIG [Member] | BIG 1 [Member] | Option ARM [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	47		71
First Lien [Member] | United States [Member] | BIG [Member] | BIG 1 [Member] | Subprime [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	156		297
First Lien [Member] | United States [Member] | BIG [Member] | BIG 2 [Member] | Prime [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	33		321
First Lien [Member] | United States [Member] | BIG [Member] | BIG 2 [Member] | Alt-A [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	531		1,137
First Lien [Member] | United States [Member] | BIG [Member] | BIG 2 [Member] | Option ARM [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	18		60
First Lien [Member] | United States [Member] | BIG [Member] | BIG 2 [Member] | Subprime [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	654		908
First Lien [Member] | United States [Member] | BIG [Member] | BIG 3 [Member] | Prime [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	252		30
First Lien [Member] | United States [Member] | BIG [Member] | BIG 3 [Member] | Alt-A [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	725		935
First Lien [Member] | United States [Member] | BIG [Member] | BIG 3 [Member] | Option ARM [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	118		467
First Lien [Member] | United States [Member] | BIG [Member] | BIG 3 [Member] | Subprime [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	765		740
Second Lien [Member] | United States [Member] | Closed-end [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	218		244
Second Lien [Member] | United States [Member] | HELOCs [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,738		2,279
Second Lien [Member] | United States [Member] | BIG [Member] | Closed-end [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	134		146
Second Lien [Member] | United States [Member] | BIG [Member] | HELOCs [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,557		1,897
Second Lien [Member] | United States [Member] | BIG [Member] | BIG 1 [Member] | Closed-end [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	0		8
Second Lien [Member] | United States [Member] | BIG [Member] | BIG 1 [Member] | HELOCs [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,012		1,499
Second Lien [Member] | United States [Member] | BIG [Member] | BIG 2 [Member] | Closed-end [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	19		20
Second Lien [Member] | United States [Member] | BIG [Member] | BIG 2 [Member] | HELOCs [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	36		20
Second Lien [Member] | United States [Member] | BIG [Member] | BIG 3 [Member] | Closed-end [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	115		118
Second Lien [Member] | United States [Member] | BIG [Member] | BIG 3 [Member] | HELOCs [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 509		$ 378

[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.