Outstanding Exposure - Components of BIG Net Par Outstanding (Details) - USD ($) $ in Millions	Sep. 30, 2015		Dec. 31, 2014
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 372,361	[1],[2]	$ 403,729	[3]
Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,750		3,133
Trust Preferred Securities (TruPS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	4,647		4,326
Student Loan [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,823		1,857
Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	23,906		31,514
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	17,611		18,247
BIG [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	216		598
BIG [Member] | Trust Preferred Securities (TruPS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	840		1,333
BIG [Member] | Student Loan [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	165		195
BIG [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,266		1,224
BIG [Member] | BIG 1 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	10,799		11,865
BIG [Member] | BIG 1 [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	0		0
BIG [Member] | BIG 1 [Member] | Trust Preferred Securities (TruPS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	549		997
BIG [Member] | BIG 1 [Member] | Student Loan [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	0		14
BIG [Member] | BIG 1 [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,061		1,007
BIG [Member] | BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	4,020		2,689
BIG [Member] | BIG 2 [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	0		0
BIG [Member] | BIG 2 [Member] | Trust Preferred Securities (TruPS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	291		0
BIG [Member] | BIG 2 [Member] | Student Loan [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	80		68
BIG [Member] | BIG 2 [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	165		172
BIG [Member] | BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,792		3,693
BIG [Member] | BIG 3 [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	216		598
BIG [Member] | BIG 3 [Member] | Trust Preferred Securities (TruPS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	0		336
BIG [Member] | BIG 3 [Member] | Student Loan [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	85		113
BIG [Member] | BIG 3 [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	40		45
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	300,732	[1],[2]	322,123	[3]
United States [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	8,400		9,417
United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	9,480		7,850
United States [Member] | BIG [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	4,130		5,643
United States [Member] | BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	7,132		6,577
United States [Member] | BIG [Member] | BIG 1 [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,144		1,868
United States [Member] | BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,212		1,156
United States [Member] | BIG [Member] | BIG 2 [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	671		1,291
United States [Member] | BIG [Member] | BIG 3 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	136		117
United States [Member] | BIG [Member] | BIG 3 [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,315		2,484
Non United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	30,103	[1],[2]	31,359	[3]
Non United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,514		1,404
Non United States [Member] | BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	913		1,402
Non United States [Member] | BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	601		2
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	465		471
First Lien [Member] | United States [Member] | Alt-A [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,189		2,532
First Lien [Member] | United States [Member] | Option ARM [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	308		407
First Lien [Member] | United States [Member] | Subprime [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	3,759		4,051
First Lien [Member] | United States [Member] | BIG [Member] | Prime [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	312		353
First Lien [Member] | United States [Member] | BIG [Member] | Alt-A [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	821		1,841
First Lien [Member] | United States [Member] | BIG [Member] | Option ARM [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	151		183
First Lien [Member] | United States [Member] | BIG [Member] | Subprime [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,336		1,575
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	228		68
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	106		585
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	48		47
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	153		156
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	57		33
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	77		531
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	8		18
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	474		654
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	27		252
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	638		725
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	95		118
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	709		765
Second Lien [Member] | United States [Member] | Closed-end [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	203		218
Second Lien [Member] | United States [Member] | HELOCs [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,476		1,738
Second Lien [Member] | United States [Member] | BIG [Member] | Closed-end [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	128		134
Second Lien [Member] | United States [Member] | BIG [Member] | HELOCs [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,382		1,557
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		0
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	609		1,012
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		19
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		36
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	109		115
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	$ 737		$ 509

[1] Excludes $1.6 billion of loss mitigation securities insured and held by the Company as of September 30, 2015, which are primarily BIG.

[2] The September 30, 2015 amounts include $12.4 billion of net par acquired from Radian Asset.

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