Outstanding Exposure - Components of BIG Net Par Outstanding (Details) - USD ($) $ in Millions	Dec. 31, 2016		Dec. 31, 2015
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 296,318	[1]	$ 358,571	[2]
Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,057		2,750
Trust preferred securities (TruPS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,892		4,379
Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	15,553		22,932
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	271,179		321,443
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	25,139		37,128
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	13,074	[1]	15,183	[2]
BIG [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	126		216
BIG [Member] | Trust preferred securities (TruPS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	430		806
BIG [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	645		1,026
BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	8,722		9,162
BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	4,352		6,021
BIG [Member] | BIG 1 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	4,495		8,023
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	304		679
BIG [Member] | BIG 1 [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	304		684
BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	3,690		5,640
BIG [Member] | BIG 1 [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	805		2,383
BIG [Member] | BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	4,059		4,129
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	126		127
BIG [Member] | BIG 2 [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	263		219
BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	3,177		3,386
BIG [Member] | BIG 2 [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	882		743
BIG [Member] | BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	4,520		3,031
BIG [Member] | BIG 3 [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	126		216
BIG [Member] | BIG 3 [Member] | Trust preferred securities (TruPS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	0		0
BIG [Member] | BIG 3 [Member] | Other structured finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	78		123
BIG [Member] | BIG 3 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,855		136
BIG [Member] | BIG 3 [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,665		2,895
United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	266,855
United States [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	5,637		7,067
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	244,798	[1]	291,866	[2]
United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	22,057	[1]	31,770	[2]
United States [Member] | BIG [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	3,151		3,973
United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	7,380	[1]	7,784	[2]
United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	4,059	[1]	5,469	[2]
United States [Member] | BIG [Member] | BIG 1 [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	197		1,020
United States [Member] | BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,402		4,765
United States [Member] | BIG [Member] | BIG 2 [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	493		397
United States [Member] | BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	3,123		2,883
United States [Member] | BIG [Member] | BIG 3 [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,461		2,556
United States [Member] | BIG [Member] | BIG 3 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,855		136
Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	29,463
Non United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	26,381	[1]	29,577	[2]
Non United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	3,082	[1]	5,358	[2]
Non United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,342	[1]	1,378	[2]
Non United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	293	[1]	552	[2]
Non United States [Member] | BIG [Member] | BIG 1 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,288		875
Non United States [Member] | BIG [Member] | BIG 2 [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	54		503
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

[1] The December 31, 2016 amounts include $2.9 billion of net par from the CIFG Acquisition.

[2] The December 31, 2015 amounts include $10.9 billion of net par from the Radian Asset Acquisition.