Outstanding Exposure - Geographic Distribution of Net Par Outstanding (Details) $ in Millions	Dec. 31, 2016 USD ($) risk		Dec. 31, 2015 USD ($)
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	11,499
Net Par Outstanding	$ 296,318	[1]	$ 358,571	[2]
Percent of Total Net Par Outstanding	100.00%
United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	11,284
Net Par Outstanding	$ 266,855
Percent of Total Net Par Outstanding	90.10%
Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	215
Net Par Outstanding	$ 29,463
Percent of Total Net Par Outstanding	9.90%
United Kingdom [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	112
Net Par Outstanding	$ 15,940
Percent of Total Net Par Outstanding	5.40%
Australia [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	18
Net Par Outstanding	$ 3,036
Percent of Total Net Par Outstanding	1.00%
Canada [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	9
Net Par Outstanding	$ 2,730
Percent of Total Net Par Outstanding	0.90%
France [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	14
Net Par Outstanding	$ 1,809
Percent of Total Net Par Outstanding	0.60%
Italy [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	9
Net Par Outstanding	$ 1,311
Percent of Total Net Par Outstanding	0.40%
Other Countries [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	53
Net Par Outstanding	$ 4,637
Percent of Total Net Par Outstanding	1.60%
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 271,179		321,443
Public Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	10,674
Net Par Outstanding	$ 244,798	[1]	291,866	[2]
Percent of Total Net Par Outstanding	82.70%
Public Finance [Member] | California [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	1,459
Net Par Outstanding	$ 42,404
Percent of Total Net Par Outstanding	14.30%
Public Finance [Member] | Texas [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	1,271
Net Par Outstanding	$ 20,599
Percent of Total Net Par Outstanding	7.00%
Public Finance [Member] | Pennsylvania [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	852
Net Par Outstanding	$ 20,232
Percent of Total Net Par Outstanding	6.80%
Public Finance [Member] | New York [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	935
Net Par Outstanding	$ 19,637
Percent of Total Net Par Outstanding	6.60%
Public Finance [Member] | Illinois [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	776
Net Par Outstanding	$ 17,967
Percent of Total Net Par Outstanding	6.10%
Public Finance [Member] | Florida [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	324
Net Par Outstanding	$ 12,643
Percent of Total Net Par Outstanding	4.30%
Public Finance [Member] | New Jersey [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	495
Net Par Outstanding	$ 12,560
Percent of Total Net Par Outstanding	4.20%
Public Finance [Member] | Michigan [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	506
Net Par Outstanding	$ 7,985
Percent of Total Net Par Outstanding	2.70%
Public Finance [Member] | Georgia [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	172
Net Par Outstanding	$ 6,372
Percent of Total Net Par Outstanding	2.20%
Public Finance [Member] | Ohio [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	409
Net Par Outstanding	$ 5,554
Percent of Total Net Par Outstanding	1.90%
Public Finance [Member] | Other States and U.S. Territories [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	3,475
Net Par Outstanding	$ 78,845
Percent of Total Net Par Outstanding	26.60%
Public Finance [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 26,381	[1]	29,577	[2]
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 25,139		37,128
Structured Finance [Member] | United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	610
Net Par Outstanding	$ 22,057	[1]	31,770	[2]
Percent of Total Net Par Outstanding	7.40%
Structured Finance [Member] | Non United States [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 3,082	[1]	$ 5,358	[2]

[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.