Financial Guaranty Insurance Losses - Narrative (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2015		Dec. 31, 2014
Guarantor Obligations [Line Items]
Financial guaranty insurance expected LAE reserve, amount	$ 10		$ 12
Net par amount outstanding	372,361	[1],[2]	$ 403,729	[3]
Guaranteed investment contract, aggregate accreted balance	1,900
Assets of GIC issuers, aggregate market value	$ 900
Variable rate demand obligations [Member]
Guarantor Obligations [Line Items]
Rate basis for bank bond rate	prime rate
Minimum [Member]
Guarantor Obligations [Line Items]
Weighted average risk-free rates for U.S. dollar denominated financial guaranty insurance obligations	0.00%		0.00%
Minimum [Member] | Variable rate demand obligations [Member]
Guarantor Obligations [Line Items]
Bank bond rate (as a percent)	2.00%
Threshold period of bonds held by bank for right of accelerated repayment (in days)	90 days
Maximum [Member]
Guarantor Obligations [Line Items]
Weighted average risk-free rates for U.S. dollar denominated financial guaranty insurance obligations	3.34%		2.95%
Maximum [Member] | Variable rate demand obligations [Member]
Guarantor Obligations [Line Items]
Bank bond rate (as a percent)	3.00%
Bank bond capped rate (as a percent)	25.00%
Threshold period of bonds held by bank for right of accelerated repayment (in days)	180 days
Cash [Member] | Minimum [Member]
Guarantor Obligations [Line Items]
Collateralization as a percentage of GIC balance	100.00%
Asset-backed Securities [Member] | Maximum [Member]
Guarantor Obligations [Line Items]
Collateralization as a percentage of GIC balance	108.00%
Termination of Swap Obligation due to Rating Downgrade [Member] | AGM [Member]
Guarantor Obligations [Line Items]
Maximum possible estimated termination payment on swap obligations after taking account the rating downgrade in January 2013	$ 157
Termination of Swap Obligation due to Further Rating Downgrade [Member] | AGM [Member]
Guarantor Obligations [Line Items]
Maximum possible estimated termination payment on swap obligations after taking account the rating downgrade in January 2013	399
Variable rate demand obligations [Member] | AGM and AGC [Member]
Guarantor Obligations [Line Items]
Net par amount outstanding	5,800
Internal Credit, B Minus Rating [Member] | Variable rate demand obligations [Member] | AGM and AGC [Member]
Guarantor Obligations [Line Items]
Net par amount outstanding	$ 300

[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.