Contracts Accounted for as Insurance - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016		Dec. 31, 2015
Guarantor Obligations [Line Items]
Weighted average risk-free rates for U.S. dollar denominated financial guaranty insurance obligations	2.74%		2.37%
Net par amount outstanding	$ 296,318	[1]	$ 358,571	[2]
Variable Rate Demand Obligation [Member]
Guarantor Obligations [Line Items]
Rate basis for bank bond rate	prime rate
Bonds held by bank, minimum installment payment period	5 years
Minimum [Member]
Guarantor Obligations [Line Items]
Financial Guarantee Insurance Contracts, Claim Liability, Risk Free Discount Rate	0.00%		0.00%
Minimum [Member] | Variable Rate Demand Obligation [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]
Financial Guarantee Insurance Contracts, Claim Liability, Risk Free Discount Rate	3.23%		3.25%
Maximum [Member] | Variable Rate Demand Obligation [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
Termination of Swap Obligation due to Rating Downgrade [Member] | Maximum [Member] | AGM [Member]
Guarantor Obligations [Line Items]
Increase in losses as a result of an adverse outcome, minimum	$ 125
Termination of Swap Obligation due to Further Rating Downgrade [Member] | Maximum [Member] | AGM [Member]
Guarantor Obligations [Line Items]
Increase in losses as a result of an adverse outcome, minimum	291
Variable Rate Demand Obligation [Member] | AGM and AGC [Member]
Guarantor Obligations [Line Items]
Net par amount outstanding	4,900
Internal Credit, B Minus Rating [Member] | Variable Rate Demand Obligation [Member] | AGM and AGC [Member]
Guarantor Obligations [Line Items]
Net par amount outstanding	$ 300
Foreign Currency Concentration Risk [Member] | Premiums Receivable [Member]
Guarantor Obligations [Line Items]
Percentage of installment premiums denominated in currencies other than the U.S. dollar	50.00%		52.00%

[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.