Reinsurance and Other Monoline Exposures - Narrative (Details)		3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 01, 2016 USD ($)	Sep. 30, 2017 USD ($) commutation	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Ceded Credit Risk [Line Items]
Derivative losses on reinsurance derivatives		$ 200,000	$ 25,000,000	$ 800,000	$ 26,000,000
Fixed-maturity securities		11,495,000,000		11,495,000,000		$ 10,823,000,000
Increase (decrease) in net par outstanding		$ 3,455,000,000	28,000,000	$ 4,628,000,000	28,000,000
Commutation agreement exposure, percent (over)		97.00%		97.00%
Commutation gains (losses)		$ 255,000,000	$ 8,000,000	$ 328,000,000	$ 8,000,000
Number of Commutation Agreements | commutation		2
Assured Guaranty Re [Member]
Ceded Credit Risk [Line Items]
Amounts could be required to pay if third party exercised right to recapture business				49,000,000
AGC [Member]
Ceded Credit Risk [Line Items]
Amounts could be required to pay if third party exercised right to recapture business				16,000,000
Fixed Maturities [Member]
Ceded Credit Risk [Line Items]
Fixed-maturity securities		$ 10,546,000,000		10,546,000,000		10,233,000,000
Fixed Maturities [Member] | National [Member]
Ceded Credit Risk [Line Items]
Fixed-maturity securities		98,000,000		98,000,000
Fixed Maturities [Member] | Ambac [Member]
Ceded Credit Risk [Line Items]
Fixed-maturity securities		69,000,000		69,000,000
Fixed Maturities [Member] | Other Reinsurer [Member]
Ceded Credit Risk [Line Items]
Fixed-maturity securities		8,000,000		8,000,000
Excess of Loss Reinsurance Facility [Member] | AGM, AGC and MAC [Member]
Ceded Credit Risk [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	$ 360,000,000
Minimum net losses required for attachment of excess of loss reinsurance facility	1,250,000,000.00
Amount of losses covered under the facility	400,000,000
Remaining amount of losses covered under the facility	$ 40,000,000
Premiums paid during the period				9,000,000		$ 9,000,000
Uncollateralized [Member] | AM Best, A- Rating [Member]
Ceded Credit Risk [Line Items]
Guaranty Liabilities		5,000,000		5,000,000
Uncollateralized [Member] | Standard & Poor's, A Rating [Member]
Ceded Credit Risk [Line Items]
Guaranty Liabilities		$ 5,000,000		$ 5,000,000