Reinsurance and Other Monoline Exposures - Narrative (Details)		3 Months Ended	12 Months Ended				24 Months Ended
	Feb. 02, 2018 USD ($)	Sep. 30, 2017 commutation	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2018 USD ($)	Jan. 01, 2016 USD ($)
Ceded Credit Risk [Line Items]
Number of Commutation Agreements | commutation		2
Commutation Agreement Exposure, Percent				97.00%				97.00%
Gain (loss) on derivatives				$ (800,000)	$ (27,000,000)	$ (3,000,000)
Document Fiscal Year Focus				2017
Fixed-maturity securities				$ 11,301,000,000	10,823,000,000			$ 11,301,000,000
Reinsurance balances payable, net				61,000,000	64,000,000			61,000,000
Assured Guaranty Re [Member]
Ceded Credit Risk [Line Items]
Amounts could be required to pay if third party exercised right to recapture business				46,000,000
AGC [Member]
Ceded Credit Risk [Line Items]
Amounts could be required to pay if third party exercised right to recapture business				15,000,000
Fixed Maturities [Member]
Ceded Credit Risk [Line Items]
Fixed-maturity securities				10,674,000,000	$ 10,233,000,000			10,674,000,000
Fixed Maturities [Member] | National [Member]
Ceded Credit Risk [Line Items]
Fixed-maturity securities				91,000,000				91,000,000
Fixed Maturities [Member] | Ambac [Member]
Ceded Credit Risk [Line Items]
Fixed-maturity securities				68,000,000				68,000,000
Fixed Maturities [Member] | Other [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									$ 800,000,000.0	$ 1,250,000,000.00
Amount of losses covered under the facility									400,000,000	$ 400,000,000
Remaining amount of losses covered under the facility									220,000,000
Premiums paid during the period				9,000,000
Remaining insurance premium payable									$ 3,200,000
Excess of Loss Reinsurance Facility [Member] | AGM, AGC and MAC [Member] | Subsequent Event [Member]
Ceded Credit Risk [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							$ 180,000,000
Premiums paid during the period			$ 3,200,000
Reinsurance of SGI Insured Portfolio [Member] | AGC [Member] | Subsequent Event [Member]
Ceded Credit Risk [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	$ 14,500,000,000.0
Premiums paid during the period	$ 360,000,000
Reinsurance Retention Policy, Quota Share Basis, Percent	100.00%
Reinsurance balances payable, net	$ 55,000,000
Standard & Poor's, A Rating [Member] | Uncollateralized [Member]
Ceded Credit Risk [Line Items]
Guaranty Liabilities				4,000,000				4,000,000
AM Best, A- Rating [Member] | Uncollateralized [Member]
Ceded Credit Risk [Line Items]
Guaranty Liabilities				$ 9,000,000				$ 9,000,000