Reinsurance and Other Monoline Exposures - Narrative (Details) - USD ($)			3 Months Ended		12 Months Ended
	Jan. 01, 2016	Jan. 01, 2014	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Ceded Credit Risk [Line Items]
Derivative losses on reinsurance derivatives			$ 600,000	$ 300,000
Fixed-maturity securities			11,168,000,000		$ 10,823,000,000
Net par due to commutations			$ 1,000,000,000
Commutation agreement exposure, percent (over)			97.00%
Gain recorded to other income from commutations			$ 73,000,000
Assured Guaranty Re [Member]
Ceded Credit Risk [Line Items]
Amounts could be required to pay if third party exercised right to recapture business			42,000,000
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			10,479,000,000		10,233,000,000
Fixed Maturities [Member] | National [Member]
Ceded Credit Risk [Line Items]
Fixed-maturity securities			106,000,000
Fixed Maturities [Member] | Ambac [Member]
Ceded Credit Risk [Line Items]
Fixed-maturity securities			81,000,000
Fixed Maturities [Member] | Other [Member]
Ceded Credit Risk [Line Items]
Fixed-maturity securities			8,000,000
Fixed Maturities [Member] | FGIC UK Limited [Member]
Ceded Credit Risk [Line Items]
Fixed-maturity securities			135,000,000
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	$ 450,000,000
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