Reinsurance and Other Monoline Exposures - Narrative (Details) - USD ($)			6 Months Ended	12 Months Ended
	Jan. 01, 2016	Jan. 01, 2014	Jun. 30, 2016	Dec. 31, 2014	Dec. 31, 2015
Ceded Credit Risk [Line Items]
Fixed-maturity securities			$ 10,545,000,000		$ 11,023,000,000
Assured Guaranty Re [Member]
Ceded Credit Risk [Line Items]
Amounts could be required to pay if third party exercised right to recapture business			49,000,000
AGC [Member]
Ceded Credit Risk [Line Items]
Amounts could be required to pay if third party exercised right to recapture business			32,000,000
Fixed Maturities [Member]
Ceded Credit Risk [Line Items]
Fixed-maturity securities			9,960,000,000		$ 10,627,000,000
Fixed Maturities [Member] | National [Member]
Ceded Credit Risk [Line Items]
Fixed-maturity securities			155,000,000
Fixed Maturities [Member] | Ambac Assurance Corporation [Member]
Ceded Credit Risk [Line Items]
Fixed-maturity securities			139,000,000
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			127,000,000
Fixed Maturities [Member] | MBIA [Member]
Ceded Credit Risk [Line Items]
Fixed-maturity securities			243,000,000
Excess of Loss Reinsurance Facility [Member] | AGM, AGC and MAC [Member]
Ceded Credit Risk [Line Items]
Minimum net losses required for attachment of excess of loss reinsurance facility	$ 1,250,000,000.00	$ 1,500,000,000
Amount of losses covered under the facility	400,000,000	500,000,000
Reinsurance retention policy, excess retention, amount reinsured	360,000,000	$ 450,000,000
Remaining amount of losses covered under the facility	40,000,000
Premiums paid during the period			$ 9,000,000	$ 19,000,000
Fixed maturity securities held for benefit of reinsured companies	$ 9,000,000