Reinsurance - Narrative (Details) - USD ($)		9 Months Ended	12 Months Ended	24 Months Ended
	Jun. 01, 2018	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2019	Jun. 30, 2019	Sep. 30, 2018	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Ceded Credit Risk [Line Items]
Net expected loss to be paid after recoveries for R&W		$ 718,000,000	$ 1,183,000,000		$ 960,000,000	$ 1,191,000,000	$ 1,432,000,000		$ 1,303,000,000
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		291,000,000
Reinsurance of SGI Insured Portfolio [Member]
Ceded Credit Risk [Line Items]
Net expected loss to be paid after recoveries for R&W	$ 131,000,000		0		$ 0		$ 0
Reinsurance of SGI Insured Portfolio [Member] | Assured Guaranty Re [Member]
Ceded Credit Risk [Line Items]
Net expected loss to be paid after recoveries for R&W	$ 131,000,000
Reinsurance of SGI Insured Portfolio [Member] | AGC [Member]
Ceded Credit Risk [Line Items]
Reinsurance retention policy quota share basis percent	100.00%
Reinsurance retention policy, excess retention, amount reinsured	$ 12,000,000,000
Premiums paid during the period	330,000,000
Prepaid reinsurance premiums	363,000,000
Future insurance installment premiums	45,000,000
Professional fees paid to advisors	4,000,000
Excess of Loss Reinsurance Facility [Member] | AGM, AGC and MAC [Member]
Ceded Credit Risk [Line Items]
Premiums paid during the period		3,200,000	$ 3,200,000
Remaining amount of losses covered under the facility								$ 220,000,000
Amount of losses covered under the facility								400,000,000
Minimum net losses required for attachment of excess of loss reinsurance facility								$ 800,000,000
Uncollateralized [Member] | AM Best, A- Rating [Member]
Ceded Credit Risk [Line Items]
Guaranty Liabilities		11,000,000
Uncollateralized [Member] | Standard & Poor's, A Rating [Member]
Ceded Credit Risk [Line Items]
Guaranty Liabilities		$ 300,000
Forecast [Member] | Excess of Loss Reinsurance Facility [Member] | AGM, AGC and MAC [Member]
Ceded Credit Risk [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				$ 180,000,000
Credit derivatives [Member] | Reinsurance of SGI Insured Portfolio [Member] | AGC [Member]
Ceded Credit Risk [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	1,500,000,000
Future insurance installment premiums	$ 17,000,000