Reinsurance and Other Monoline Exposures - Narrative (Details) - USD ($)			3 Months Ended		12 Months Ended		24 Months Ended
	Feb. 02, 2018	Jan. 01, 2016	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Jan. 01, 2018
Ceded Credit Risk [Line Items]
Increase (decrease) in net par outstanding			$ 42,000,000	$ 1,173,000,000
Commutation agreement exposure, percent (over)				97.00%
Commutation gains (losses)			1,000,000	$ 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			44,000,000
AGC [Member]
Ceded Credit Risk [Line Items]
Amounts could be required to pay if third party exercised right to recapture business			17,000,000
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						$ 800,000,000.0
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
Cash held in trust accounts for benefits of reinsured companies								$ 3,200,000
Reinsurance of SGI Insured Portfolio [Member] | AGC [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 payable	$ 55,000,000
Reinsurance retention policy, quota share basis, percent	100.00%
Uncollateralized [Member] | AM Best, A- Rating [Member]
Ceded Credit Risk [Line Items]
Guaranty Liabilities			9,000,000
Uncollateralized [Member] | Standard & Poor's, A Rating [Member]
Ceded Credit Risk [Line Items]
Guaranty Liabilities			$ 4,000,000
Scenario, 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
Premiums paid during the period					$ 3,200,000