Reinsurance - Narrative (Details) - USD ($)	3 Months Ended		12 Months Ended	24 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2019	Jan. 01, 2018
Ceded Credit Risk [Line Items]
Increase (decrease) in net unearned premium reserve		$ 4,000,000
Increase (decrease) in net par outstanding		$ 42,000,000
Assured Guaranty Re [Member]
Ceded Credit Risk [Line Items]
Amounts could be required to pay if third party exercised right to recapture business	$ 41,000,000
AGC [Member]
Ceded Credit Risk [Line Items]
Amounts could be required to pay if third party exercised right to recapture business	312,000,000
Excess of Loss Reinsurance Facility [Member] | AGM, AGC and MAC [Member]
Ceded Credit Risk [Line Items]
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
Remaining amount of losses covered under the facility					$ 220,000,000
Premiums paid during the period	3,200,000		$ 3,200,000
Uncollateralized [Member] | AM Best, A- Rating [Member]
Ceded Credit Risk [Line Items]
Guaranty Liabilities	10,000,000
Uncollateralized [Member] | Standard & Poor's, A Rating [Member]
Ceded Credit Risk [Line Items]
Guaranty Liabilities	$ 1,000,000
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