Reinsurance and Other Monoline Exposures - Narrative (Details)		3 Months Ended		9 Months Ended	12 Months Ended		24 Months Ended
	Jan. 01, 2016 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 commutation	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($)	Jun. 01, 2018 USD ($)	Jan. 01, 2018 USD ($)	Mar. 31, 2017
Ceded Credit Risk [Line Items]
Commutation agreement, reassumed portion, percent of total portfolio		12.00%
Number of commutation agreements | commutation			2
Commutation agreement exposure, percent (over)										97.00%
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				347,000,000
Reinsurance of SGI Insured Portfolio [Member] | AGC [Member]
Ceded Credit Risk [Line Items]
Reinsurance retention policy, quota share basis, percent								100.00%
Total net exposure								$ 11,000,000,000
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								$ 400,000,000
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
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
Uncollateralized [Member] | AM Best, A- Rating [Member]
Ceded Credit Risk [Line Items]
Guaranty Liabilities		$ 8,000,000		8,000,000
Uncollateralized [Member] | Standard & Poor's, A Rating [Member]
Ceded Credit Risk [Line Items]
Guaranty Liabilities		$ 3,000,000		$ 3,000,000
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