Reinsurance - Narrative (Details) - USD ($)	12 Months Ended			24 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2019	Dec. 31, 2017	Jun. 01, 2018	Jan. 01, 2018	Jan. 01, 2016
Ceded Credit Risk [Line Items]
Gain (loss) on derivatives	$ 3,000,000	$ (1,000,000)	$ (27,000,000)
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	326,000,000
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							800,000,000.0	$ 1,250,000,000.00
Remaining amount of losses covered under the facility							220,000,000
Premiums paid during the period	$ 3,200,000	$ 9,000,000
Remaining insurance premium payable							$ 3,200,000
Excess of Loss Reinsurance Facility [Member] | AGM, AGC and MAC [Member] | Subsequent Event [Member]
Ceded Credit Risk [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				$ 180,000,000
Reinsurance of SGI Insured Portfolio [Member] | AGC [Member]
Ceded Credit Risk [Line Items]
Reinsurance Retention Policy, Quota Share Basis, Percent	100.00%					100.00%
Insured Financial Obligations, Assumed New Business, Outstanding Principal, Net						$ 11,000,000,000
Standard & Poor's, A Rating [Member] | Uncollateralized [Member]
Ceded Credit Risk [Line Items]
Guaranty Liabilities	$ 2,000,000
AM Best, A- Rating [Member] | Uncollateralized [Member]
Ceded Credit Risk [Line Items]
Guaranty Liabilities	$ 8,000,000