Reinsurance - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Jun. 30, 2020	Sep. 30, 2020
Ceded Credit Risk [Line Items]
Insured financial obligations, net par exposures reassumed	$ 118.0
Assured Guaranty Re
Ceded Credit Risk [Line Items]
Amounts could be required to pay if third party exercised right to recapture business		$ 42.0
AGC
Ceded Credit Risk [Line Items]
Amounts could be required to pay if third party exercised right to recapture business		247.0
Standard & Poor's, A Rating | Uncollateralized
Ceded Credit Risk [Line Items]
Guaranty liabilities		2.3
AM Best, A- Rating | Uncollateralized
Ceded Credit Risk [Line Items]
Guaranty liabilities		$ 13.0