Summary of Significant Accounting Policies (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2020 USD ($) Segment	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Description of Business [Abstract]
Number of reportable segments | Segment	1
Recently Adopted Accounting Pronouncements [Abstract]
Cumulative effect adjustment to opening balance of retained earnings, net of tax	$ (344,664)	$ (336,038)	$ (364,316)	$ (363,430)	$ (367,416)	$ (356,082)
ASU 2016-02 [Member]
Recently Adopted Accounting Pronouncements [Abstract]
Right-of-use asset	145
Lease liability	145
ASU 2016-13 [Member]
Deductible Receivables [Abstract]
Allowance for credit losses, before tax	16,500		15,000
Allowance for credit losses, net of tax	13,035		11,850
Commercial Mortgage Loans [Member]
Investments [Abstract]
Allowance for credit loss	195
Retained Earnings [Member]
Recently Adopted Accounting Pronouncements [Abstract]
Cumulative effect adjustment to opening balance of retained earnings, net of tax	$ (275,710)	$ (273,846)	(300,988)	$ (299,553)	$ (304,513)	$ (308,075)
Retained Earnings [Member] | ASU 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Recently Adopted Accounting Pronouncements [Abstract]
Cumulative effect adjustment to opening balance of retained earnings, before tax			15,545
Cumulative effect adjustment to opening balance of retained earnings, net of tax			$ 12,281