Investments - Commercial Loan Participations, Allowance for Credit Loss (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201613Member		us-gaap:AccountingStandardsUpdate201602Member	us-gaap:AccountingStandardsUpdate201613Member
Commercial mortgage participations
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans past due, allowance	$ 1,000				$ 1,000
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses beginning balance	3,838	$ 0	$ 0	$ 0	0	$ 0
Provision (reversal) for credit losses	(119)		0		3,384	0
Loans charge-off	0		0		0	0
Allowance for credit losses ending balance	3,719		0		3,719	0
Commercial mortgage participations | Cumulative Effect, Period Of Adoption, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses beginning balance	$ 0	335	$ 0	$ 0	335	$ 0
Commercial mortgage participations | Cumulative Effect, Period of Adoption, Adjustment, Net of Tax
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses beginning balance		$ 265			$ 265