Investments - Commercial Mortgage Loan Participations (Details) - Commercial mortgage participations $ in Thousands	Sep. 30, 2020 USD ($) loan	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 168,621		$ 137,692
Carrying value, gross, percent	102.00%		100.00%
Allowance for credit losses	$ (3,719)	$ (3,838)	$ 0	$ 0	$ 0	$ 0
Allowance for credit losses, percent	(2.00%)		0.00%
Carrying value, net of valuation allowance	$ 164,902		$ 137,692
Carrying value, net, percent	100.00%		100.00%
Number of loans | loan	25
South Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 52,397		$ 50,867
Carrying value, gross, percent	32.00%		37.00%
Pacific
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 62,297		$ 36,546
Carrying value, gross, percent	38.00%		27.00%
Middle Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 28,268		$ 25,328
Carrying value, gross, percent	17.00%		18.00%
East North Central
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 10,597		$ 10,568
Carrying value, gross, percent	6.00%		8.00%
West South Central
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 8,971		$ 8,072
Carrying value, gross, percent	5.00%		6.00%
East South Central
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 4,700		$ 4,676
Carrying value, gross, percent	3.00%		3.00%
West North Central
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 1,391		$ 0
Carrying value, gross, percent	1.00%		0.00%
New England
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 0		$ 1,635
Carrying value, gross, percent	0.00%		1.00%
Office
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 41,622		$ 42,350
Carrying value, gross, percent	25.00%		31.00%
Mixed use
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 48,348		$ 27,501
Carrying value, gross, percent	29.00%		20.00%
Hospitality
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 23,030		$ 22,324
Carrying value, gross, percent	14.00%		16.00%
Industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 17,885		$ 17,612
Carrying value, gross, percent	11.00%		13.00%
Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 19,137		$ 17,318
Carrying value, gross, percent	12.00%		12.00%
Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 18,599		$ 10,587
Carrying value, gross, percent	11.00%		8.00%