Investments - Commercial Mortgage Loan Participations (Details) - Commercial mortgage participations - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 138,256		$ 164,107
Carrying value, gross, percent	101.00%		102.00%
Allowance for credit losses	$ (1,184)	$ (1,639)	$ (3,505)	$ (3,719)	$ (3,838)	$ 0
Allowance for credit losses, percent	(1.00%)		(2.00%)
Carrying value, net of valuation allowance	$ 137,072		$ 160,602
Carrying value, net, percent	100.00%		100.00%
California
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 64,519		$ 61,610
Carrying value, gross, percent	47.00%		38.00%
Virginia
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 0		$ 27,019
Carrying value, gross, percent	0.00%		17.00%
New York
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 18,111		$ 16,602
Carrying value, gross, percent	13.00%		10.00%
Pennsylvania
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 11,672		$ 11,314
Carrying value, gross, percent	9.00%		7.00%
Indiana
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 9,717		$ 9,717
Carrying value, gross, percent	7.00%		6.00%
Florida
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 8,182		$ 12,420
Carrying value, gross, percent	6.00%		8.00%
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 26,055		$ 25,425
Carrying value, gross, percent	19.00%		16.00%
Mixed use
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 54,042		$ 49,002
Carrying value, gross, percent	39.00%		31.00%
Office
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 9,137		$ 36,153
Carrying value, gross, percent	7.00%		22.00%
Hospitality
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 22,550		$ 22,605
Carrying value, gross, percent	17.00%		14.00%
Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 19,754		$ 19,319
Carrying value, gross, percent	14.00%		12.00%
Industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 17,900		$ 17,900
Carrying value, gross, percent	13.00%		11.00%
Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 14,873		$ 19,128
Carrying value, gross, percent	11.00%		12.00%