Fair Value of Financial Instruments - Carrying Value of Commercial Mortgage Loans and Additional Information (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Total amortized cost	$ 47,449	$ 47,690
Allowance for mortgage loan losses	(76)	(76)
Mortgage loans, net	47,373	47,614
Total mortgage loans | Mortgage Loans by Region
Financing Receivable, Credit Quality Indicator [Line Items]
Mortgage loans, net	$ 47,449	$ 47,690
Percent of Total	100.00%	100.00%
Total mortgage loans | Mortgage Loans by Region | East North Central
Financing Receivable, Credit Quality Indicator [Line Items]
Mortgage loans, net	$ 3,245	$ 3,245
Percent of Total	6.80%	6.80%
Total mortgage loans | Mortgage Loans by Region | Southern Atlantic
Financing Receivable, Credit Quality Indicator [Line Items]
Mortgage loans, net	$ 9,666	$ 9,752
Percent of Total	20.40%	20.50%
Total mortgage loans | Mortgage Loans by Region | East South Central
Financing Receivable, Credit Quality Indicator [Line Items]
Mortgage loans, net	$ 8,114	$ 8,197
Percent of Total	17.10%	17.20%
Total mortgage loans | Mortgage Loans by Region | New England
Financing Receivable, Credit Quality Indicator [Line Items]
Mortgage loans, net	$ 6,588	$ 6,588
Percent of Total	13.90%	13.80%
Total mortgage loans | Mortgage Loans by Region | Middle Atlantic
Financing Receivable, Credit Quality Indicator [Line Items]
Mortgage loans, net	$ 14,863	$ 14,936
Percent of Total	31.30%	31.20%
Total mortgage loans | Mortgage Loans by Region | Mountain
Financing Receivable, Credit Quality Indicator [Line Items]
Mortgage loans, net	$ 2,227	$ 2,227
Percent of Total	4.70%	4.70%
Total mortgage loans | Mortgage Loans by Region | West North Central
Financing Receivable, Credit Quality Indicator [Line Items]
Mortgage loans, net	$ 2,746	$ 2,745
Percent of Total	5.80%	5.80%
Total mortgage loans | Mortgage Loans by Property Type
Financing Receivable, Credit Quality Indicator [Line Items]
Mortgage loans, net	$ 47,449	$ 47,690
Percent of Total	100.00%	100.00%
Total mortgage loans | Mortgage Loans by Property Type | Multifamily
Financing Receivable, Credit Quality Indicator [Line Items]
Mortgage loans, net	$ 17,012	$ 17,038
Percent of Total	35.90%	35.70%
Total mortgage loans | Mortgage Loans by Property Type | Office
Financing Receivable, Credit Quality Indicator [Line Items]
Mortgage loans, net	$ 11,718	$ 11,861
Percent of Total	24.70%	24.90%
Total mortgage loans | Mortgage Loans by Property Type | Industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Mortgage loans, net	$ 10,125	$ 10,124
Percent of Total	21.30%	21.20%
Total mortgage loans | Mortgage Loans by Property Type | Retail
Financing Receivable, Credit Quality Indicator [Line Items]
Mortgage loans, net	$ 2,227	$ 2,227
Percent of Total	4.70%	4.70%
Total mortgage loans | Mortgage Loans by Property Type | Mixed use/Other
Financing Receivable, Credit Quality Indicator [Line Items]
Mortgage loans, net	$ 6,367	$ 6,440
Percent of Total	13.40%	13.50%
Less than 65%
Financing Receivable, Credit Quality Indicator [Line Items]
Total amortized cost	$ 30,145	$ 30,361
65%-75%
Financing Receivable, Credit Quality Indicator [Line Items]
Total amortized cost	$ 17,304	$ 17,329