Mortgage Loans and Interest Rate Swaps (Details) $ in Thousands		3 Months Ended
	Jan. 23, 2020 USD ($) ft² subsidiary building	Mar. 31, 2021 USD ($) derivative item	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Nov. 30, 2020 USD ($)
Long-Term Debt
Loans, net of debt issuance costs		$ 159,463		$ 160,655	$ 161,048
Interest rate swap agreement
Debt disclosures
Number of agreements containing credit risk related contingent features | item		0
Loss expected to be reclassified over next twelve months from accumulated other comprehensive loss to interest expense		$ (1,972)
Recognized net gains (losses) (included in other comprehensive loss), before taxes, on interest rate swap agreements		3,479	$ (6,467)
Notional amount of interest rate swap agreement		$ 97,142		97,868	98,108
Number of interest rate swap derivatives | derivative		13
Interest expense		$ 488	$ 135
Net derivative liability fair value		5,287
Interest rate swap agreement | Other assets
Debt disclosures
Derivative asset fair value		249
Interest rate swap agreement | Other liabilities
Debt disclosures
Derivative liability fair value		5,536
Nonrecourse mortgage loans
Long-Term Debt
Loans, prior to debt issuance costs		161,501		162,768	163,185
Debt issuance costs, net		(2,038)		(2,113)	(2,137)
Loans, net of debt issuance costs		$ 159,463		$ 160,655	$ 161,048
Weighted average interest rate		4.18%		4.18%	4.18%
4.72%, due October 3, 2022
Long-Term Debt
Loans, prior to debt issuance costs		$ 4,034		$ 4,061	$ 4,070
Interest rate (as a percent)		4.72%		4.72%	4.72%
4.39%, due January 2, 2025
Long-Term Debt
Loans, prior to debt issuance costs		$ 18,298		$ 18,453	$ 18,503
Interest rate (as a percent)		4.39%		4.39%	4.39%
4.17%, due May 1, 2026
Long-Term Debt
Loans, prior to debt issuance costs		$ 12,594		$ 12,696	$ 12,730
Interest rate (as a percent)		4.17%		4.17%	4.17%
3.79%, November 17, 2026
Long-Term Debt
Loans, prior to debt issuance costs		$ 23,724		$ 23,911	$ 23,973
Interest rate (as a percent)		3.79%		3.79%	3.79%
4.39%, due August 1, 2027
Long-Term Debt
Loans, prior to debt issuance costs		$ 9,683		$ 9,750	$ 9,772
Interest rate (as a percent)		4.39%		4.39%	4.39%
3.97%, due September 1, 2027
Long-Term Debt
Loans, prior to debt issuance costs		$ 11,358		$ 11,419	$ 11,439
Interest rate (as a percent)		3.97%		3.97%	3.97%
4.57%, due February 1, 2028
Long-Term Debt
Loans, prior to debt issuance costs		$ 17,486		$ 17,601	$ 17,639
Interest rate (as a percent)		4.57%		4.57%	4.57%
5.09%, due July 1, 2029 GCD mortgage loan
Long-Term Debt
Loans, prior to debt issuance costs		$ 5,092		$ 5,214	$ 5,254
Interest rate (as a percent)		5.09%		5.09%	5.09%
5.09%, due July 1, 2029 TD mortgage Loan
Long-Term Debt
Loans, prior to debt issuance costs		$ 3,568		$ 3,653	$ 3,681
Interest rate (as a percent)		5.09%		5.09%	5.09%
3.60%, due January 2, 2030
Long-Term Debt
Loans, prior to debt issuance costs		$ 6,309		$ 6,350	$ 6,364
Interest rate (as a percent)		3.60%		3.60%	3.60%
3.48% due February 1 2030
Long-Term Debt
Loans, prior to debt issuance costs		$ 14,584		$ 14,682	$ 14,714
Interest rate (as a percent)	3.48%	3.48%		3.48%	3.48%
Debt disclosures
Term of debt	10 years
Amortization period of debt	25 years
New mortgage	$ 15,000
Number of buildings used as collateral | building	2
Area of collateralized properties completed (in square feet) | ft²	254,000
Repayment of debt	$ 3,191
Number of subsidiaries which are party to the mortgage | subsidiary	2
3.50%, due July 1, 2030
Long-Term Debt
Loans, prior to debt issuance costs		$ 5,014		$ 5,046	$ 5,057
Interest rate (as a percent)		3.50%
4.33%, due August 1, 2030
Long-Term Debt
Loans, prior to debt issuance costs		$ 16,151		$ 16,244	$ 16,274
Interest rate (as a percent)		4.33%		4.33%	4.33%
4.51%, due April 1, 2034
Long-Term Debt
Loans, prior to debt issuance costs		$ 13,606		$ 13,688	$ 13,715
Interest rate (as a percent)		4.51%		4.51%	4.51%