Schedule III: Real Estate and Accumulated Depreciation (Rollforward of Real Estate and Accumulated Deprecation) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
REAL ESTATE:
Balance at beginning of period	$ 1,041,342	$ 1,027,691	$ 1,037,368
Subtotal real estate	1,071,919	1,041,342	1,041,281
Less: Fully depreciated assets	(876)	0	(13,590)
Balance at end of the period	1,071,043	1,041,342	1,027,691
ACCUMULATED DEPRECIATION:
Balance at beginning of period	324,499	297,421	283,044
Additions charged to operating expenses	26,499	27,078	27,967
Subtotal of accumulated depreciation	350,998	324,499	311,011
Less: Fully depreciated assets	(876)	0	(13,590)
Balance at end of the period	350,122	324,499	297,421
Land
REAL ESTATE:
Changes during the period	0	0	0
Buildings and leasehold improvements
REAL ESTATE:
Changes during the period	31,134	5,579	3,218
Development and construction in progress
REAL ESTATE:
Changes during the period	$ (557)	$ 8,072	$ 695