Schedule III: Real Estate and Accumulated Depreciation (Rollforward of Real Estate and Accumulated Deprecation) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
REAL ESTATE:
Balance at beginning of period	$ 1,037,368	$ 1,033,551	$ 1,029,472
Subtotal real estate	1,041,281	1,037,368	1,035,230
Less: Fully depreciated assets	(13,590)	0	(1,679)
Balance at end of the period	1,027,691	1,037,368	1,033,551
ACCUMULATED DEPRECIATION:
Balance at beginning of period	283,044	252,737	225,533
Additions charged to operating expenses	27,967	30,307	28,883
Subtotal of accumulated depreciation	311,011	283,044	254,416
Less: Fully depreciated assets	(13,590)	0	(1,679)
Balance at end of the period	297,421	283,044	252,737
Land
REAL ESTATE:
Changes during the period	0	0	0
Buildings and leasehold improvements
REAL ESTATE:
Changes during the period	3,218	3,046	12,464
Development and construction in progress
REAL ESTATE:
Changes during the period	$ 695	$ 771
Changes during the period			$ (6,706)