Schedule III: Real Estate and Accumulated Depreciation (Rollforward of Real Estate and Accumulated Deprecation) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
REAL ESTATE:
Balance at beginning of period	$ 1,033,551	$ 1,029,472	$ 993,927
Subtotal real estate	1,037,368	1,035,230	1,040,662
Less: Fully depreciated assets	0	(1,679)	(11,190)
Balance at end of the period	1,037,368	1,033,551	1,029,472
ACCUMULATED DEPRECIATION:
Balance at beginning of period	252,737	225,533	210,025
Additions charged to operating expenses	30,307	28,883	26,698
Subtotal of accumulated depreciation	283,044	254,416	236,723
Less: Fully depreciated assets	0	(1,679)	(11,190)
Balance at end of the period	283,044	252,737	225,533
Land
REAL ESTATE:
Changes during the period	0	0	0
Buildings and leasehold improvements
REAL ESTATE:
Changes during the period	3,046	12,464	112,538
Development and construction in progress
REAL ESTATE:
Changes during the period	$ 771
Changes during the period		$ (6,706)	$ (65,803)