Real Estate and Accumulated Depreciation - Changes in Total Real Estate Investments and Related Accumulated Depreciation (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Total real estate investments:
Balance, beginning of year	$ 1,700,813	$ 1,622,710	$ 1,560,159
Acquisitions	77,258	48,123	4,894
Improvements	42,640	35,826	70,067
Retirements	(17,511)	(5,846)	(1,981)
Transfers to assets held for sale	0	0	(10,429)
Balance, end of year	1,803,200	1,700,813	1,622,710
Total accumulated depreciation:
Balance, beginning of year	458,279	425,370	396,617
Depreciation expense	40,197	38,755	37,698
Retirements	(10,310)	(5,846)	(1,911)
Transfers to assets held for sale	0	0	(7,034)
Balance, end of year	$ 488,166	$ 458,279	$ 425,370