Real Estate Investment and Accumulated Depreciation (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEC Schedule III, Real Estate and Accumulated Depreciation Disclosure [Abstract]
Balance at beginning of year	$ 2,529,641	$ 2,510,579	$ 2,488,361
Acquisitions	0	0	0
Improvements	40,749	21,899	22,302
Disposals		(2,837)	(84)
Balance at end of year	2,570,390	2,529,641	2,510,579
Balance at beginning of year	818,323	754,861	691,691
Depreciation	65,122	63,718	63,180
Disposals		(256)	(10)
Balance at end of year	$ 883,445	$ 818,323	$ 754,861