Real Estate and Accumulated Depreciation - Changes in Total Real Estate Investments and Related Accumulated Depreciation (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Total real estate investments:
Balance, beginning of year	$ 2,081,597	$ 1,948,165	$ 1,803,200
Acquisitions	0	0	48,579
Improvements	45,396	135,966	98,917
Retirements	(2,197)	(2,534)	(2,531)
Balance, end of year	2,124,796	2,081,597	1,948,165
Total accumulated depreciation:
Balance, beginning of year	563,474	525,518	488,166
Depreciation expense	45,865	40,490	39,768
Retirements	(1,633)	(2,534)	(2,416)
Balance, end of year	$ 607,706	$ 563,474	$ 525,518