Changes in Total Real Estate (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Carrying amount of real estate investments [Roll Forward]
Gross real estate, beginning balance	$ 4,685,336	$ 4,477,599	$ 4,387,913
Acquisitions	142,484	120,448	23,900
Improvements	126,050	119,437	93,799
Dispositions and other	(38,057)	(32,148)	(28,013)
Gross real estate, ending balance	$ 4,915,813	$ 4,685,336	$ 4,477,599