Deferred Revenue-Entry of Right-to-Use Contracts and Deferred Commission Expense - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Deferred Revenue [Roll Forward]
Deferred revenue—upfront payments from right-to-use contracts, as of January 1,	$ 81,484	$ 78,405
Right-to-use contracts current period, gross	14,132	12,327	$ 12,783
Revenue recognized from right-to-use contract upfront payments	(10,020)	(9,248)	(8,552)
Right-to-use contract upfront payments, deferred, net	4,112	3,079
Deferred revenue—upfront payments from right-to-use contracts, as of December 31,	85,596	81,484	78,405
Movement in Deferred Costs [Roll Forward]
Deferred commission expense, as of January 1,	31,375	30,865
Deferred commission expense	4,577	4,659
Commission expense recognized	(4,509)	(4,149)	(3,595)
Net increase in deferred commission expense	68	510
Deferred commission expense, as of December 31,	$ 31,443	$ 31,375	$ 30,865