Deferred Revenue-Right-to-use contracts and Deferred commission expense - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Movement in Deferred Revenue [Roll Forward]
Deferred revenue - right-to-use contracts, as of January 1			$ 74,174	$ 68,673
Right-to-use contracts current period, gross	$ 3,578	$ 3,263	6,375	6,344
Revenue recognized from right-to-use contract upfront payments			(4,147)	(4,029)
Right-to-use contract upfront payments, deferred, net			2,228	2,315
Deferred revenue - right-to-use contracts, as of June 30	76,402	70,988	76,402	70,988
Movement in Deferred Costs [Roll Forward]
Deferred commission expense, as of January 1			28,589	25,251
Deferred commission expense			3,067	2,652
Commission expense recognized			(1,696)	(1,318)
Net increase in deferred commission expense			1,371	1,334
Deferred commission expense, as of June 30	$ 29,960	$ 26,585	$ 29,960	$ 26,585