Deferred Revenue-Right-to-use contracts and Deferred commission expense - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 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	$ 3,889	$ 4,168	10,264	10,512
Revenue recognized from right-to-use contract upfront payments			(6,335)	(6,209)
Right-to-use contract upfront payments, deferred, net			3,929	4,303
Deferred revenue - right-to-use contracts, as of June 30	78,103	72,976	78,103	72,976
Movement in Deferred Costs [Roll Forward]
Deferred commission expense, as of January 1			28,589	25,251
Deferred commission expense			4,821	4,734
Commission expense recognized			(2,629)	(2,100)
Net increase in deferred commission expense			2,192	2,634
Deferred commission expense, as of June 30	$ 30,781	$ 27,885	$ 30,781	$ 27,885