Deferred Revenue-Right-to-use contracts and Deferred commission expense - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Deferred Revenue Arrangement [Line Items]
Deferred revenue - right-to-use contracts, as of January 1			$ 62,979	$ 56,285
Right To Use Contracts Current Period Gross	3,361	2,942	6,192	5,186
Recognition of Deferred Revenue			(3,602)	(3,295)
Deferred Revenue, Period Increase (Decrease)			2,590	1,891
Deferred revenue - right-to-use contracts, as of June 30	65,569	58,176	65,569	58,176
Deferred commission expense, as of January 1			22,841	19,687
Increase (Decrease) in Deferred Charges			2,393	2,010
Commission expense recognized related to right-to-use contracts			(1,275)	(1,113)
Sales And Marketing Deferred Commissions Net	655	655	1,118	897
Deferred commission expense, as of June 30	$ 23,959	$ 20,584	$ 23,959	$ 20,584
Minimum
Deferred Revenue Arrangement [Line Items]
Deferred Revenue Estimated Period Of Recognition	1 year		1 year
Maximum
Deferred Revenue Arrangement [Line Items]
Deferred Revenue Estimated Period Of Recognition	31 years		31 years