Deferred Revenue-Right-to-use contracts and Deferred commission expense - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 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	3,707	4,494	9,899	9,680
Recognition of Deferred Revenue			(5,453)	(5,000)
Deferred Revenue, Period Increase (Decrease)			4,446	4,680
Deferred revenue - right-to-use contracts, as of June 30	67,425	60,965	67,425	60,965
Deferred commission expense, as of January 1			22,841	19,687
Increase (Decrease) in Deferred Charges			3,757	3,875
Commission expense recognized related to right-to-use contracts			(1,933)	(1,701)
Sales And Marketing Deferred Commissions Net	706	1,277	1,824	2,174
Deferred commission expense, as of June 30	$ 24,665	$ 21,861	$ 24,665	$ 21,861
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