Deferred Revenue-Right-to-use contracts and Deferred commission expense - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011
Deferred Revenue Arrangement [Line Items]
Deferred Revenue	$ 64,019	$ 56,892	$ 62,979	$ 56,285
Right-to-use contracts current period, gross	2,831	2,244
Recognition of Deferred Revenue	(1,791)	(1,637)
Deferred Revenue, Period Increase (Decrease)	1,040	607
Deferred Costs	23,305	19,929	22,842	19,687
Increase (Decrease) in Deferred Charges	1,093	794
Commission expense recognized related to right-to-use contracts	(630)	(552)
Sales And Marketing Deferred Commissions Net	$ 463	$ 242
Minimum
Deferred Revenue Arrangement [Line Items]
Deferred Revenue Estimated Period Of Recognition	1 year
Maximum
Deferred Revenue Arrangement [Line Items]
Deferred Revenue Estimated Period Of Recognition	31 years