Components of Change in Deferred Revenue-Entry of Right-to-Use Contracts and Deferred Commission Expense (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Deferred Revenue Arrangement [Line Items]
Right To Use Contracts Current Period Gross	$ 13,433	$ 17,856	$ 19,496
Deferred revenue - entry of right-to-use contracts, beginning balance	56,285	44,349
Net increase in deferred revenue	6,694	11,936
Deferred revenue - entry of right-to-use contracts, ending balance	62,979	56,285	44,349
Deferred commission expense, beginning balance	19,687	14,898
Costs deferred	5,465	6,735	6,957
Commission expense recognized	(2,310)	(1,946)	(1,432)
Net increase in deferred commission expense	3,155	4,789	5,525
Deferred commission expense, ending balance	22,842	19,687	14,898
Recognition of Deferred Revenue	$ (6,739)	$ (5,920)	$ (4,640)