Components of the Change in Deferred Revenue-Entry of Right-to-Use Contracts and Deferred Commission Expense (Detail) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Deferred Revenue Arrangement [Line Items]
Deferred revenue - entry of right-to-use contracts, beginning balance			$ 44,349	$ 29,493
Deferral of new right-to-use contracts	4,857	5,681	8,710	10,618
Deferred revenue recognized			(2,800)	(2,119)
Net increase in deferred revenue	3,414	4,551	5,910	8,499
Deferred revenue - entry of right-to-use contracts, ending balance	50,259	37,992	50,259	37,992
Deferred commission expense, beginning balance			14,898	9,373
Costs deferred			3,256	3,708
Commission expense recognized			(909)	(638)
Net increase in deferred commission expense			2,347	3,070
Deferred commission expense, ending balance	$ 17,245	$ 12,443	$ 17,245	$ 12,443