Deferred Revenue-Right-to-use contracts and Deferred commission expense - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Dec. 31, 2010
Deferred Revenue Arrangement [Line Items]
Deferred Revenue	$ 60,965,000	$ 53,117,000	$ 60,965,000	$ 53,117,000	$ 56,285,000	$ 44,349,000
Right-to-use contracts current period, gross	4,494,000	4,386,000	9,680,000	13,096,000
Recognition of Deferred Revenue			(5,000,000)	(4,328,000)
Deferred Revenue, Period Increase (Decrease)			4,680,000	8,768,000
Deferred Costs	21,861,000	18,393,000	21,861,000	18,393,000	19,687,000	14,898,000
Increase (Decrease) in Deferred Charges			3,875,000	4,908,000
Commission expense recognized related to right-to-use contracts			(1,701,000)	(1,413,000)
Sales And Marketing Deferred Commissions Net	$ 1,277,000	$ 1,148,000	$ 2,174,000	$ 3,495,000
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