SELECTED QUARTERLY INFORMATION - UNAUDITED (Selected Quarterly Information) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Selected Quarterly Financial Information [Line Items]
Total revenue, net	$ 214,432		$ 210,084		$ 210,170		$ 196,185		$ 203,523		$ 202,185		$ 193,329		$ 181,041		$ 830,871	$ 780,078	$ 732,068
Gross margin	132,502		130,536		131,896		121,510		120,783		123,534		120,491		116,120
Net income	$ 12,786		$ 13,211		$ 8,514		$ 6,693		$ 4,560		$ 11,243		$ 699		$ 11,487		$ 41,204	$ 27,989	$ 65,669
Basic net income (loss) per common share	$ 0.46	[1],[2]	$ 0.46	[1],[2]	$ 0.30	[1],[2]	$ 0.24	[1],[2]	$ 0.16	[1],[2]	$ 0.39	[1],[2]	$ 0.02	[1],[2]	$ 0.39	[1],[2]
Diluted net income (loss) per common share	$ 0.46	[1],[2]	$ 0.46	[1],[2]	$ 0.30	[1],[2]	$ 0.23	[1],[2]	$ 0.16	[1],[2]	$ 0.39	[1],[2]	$ 0.02	[1],[2]	$ 0.38	[1],[2]
Maximum
Selected Quarterly Financial Information [Line Items]
Impact of restatement on earnings per share (in dollars per share)																	$ 0.01

[1] Certain of the Company’s unvested restricted share units contain rights to receive nonforfeitable dividends, and thus, are participating securities requiring the two-class method of computing earnings per share. The participating securities had an insignificant impact on the calculation of earnings per share (impacts the rounding by less than $0.01 per share) on all of the periods presented; therefore, the Company does not present the full calculation.

[2] Per common share amounts for the quarters and full years have been calculated separately. Accordingly, quarterly amounts do not necessarily add to the annual amount because of differences in the weighted average common shares outstanding during each period principally due to the effect of the Company’s issuing shares of its common stock during the year.