Selected Quarterly Information (Selected Quarterly Information) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2010		Sep. 30, 2010		Jun. 30, 2010		Mar. 31, 2010		Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Total Revenue	$ 203,523		$ 202,185		$ 193,329		$ 181,041		$ 194,134		$ 186,641		$ 178,595		$ 172,698		$ 780,078	$ 732,068	$ 682,487
Gross Margin	120,783		123,534		120,491		116,120		122,828		117,447		114,131		109,474
Net income (loss)	$ 4,560		$ 11,243		$ 699		$ 11,487		$ 18,807		$ 16,483		$ 15,157		$ 15,222		$ 27,989	$ 65,669	$ 50,955
Basic net income (loss) per common share	$ 0.16	[1],[2]	$ 0.39	[1],[2]	$ 0.02	[1],[2]	$ 0.39	[1],[2]	$ 0.64	[1],[2]	$ 0.56	[1],[2]	$ 0.51	[1],[2]	$ 0.51	[1],[2]
Diluted net income (loss) per common share	$ 0.16	[1],[2]	$ 0.39	[1],[2]	$ 0.02	[1],[2]	$ 0.38	[1],[2]	$ 0.63	[1],[2]	$ 0.55	[1],[2]	$ 0.50	[1],[2]	$ 0.50	[1],[2]
Maximum [Member]
Impact of restatement on earnings per share																	$ 0.01

[1] 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.

[2] 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.