Quarterly Financial Data (Unaudited) (Details) (USD $)	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
Selected Quarterly Financial Information [Abstract]
Revenues	$ 495,114,000		$ 471,821,000	$ 472,903,000	$ 536,381,000	$ 435,526,000	$ 414,146,000	$ 412,418,000	$ 457,662,000	$ 1,976,219,000	$ 1,719,752,000	$ 1,653,096,000
Gross profit	89,946,000	[1]	87,029,000	83,841,000	90,959,000	80,808,000	73,686,000	71,357,000	72,685,000	351,775,000	298,536,000	287,967,000
Operating income	16,691,000		14,094,000	11,400,000	15,129,000	12,652,000	12,078,000	8,569,000	3,761,000	57,314,000	37,060,000	27,033,000
Net income	10,844,000		4,099,000	6,741,000	8,786,000	7,789,000	7,234,000	5,118,000	2,299,000	30,470,000	22,440,000	16,574,000
Basic net income per share (in dollars per share)	$ 0.42		$ 0.16	$ 0.25	$ 0.33	$ 0.30	$ 0.28	$ 0.20	$ 0.09	$ 1.16	$ 0.86	$ 0.65
Diluted net income per share (in dollars per share)	$ 0.42		$ 0.16	$ 0.25	$ 0.33	$ 0.30	$ 0.28	$ 0.20	$ 0.09	$ 1.16	$ 0.86	$ 0.65
Adjustment related to the reversal of sales taxes accrued	$ 2,500,000

[1] Included in the results for the fourth quarter of 2011 is a $2.5 million adjustment related to the reversal of Pennsylvania sales taxes accrued in prior periods. Please read Note 12 "Commitments and Contingencies" for additional information.