Quarterly Financial Data (Unaudited) (Details) (USD $)	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 [Abstract]
Revenues	$ 532,438,000		$ 511,953,000	$ 519,256,000		$ 595,177,000	$ 495,114,000		$ 471,821,000		$ 472,903,000	$ 536,381,000	$ 2,158,824,000	$ 1,976,219,000	$ 1,719,752,000
Gross profit	93,503,000		98,420,000	87,294,000	[1]	103,004,000	89,946,000	[2]	87,029,000		83,841,000	90,959,000	382,221,000	351,775,000	298,536,000
Operating income	15,893,000	[3]	19,140,000	9,415,000		23,046,000	16,691,000		14,094,000		11,400,000	15,129,000	67,494,000	57,314,000	37,060,000
Net income	9,445,000		11,452,000	5,621,000		13,884,000	10,844,000		4,099,000	[4]	6,741,000	8,786,000	40,402,000	30,470,000	22,440,000
Basic net income per share (in dollars per share)	$ 0.34	[5]	$ 0.45	$ 0.22		$ 0.54	$ 0.42		$ 0.16		$ 0.25	$ 0.33	$ 1.57	$ 1.16	$ 0.86
Diluted net income per share (in dollars per share)	$ 0.34	[5]	$ 0.45	$ 0.22		$ 0.54	$ 0.42		$ 0.16		$ 0.25	$ 0.33	$ 1.56	$ 1.16	$ 0.86
Impairment charge	4,191,000												4,191,000	0	0
Adjustment related to the reversal of sales taxes accrued							2,500,000
Loss related to the exchange of an aircraft									4,400,000
Loss related to a settlement with the State of California									$ 3,100,000

[1] Included in the results for the second quarter of 2012 is a $2.9 million reduction to payroll tax expense related to a refund of Pennsylvania sales taxes. Please read Note 13, "Commitments and Contingencies," for additional information.

[2] 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 13, "Commitments and Contingencies," for additional information.

[3] Included in the results for the fourth quarter of 2012 is a $4.2 million impairment charge, related to our Performance Management reporting unit. Please read Note 5, "Goodwill and Other Intangible Assets," for additional information.

[4] Included in the results for the third quarter of 2011 is a $4.4 million loss related to the exchange of an aircraft and a $3.1 million loss related to a settlement with the State of California. Please read Note 13, "Commitments and Contingencies," for additional information on the settlement with the State of California.

[5] Under the two-class earnings per share method, undistributed losses resulting from dividends exceeding net income are not allocated to participating securities. This resulted in a $0.03 earnings per share decrease in the fourth quarter of 2012. Please read Note 11, "Net Income Per Share," for additional information.