Quarterly Financial Data (Details) (USD $) In Thousands, except Per Share data	3 Months Ended								12 Months Ended
	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2009	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2009
Quarterly Financial Data Details [Abstract]
Revenues	$ 421,318	$ 410,736	$ 367,112	$ 366,174	$ 358,947	$ 347,687	$ 286,597	$ 236,111	$ 1,565,340	$ 1,229,342	$ 941,332
Gross profit	50,440	54,087	50,986	50,072	45,535	43,937	39,522	36,009	205,585	165,003	134,655
Operating Income Loss	15,560	18,701	16,869	17,536	13,757	15,064	12,664	12,733	68,666	54,218	40,463
Net Income	$ 10,030	$ 11,390	$ 10,332	$ 10,828	$ 8,798	$ 9,137	$ 7,671	$ 7,834	$ 42,580	$ 33,440	$ 24,905
Basic EPS	$ 0.80	$ 0.91	$ 0.83	$ 0.87	$ 0.71	$ 0.74	$ 0.63	$ 0.64	$ 3.42	$ 2.73	$ 2.06
Diluted EPS	$ 0.80	$ 0.91	$ 0.83	$ 0.87	$ 0.71	$ 0.74	$ 0.62	$ 0.63	$ 3.40	$ 2.70	$ 2.02
Weighted average common shares outstanding:
Weighted Average Number of Shares Outstanding, Basic	12,497	12,484	12,462	12,415	12,318	12,265	12,207	12,173	12,464	12,241	12,088
Weighted Average Number of Shares Outstanding, Diluted	12,531	12,526	12,511	12,483	12,437	12,408	12,376	12,356	12,513	12,395	12,306