Quarterly Financial Data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010
Quarterly Financial Data Details [Abstract]
Revenues	$ 551,406	$ 554,669	$ 507,170	$ 461,901	$ 421,318	$ 410,736	$ 367,112	$ 366,174	$ 2,075,146	$ 1,565,340	$ 1,229,342
Gross profit	68,265	69,519	66,618	62,514	50,440	54,087	50,986	50,072	266,916	205,585	165,003
Operating Income Loss	$ 19,654	$ 23,287	$ 21,411	$ 21,415	$ 15,560	$ 18,701	$ 16,869	$ 17,536	$ 85,767	$ 68,666	$ 54,218
Basic EPS	$ 0.99	$ 1.15	$ 1.04	$ 1.05	$ 0.80	$ 0.91	$ 0.83	$ 0.87	$ 4.24	$ 3.42	$ 2.73
Diluted EPS	$ 0.99	$ 1.15	$ 1.04	$ 1.05	$ 0.80	$ 0.91	$ 0.83	$ 0.87	$ 4.23	$ 3.40	$ 2.70
Weighted average common shares outstanding:
Weighted Average Number of Shares Outstanding, Basic	12,636	12,628	12,619	12,581	12,497	12,484	12,462	12,415	12,616	12,464	12,241
Weighted Average Number of Shares Outstanding, Diluted	12,672	12,662	12,648	12,605	12,531	12,526	12,511	12,483	12,647	12,513	12,395