Quarterly Financial Data Schedule of Quarterly Financial Information) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011
Quarterly Financial Data [Abstract]
Revenues	$ 605,080	$ 606,443	$ 563,114	$ 572,848	$ 551,406	$ 554,669	$ 507,170	$ 461,901	$ 2,347,485	$ 2,075,146	$ 1,565,340
Gross profit	75,219	77,511	74,679	76,929	68,265	69,519	66,618	62,514	304,338	266,916	205,585
Operating Income Loss	22,473	26,534	24,238	27,077	19,654	23,287	21,411	21,415	100,323	85,767	68,666
Net income	$ 14,216	$ 16,781	$ 15,100	$ 16,751	$ 12,600	$ 14,503	$ 13,178	$ 13,196	$ 62,849	$ 53,477	$ 42,580
Basic EPS	$ 1.12	$ 1.32	$ 1.19	$ 1.32	$ 0.99	$ 1.15	$ 1.04	$ 1.05	$ 4.96	$ 4.24	$ 3.42
Diluted EPS	$ 1.12	$ 1.32	$ 1.19	$ 1.32	$ 0.99	$ 1.15	$ 1.04	$ 1.05	$ 4.95	$ 4.23	$ 3.40
Weighted average common shares outstanding:
Weighted Average Number of Shares Outstanding, Basic	12,683	12,679	12,674	12,665	12,636	12,628	12,619	12,581	12,675	12,616	12,464
Weighted Average Number of Shares Outstanding, Diluted	12,721	12,713	12,709	12,695	12,672	12,662	12,648	12,605	12,709	12,647	12,513