EARNINGS PER SHARE (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended				6 Months Ended
	Mar. 31, 2012		Mar. 31, 2011		Mar. 31, 2012		Mar. 31, 2011
Earnings Per Share, Basic [Line Items]
Net income	$ 54,535		$ 63,927		$ 111,892		$ 88,436
Basic earnings per share
Weighted average shares of common stock outstanding-basic	41,509,000		41,352,000		41,472,000		41,316,000
Basic earnings per common share	$ 1.31		$ 1.55		$ 2.70		$ 2.14
Diluted earnings per share
Incremental shares	202,000	[1]	201,000	[1]	201,000	[1]	200,000	[1]
Weighted average shares of common stock outstanding-diluted	41,711,000		41,553,000		41,673,000		41,516,000
Diluted earnings per common share	$ 1.31	[2]	$ 1.54	[2]	$ 2.68	[2]	$ 2.13	[2]
Antidilutive securities excluded from computation of earnings per share, amount	0		0		0		0

[1] Incremental shares consist of stock options, stock awards and performance units.

[2] There were no anti-dilutive shares excluded from the calculation of diluted earnings per share for the three and six months ended March 31, 2012 and 2011.