Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net income	$ 102,098	$ 87,898	$ 105,196
Net gains (losses) on derivative instruments (net of tax of $(10,746), $4,783 and $7,712, respectively)	15,153	(6,744)	(10,874)
Reclassifications of net losses on derivative instruments (net of tax of $(334), $(766) and $(483), respectively)	471	1,111	681
Benefit plans (net of tax of $(3,325), $1,948 and $479, respectively)	4,689	(2,745)	(674)
Reclassifications of benefit plans actuarial losses and prior service costs to net income (net of tax of $(555), $(333) and $(304), respectively)	784	394	430
Other comprehensive income (loss)	21,097	(7,984)	(10,437)
Comprehensive income	$ 123,195	$ 79,914	$ 94,759