Condensed Consolidated Statements of Comprehensive Income (unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013	Mar. 31, 2012
Net income	$ 58,259	$ 47,871	$ 95,071	$ 80,539
Net gains in fair value of derivative instruments (net of tax $(2,538), $(1,869), $(4,337) and $(592), respectively)	3,578	2,635	6,115	834
Reclassifications of net losses on derivative instruments (net of tax of $(84), $(403), $(168) and $(524), respectively)	118	570	237	740
Benefit plans (net of tax of $(139), $(81), $(278) and $(162),respectively)	198	77	393	216
Other comprehensive income (loss)	3,894	3,282	6,745	1,790
Comprehensive income	$ 62,153	$ 51,153	$ 101,816	$ 82,329