Condensed Consolidated Statements of Comprehensive Income (unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Statement of Comprehensive Income [Abstract]
Net income	$ 76,110	$ 58,259	$ 121,396	$ 95,071
Other comprehensive income:
Net gains in fair value of derivative instruments (net of tax of $0, $(2,538), $0 and $(4,337), respectively)	0	3,578	0	6,115
Reclassifications of net losses on derivative instruments (net of tax of $(278), $(84), $(556) and $(168), respectively)	392	118	783	237
Benefit plans reclassifications of actuarial losses and prior service costs (net of tax of $(67), $(139), $(137) and $(278), respectively)	95	198	193	393
Other comprehensive income	487	3,894	976	6,745
Comprehensive income	$ 76,597	$ 62,153	$ 122,372	$ 101,816