Condensed Consolidated Statements of Comprehensive Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 65,125	$ 63,294	$ 109,390	$ 86,645
Other comprehensive income (loss):
Net losses on derivative instruments (net of tax of $0, $13,348, $0, and $12,016, respectively)	0	(18,820)	0	(16,943)
Reclassifications of net losses on derivative instruments (net of tax of $(341), $(253), $(692), and $(529), respectively)	481	356	976	746
Benefit plans reclassifications of actuarial losses and prior service costs (net of tax of $(169), $(114), $(338), and $(227), respectively)	239	160	478	320
Other comprehensive income (loss)	720	(18,304)	1,454	(15,877)
Comprehensive income	$ 65,845	$ 44,990	$ 110,844	$ 70,768