Condensed Consolidated Statements of Comprehensive Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 89,184	$ 65,125	$ 157,487	$ 109,390
Other comprehensive income:
Reclassifications of net losses on derivative instruments (net of tax of $(280), $(341), $(559) and $(692), respectively)	592	481	1,184	976
Benefit plans reclassifications of actuarial losses and net prior service credits (net of tax of $(104), $(169), $(208) and $(338), respectively)	220	239	440	478
Other comprehensive income	812	720	1,624	1,454
Comprehensive income	$ 89,996	$ 65,845	$ 159,111	$ 110,844