CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 603,064	$ 396,421	$ 350,104
Other comprehensive income (loss), net of tax
Net unrealized holding gains (losses) on available-for-sale securities, net of tax of $(146), $1,473 and $(245)	(395)	2,564	(465)
Cash flow hedges:
Amortization and unrealized gain (loss) on interest rate agreements, net of tax of $13,017, $43,238 and $(56,723)	44,936	75,222	(98,682)
Net unrealized gains on commodity cash flow hedges, net of tax of $0, $3,183 and $13,078	0	4,982	20,455
Total other comprehensive income (loss)	44,541	82,768	(78,692)
Total comprehensive income	$ 647,605	$ 479,189	$ 271,412