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	$ 178,992	$ 164,728		$ 493,124	$ 289,760
Other comprehensive income (loss), net of tax
Net unrealized holding gains (losses) on available-for-sale securities, net of tax	(939)	1,530		(1,046)	702
Schedule of Other Comprehensive Income (Loss), Cash Flow Hedges [Line Items]
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	22,244	4,230	[1]	21,289	100,426	[1]
Total other comprehensive income (loss)	21,305	5,760		20,243	101,128
Total comprehensive income (loss)	200,297	170,488		513,367	390,888
Interest Rate Contract [Member]
Schedule of Other Comprehensive Income (Loss), Cash Flow Hedges [Line Items]
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	22,244	4,230		21,289	95,444
Commodity Contract [Member]
Schedule of Other Comprehensive Income (Loss), Cash Flow Hedges [Line Items]
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	$ 0	$ 0		$ 0	$ 4,982

[1] Utilizing an income tax rate ranging from 37 percent to 39 percent based on the effective rates in each taxing jurisdiction for the three and six-month period ended March 31, 2017.