Shareholders' Equity Components (Details) - USD ($)	9 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Shareholders' equity, beginning balance	$ 3,463,059,000
Shareholders' equity, ending balance	3,901,710,000
Shareholders' Equity Other Disclosures [Abstract]
Debt and Equity Securities Authorized for Issuance	2,500,000,000.0
Debt And Equity Securities Available For Issuance	1,600,000,000
ATM Equity Distribution Offering Price, Maximum Amount	$ 200,000,000
ATM Equity Distribution, Shares Issued	1,303,494
ATM Equity Distribution, Net Proceeds	$ 98,800,000
ATM Equity Distribution, Gross Proceeds	100,000,000.0
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Shareholders' equity, beginning balance	4,484,000	$ 4,949,000
Other comprehensive income (loss) before reclassifications	1,485,000	(1,417,000)
Amounts reclassified from accumulated other comprehensive income	68,000	(79,000)
Net current-period other comprehensive income (loss)	1,553,000	(1,496,000)
Shareholders' equity, ending balance	6,037,000	3,453,000
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Interest Rate Contract [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Shareholders' equity, beginning balance	(187,524,000)	(88,842,000)
Other comprehensive income (loss) before reclassifications	76,602,000	(88,345,000)
Amounts reclassified from accumulated other comprehensive income	286,000	260,000
Net current-period other comprehensive income (loss)	76,888,000	(88,085,000)
Shareholders' equity, ending balance	(110,636,000)	(176,927,000)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Commodity Contract [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Shareholders' equity, beginning balance	(4,982,000)	(25,437,000)
Other comprehensive income (loss) before reclassifications	9,847,000	(8,612,000)
Amounts reclassified from accumulated other comprehensive income	(4,865,000)	29,290,000
Net current-period other comprehensive income (loss)	4,982,000	20,678,000
Shareholders' equity, ending balance	0	(4,759,000)
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Shareholders' equity, beginning balance	(188,022,000)	(109,330,000)
Other comprehensive income (loss) before reclassifications	87,934,000	(98,374,000)
Amounts reclassified from accumulated other comprehensive income	(4,511,000)	29,471,000
Net current-period other comprehensive income (loss)	83,423,000	(68,903,000)
Shareholders' equity, ending balance	$ (104,599,000)	$ (178,233,000)