Shareholders' Equity Components (Details) - USD ($)	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Shareholders' equity, beginning balance	$ 3,463,059,000
Shareholders' equity, ending balance	3,698,975,000
Shareholders' Equity Other Disclosures [Abstract]
Debt and Equity Securities Authorized for Issuance	2,500,000,000.0
Debt And Equity Securities Available For Issuance	2,400,000,000
ATM Equity Distribution Offering Price, Maximum Amount	$ 200,000,000
ATM Equity Distribution, Shares Issued	690,812
ATM Equity Distribution, Net Proceeds	$ 49,400,000
ATM Equity Distribution, Gross Proceeds	50,000,000.0
ATM Equity Distribution, equity remaining	50,000,000
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	(828,000)	(768,000)
Amounts reclassified from accumulated other comprehensive income	0	0
Net current-period other comprehensive income (loss)	(828,000)	(768,000)
Shareholders' equity, ending balance	3,656,000	4,181,000
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	91,127,000	4,696,000
Amounts reclassified from accumulated other comprehensive income	87,000	87,000
Net current-period other comprehensive income (loss)	91,214,000	4,783,000
Shareholders' equity, ending balance	(96,310,000)	(84,059,000)
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	(11,656,000)
Amounts reclassified from accumulated other comprehensive income	(4,865,000)	14,009,000
Net current-period other comprehensive income (loss)	4,982,000	2,353,000
Shareholders' equity, ending balance	0	(23,084,000)
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	100,146,000	(7,728,000)
Amounts reclassified from accumulated other comprehensive income	(4,778,000)	14,096,000
Net current-period other comprehensive income (loss)	95,368,000	6,368,000
Shareholders' equity, ending balance	$ (92,654,000)	$ (102,962,000)