Shareholders' Equity Components (Details) - USD ($)	6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Shareholders' equity, beginning balance	$ 3,898,666,000
Shareholders' equity, ending balance	4,721,346,000
Shareholders' Equity Other Disclosures [Abstract]
Debt and Equity Securities Authorized for Issuance	2,500,000,000.0
Debt And Equity Securities Available For Issuance	1,200,000,000
ATM Equity Distribution Offering Price, Maximum Amount	$ 500,000,000
ATM Equity Distribution, Shares Issued	0
Block Trade, shares authorized for issuance	4,558,404
Block Trade, Gross Proceeds	$ 400,000,000
Block Trade, Net Proceeds	395,100,000
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Shareholders' equity, beginning balance	7,048,000	$ 4,484,000
Other comprehensive income (loss) before reclassifications	(167,000)	634,000
Amounts reclassified from accumulated other comprehensive income	(879,000)	68,000
Net current-period other comprehensive income (loss)	(1,046,000)	702,000
Shareholders' equity, ending balance	6,002,000	5,186,000
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	(112,302,000)	(187,524,000)
Other comprehensive income (loss) before reclassifications	20,454,000	95,271,000
Amounts reclassified from accumulated other comprehensive income	835,000	173,000
Net current-period other comprehensive income (loss)	21,289,000	95,444,000
Shareholders' equity, ending balance	(91,013,000)	(92,080,000)
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)
Other comprehensive income (loss) before reclassifications		9,847,000
Amounts reclassified from accumulated other comprehensive income		(4,865,000)
Net current-period other comprehensive income (loss)		4,982,000
Shareholders' equity, ending balance		0
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Shareholders' equity, beginning balance	(105,254,000)	(188,022,000)
Other comprehensive income (loss) before reclassifications	20,287,000	105,752,000
Amounts reclassified from accumulated other comprehensive income	(44,000)	(4,624,000)
Net current-period other comprehensive income (loss)	20,243,000	101,128,000
Shareholders' equity, ending balance	$ (85,011,000)	$ (86,894,000)