Shareholders' Equity Components (Details) - USD ($)	1 Months Ended	12 Months Ended
	Feb. 29, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Shareholders' equity, beginning balance		$ 3,463,059,000	$ 3,194,797,000	$ 3,086,232,000
Net current-period other comprehensive income (loss)		82,768,000	(78,692,000)	(96,937,000)
Shareholders' equity, ending balance		3,898,666,000	3,463,059,000	$ 3,194,797,000
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	1,360,756
ATM Equity Distribution, Net Proceeds		$ 98,800,000	$ 98,600,000
ATM Equity Distribution, Gross Proceeds		$ 100,000,000	100,000,000
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized		11,200,000.0		8,700,000.0
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized	2,500,000.0
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		2,502,000	(263,000)
Amounts reclassified from accumulated other comprehensive income		62,000	(202,000)
Net current-period other comprehensive income (loss)		2,564,000	(465,000)
Shareholders' equity, ending balance		7,048,000	4,484,000	$ 4,949,000
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		74,560,000	(99,029,000)
Amounts reclassified from accumulated other comprehensive income		662,000	347,000
Net current-period other comprehensive income (loss)		75,222,000	(98,682,000)
Shareholders' equity, ending balance		(112,302,000)	(187,524,000)	(88,842,000)
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	(11,662,000)
Amounts reclassified from accumulated other comprehensive income		(4,865,000)	32,117,000
Net current-period other comprehensive income (loss)		4,982,000	20,455,000
Shareholders' equity, ending balance		0	(4,982,000)	(25,437,000)
Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Shareholders' equity, beginning balance		(188,022,000)	(109,330,000)	(12,393,000)
Other comprehensive income (loss) before reclassifications		86,909,000	(110,954,000)
Amounts reclassified from accumulated other comprehensive income		(4,141,000)	32,262,000
Net current-period other comprehensive income (loss)		82,768,000	(78,692,000)	(96,937,000)
Shareholders' equity, ending balance		$ (105,254,000)	$ (188,022,000)	$ (109,330,000)