Shareholders' Equity Components (Details) - USD ($)	3 Months Ended	9 Months Ended
	Jun. 30, 2016	Jun. 30, 2016	Jun. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Shareholders' equity, beginning balance		$ 3,194,797,000
Shareholders' equity, ending balance	$ 3,466,724,000	$ 3,466,724,000
Shareholders' Equity Other Disclosures [Abstract]
Share Repurchase Program - Shares Purchased		0	0
Debt and Equity Securities Authorized for Issuance	2,500,000,000	$ 2,500,000,000
Debt And Equity Securities Available For Issuance	2,400,000,000	2,400,000,000
ATM Equity Distribution Offering Price, Maximum Amount	$ 200,000,000	$ 200,000,000
ATM Equity Distribution, Shares Issued	1,360,756
ATM Agent Commission on Proceeds, percentage	1.00%	1.00%
ATM Equity Distribution, Net Proceeds	$ 98,700,000
ATM Equity Distribution, Gross Proceeds	100,000,000
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Shareholders' equity, beginning balance		$ 4,949,000	$ 7,662,000
Other comprehensive income (loss) before reclassifications		(1,417,000)	30,000
Amounts reclassified from accumulated other comprehensive income		(79,000)	(326,000)
Net current-period other comprehensive income (loss)		(1,496,000)	(296,000)
Shareholders' equity, ending balance	3,453,000	3,453,000	7,366,000
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		(88,842,000)	(18,381,000)
Other comprehensive income (loss) before reclassifications		(88,345,000)	(30,436,000)
Amounts reclassified from accumulated other comprehensive income		260,000	455,000
Net current-period other comprehensive income (loss)		(88,085,000)	(29,981,000)
Shareholders' equity, ending balance	(176,927,000)	(176,927,000)	(48,362,000)
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		(25,437,000)	(1,674,000)
Other comprehensive income (loss) before reclassifications		(8,612,000)	(37,397,000)
Amounts reclassified from accumulated other comprehensive income		29,290,000	17,826,000
Net current-period other comprehensive income (loss)		20,678,000	(19,571,000)
Shareholders' equity, ending balance	(4,759,000)	(4,759,000)	(21,245,000)
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Shareholders' equity, beginning balance		(109,330,000)	(12,393,000)
Other comprehensive income (loss) before reclassifications		(98,374,000)	(67,803,000)
Amounts reclassified from accumulated other comprehensive income		29,471,000	17,955,000
Net current-period other comprehensive income (loss)		(68,903,000)	(49,848,000)
Shareholders' equity, ending balance	$ (178,233,000)	$ (178,233,000)	$ (62,241,000)