Accumulated Other Comprehensive Income Components (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Shareholders' equity, beginning balance	$ 3,086,232	$ 2,580,409	$ 2,359,243
Net current-period other comprehensive income (loss)	(96,937)	(51,271)	86,485
Shareholders' equity, ending balance	3,194,797	3,086,232	2,580,409
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Shareholders' equity, beginning balance	7,662	5,448
Other comprehensive income before reclassifications	(2,173)	3,009
Amounts reclassified from accumulated other comprehensive income	(540)	(795)
Net current-period other comprehensive income (loss)	(2,713)	2,214
Shareholders' equity, ending balance	4,949	7,662	5,448
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	(18,381)	37,906
Other comprehensive income before reclassifications	(71,003)	(58,973)
Amounts reclassified from accumulated other comprehensive income	542	2,686
Net current-period other comprehensive income (loss)	(70,461)	(56,287)
Shareholders' equity, ending balance	(88,842)	(18,381)	37,906
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	(1,674)	(4,476)
Other comprehensive income before reclassifications	(49,211)	7,904
Amounts reclassified from accumulated other comprehensive income	25,448	(5,102)
Net current-period other comprehensive income (loss)	(23,763)	2,802
Shareholders' equity, ending balance	(25,437)	(1,674)	(4,476)
Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Shareholders' equity, beginning balance	(12,393)	38,878	(47,607)
Other comprehensive income before reclassifications	(122,387)	(48,060)
Amounts reclassified from accumulated other comprehensive income	25,450	(3,211)
Net current-period other comprehensive income (loss)	(96,937)	(51,271)	86,485
Shareholders' equity, ending balance	$ (109,330)	$ (12,393)	$ 38,878