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Shareholders' Equity Components (Details) - USD ($) | 9 Months Ended
--- | ---
Jun. 30, 2017 | Jun. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |
Shareholders' equity, beginning balance | $ 3,463,059,000 |
Shareholders' equity, ending balance | 3,901,710,000 |
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 |
ATM Equity Distribution, Net Proceeds | $ 98,800,000 |
ATM Equity Distribution, Gross Proceeds | 100,000,000.0 |
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 | 1,485,000 | (1,417,000)
Amounts reclassified from accumulated other comprehensive income | 68,000 | (79,000)
Net current-period other comprehensive income (loss) | 1,553,000 | (1,496,000)
Shareholders' equity, ending balance | 6,037,000 | 3,453,000
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 | 76,602,000 | (88,345,000)
Amounts reclassified from accumulated other comprehensive income | 286,000 | 260,000
Net current-period other comprehensive income (loss) | 76,888,000 | (88,085,000)
Shareholders' equity, ending balance | (110,636,000) | (176,927,000)
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 | (8,612,000)
Amounts reclassified from accumulated other comprehensive income | (4,865,000) | 29,290,000
Net current-period other comprehensive income (loss) | 4,982,000 | 20,678,000
Shareholders' equity, ending balance | 0 | (4,759,000)
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 | 87,934,000 | (98,374,000)
Amounts reclassified from accumulated other comprehensive income | (4,511,000) | 29,471,000
Net current-period other comprehensive income (loss) | 83,423,000 | (68,903,000)
Shareholders' equity, ending balance | $ (104,599,000) | $ (178,233,000)

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