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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)

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