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DERIVATIVE INSTRUMENTS - INCOME STATEMENT RELATED DISCLOSURES (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands | 3 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Energy Services |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Amount of gain (loss) recognized in income on derivatives | $ (47,532) | $ (37,753)
Energy Services | Physical commodity contracts | Operating revenues |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Amount of gain (loss) recognized in income on derivatives | 1,210 | 1,743
Energy Services | Physical commodity contracts | Gas purchases |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Amount of gain (loss) recognized in income on derivatives | (22,697) | (8,799)
Energy Services | Financial commodity contracts | Gas purchases |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Amount of gain (loss) recognized in income on derivatives | (25,997) | (30,611)
Energy Services | Foreign currency contracts | Gas purchases |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Amount of gain (loss) recognized in income on derivatives | (48) | (86)
Natural Gas Distribution |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Amount of gain (loss) recognized in income on derivatives | (15,851) | 32,599
Natural Gas Distribution | Physical commodity contracts |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Amount of gain (loss) recognized in income on derivatives | (2,976) | 1,050
Natural Gas Distribution | Financial commodity contracts |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Amount of gain (loss) recognized in income on derivatives | (8,808) | 11,178
Natural Gas Distribution | Interest rate contracts |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Amount of gain (loss) recognized in income on derivatives | $ (4,067) | $ 20,371

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