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Derivatives And Risk Management (Energy Commodity Derivatives) (Details) frequency in Thousands, Volt in Thousands | 9 Months Ended | 12 Months Ended
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Sep. 30, 2018 Volt frequency | Dec. 31, 2017 Volt frequency
Sales [Member] | Physical [Member] | Electric Derivative [Member] |  |
Energy Commodity Derivative Volumes [Line Items] |  |
2018 | 41 | 213
2019 | 123 | 94
2020 | 0 | 0
2021 | 0 | 0
2022 | 0 | 0
Thereafter | 0 | 0
Sales [Member] | Physical [Member] | Gas Derivative [Member] |  |
Energy Commodity Derivative Volumes [Line Items] |  |
2018 | Volt | 3,101 | 3,643
2019 | Volt | 2,245 | 1,345
2020 | Volt | 1,430 | 1,430
2021 | 1,049 | 1,049
2022 | 0 | 0
Thereafter | Volt | 0 | 0
Sales [Member] | Financial [Member] | Electric Derivative [Member] |  |
Energy Commodity Derivative Volumes [Line Items] |  |
2018 | 575 | 1,739
2019 | 2,403 | 1,420
2020 | 836 | 589
2021 | 0 | 0
2022 | 0 | 0
Thereafter | 0 | 0
Sales [Member] | Financial [Member] | Gas Derivative [Member] |  |
Energy Commodity Derivative Volumes [Line Items] |  |
2018 | Volt | 20,683 | 67,375
2019 | Volt | 47,488 | 35,438
2020 | Volt | 7,995 | 915
2021 | Volt | 2,275 | 0
2022 | 0 | 0
Thereafter | Volt | 0 | 0
Purchase [Member] | Physical [Member] | Electric Derivative [Member] |  |
Energy Commodity Derivative Volumes [Line Items] |  |
2018 | 120 | 426
2019 | 204 | 235
2020 | 0 | 0
2021 | 0 | 0
2022 | 0 | 0
Thereafter | 0 | 0
Purchase [Member] | Physical [Member] | Gas Derivative [Member] |  |
Energy Commodity Derivative Volumes [Line Items] |  |
2018 | Volt | 8,109 | 10,572
2019 | Volt | 5,110 | 610
2020 | Volt | 910 | 910
2021 | Volt | 0 | 0
2022 | Volt | 0 | 0
Thereafter | Volt | 0 | 0
Purchase [Member] | Financial [Member] | Electric Derivative [Member] |  |
Energy Commodity Derivative Volumes [Line Items] |  |
2018 | 542 | 763
2019 | 901 | 737
2020 | 0 | 0
2021 | 0 | 0
2022 | 0 | 0
Thereafter | 0 | 0
Purchase [Member] | Financial [Member] | Gas Derivative [Member] |  |
Energy Commodity Derivative Volumes [Line Items] |  |
2018 | Volt | 34,905 | 107,580
2019 | Volt | 87,118 | 61,073
2020 | Volt | 31,005 | 16,590
2021 | Volt | 4,975 | 0
2022 | Volt | 0 | 0
Thereafter | Volt | 0 | 0

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