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Accounting for Derivative Instruments - Volumetric Underlying Derivative Transactions (Details) MWh in Thousands, MMBTU in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2017 MWh MMBTU | Dec. 31, 2016 MWh MMBTU
Non-trading | Natural Gas | Buy |  |
Derivatives, Fair Value [Line Items] |  |
Net notional volume | 9,136 | 8,016
Non-trading | Natural Gas Basis | Buy |  |
Derivatives, Fair Value [Line Items] |  |
Net notional volume | 0 | 0
Non-trading | Electricity | Buy |  |
Derivatives, Fair Value [Line Items] |  |
Net notional volume | MWh | 4,276 | 3,958
Trading | Natural Gas | Buy |  |
Derivatives, Fair Value [Line Items] |  |
Net notional volume | 108 |
Trading | Natural Gas | Sell |  |
Derivatives, Fair Value [Line Items] |  |
Net notional volume |  | 953
Trading | Natural Gas Basis | Buy |  |
Derivatives, Fair Value [Line Items] |  |
Net notional volume | 0 |
Trading | Natural Gas Basis | Sell |  |
Derivatives, Fair Value [Line Items] |  |
Net notional volume |  | 380

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