Accounting for Derivative Instruments - Volumetric Underlying Derivative Transactions (Details) MWh in Thousands, MMBTU in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016 MWh MMBTU	Dec. 31, 2015 MWh MMBTU
Non-trading | Natural Gas | Buy
Derivatives, Fair Value [Line Items]
Net notional volume	6,760	7,543
Non-trading | Natural Gas Basis | Buy
Derivatives, Fair Value [Line Items]
Net notional volume	0	455
Non-trading | Electricity | Buy
Derivatives, Fair Value [Line Items]
Net notional volume | MWh	3,278	1,187
Trading | Natural Gas | Buy
Derivatives, Fair Value [Line Items]
Net notional volume		8
Trading | Natural Gas | Sell
Derivatives, Fair Value [Line Items]
Net notional volume	2,366
Trading | Natural Gas Basis | Buy
Derivatives, Fair Value [Line Items]
Net notional volume	242
Trading | Natural Gas Basis | Sell
Derivatives, Fair Value [Line Items]
Net notional volume		455