Fair Value Accounting - Fair Value of Derivative Instruments as Recorded in Condensed Consolidated Balance Sheets (Details) - Price Risk Derivative - DERIVATIVES NOT DESIGNATED AS HEDGING INSTRUMENTS - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Commodity-related contracts
Commodity-related contracts, net	$ 22,260	$ 7,044
Energy risk management assets
Commodity-related contracts
FTRs in energy risk management assets	23,163	7,396
Other current liabilities
Commodity-related contracts
FTRs in other current liabilities	$ 903	$ 352