Fair Value Accounting - Significant Unobservable Inputs Used in Developing Fair Value of Level 3 Positions (Details) $ in Thousands	Sep. 30, 2018 USD ($) $ / MW	Dec. 31, 2017 USD ($) $ / MW
FAIR VALUE
FTR price - per MWh, LOW | $ / MW	(3.04)	(2.95)
FTR price - per MWh, HIGH | $ / MW	7.48	6.33
Measured On A Recurring Basis
FAIR VALUE
ASSETS	$ 23,163	$ 7,396
LIABILITIES	903	352
Measured On A Recurring Basis | Level 3
FAIR VALUE
ASSETS	23,163	7,396
LIABILITIES	$ 903	$ 352