Fair Value Accounting - Amount of Gain (Loss) Recognized in Income on Derivatives (Details) - DERIVATIVES NOT DESIGNATED AS HEDGING INSTRUMENTS - Price Risk Derivative - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Commodity contracts
Net gain (loss) recognized in income on derivatives	$ 5,763	$ 6,295	$ 29,061	$ 16,413
Accumulated deferred fuel
Commodity contracts
Unrealized gains (losses) associated with FTRs	4,500	(1,600)	6,200	800
Electric operations
Commodity contracts
Gain recognized in income on derivatives	13,665	14,062	49,656	35,172
Power purchased for utility customers
Commodity contracts
Loss recognized in income on derivatives	$ (7,902)	$ (7,767)	$ (20,595)	$ (18,759)