Accounting for Derivative Instruments and Hedging Activities (Schedule of Effects of Non-hedging Derivative Instruments on Income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized in income on derivatives	$ 41,662	$ (30,790)	$ 83,795
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized in income on derivatives	80,124	(71,830)	21,079
Not Designated as Hedging Instrument | Energy Related Derivative [Member] | Unrealized (Gain) Loss on Derivative Instruments, Net
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized in income on derivatives	23,186	(32,492)	62,318
Not Designated as Hedging Instrument | Interest Rate Contract | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized in income on derivatives	0	0	0
Not Designated as Hedging Instrument | Commodity contracts: | Electric Generation Fuel
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized in income on derivatives	26,222	(23,195)	(39,656)
Not Designated as Hedging Instrument | Commodity contracts: | Purchased Electricity
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized in income on derivatives	12,240	(17,873)	(21,998)
Not Designated as Hedging Instrument | Electric [Member] | Unrealized (Gain) Loss on Derivative Instruments, Net
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized in income on derivatives	18,476	1,702	21,477
Subsidiaries [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized in income on derivatives	$ 41,662	$ (30,790)	$ 83,795