Risk Management and Fair Values (Schedules Of Valuation Techniques) (Details)	Mar. 31, 2018 USD ($)
Fair Value of Electricity Swaps	$ 75,000,000
Maximum [Member]
Range from Average Percentage for Fair Value of Electricity Swaps	4.75%
Effect of Significant Unobservable Inputs on Fair Value of Electricity Swaps	$ 7,000,000
Minimum [Member]
Range from Average Percentage for Fair Value of Electricity Swaps	4.00%
Effect of Significant Unobservable Inputs on Fair Value of Electricity Swaps	$ 5