Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative Assets	$ 44,777	$ 74,155
Derivative Liabilities	(44,052)	(69,348)
Effect of counterparty netting in derivative assets	(43,359)	(69,247)
Effect of counterparty netting in derivative liabilities	43,359	69,247
Derivative Asset	1,418	4,908
Derivative Liability	(693)	(101)
Net derivatives as classified in the balance sheet	725	4,807
Designated as Hedging Instrument [Member] | Coal Contract [Member]
Derivative [Line Items]
Derivative Assets	9	0
Derivative Liabilities	0	(15)
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Assets	44,768	74,155
Derivative Liabilities	(44,052)	(69,333)
Not Designated as Hedging Instrument [Member] | Coal Contract [Member]
Derivative [Line Items]
Derivative Assets	43,157	68,948
Derivative Liabilities	(43,781)	(68,740)
Not Designated as Hedging Instrument [Member] | Coal Contract [Member]
Derivative [Line Items]
Derivative Assets	193	475
Derivative Liabilities	(271)	(580)
Not Designated as Hedging Instrument [Member] | Heating Oil-Diesel Purchases [Member]
Derivative [Line Items]
Derivative Assets	1,418	4,646
Derivative Liabilities	0	0
Not Designated as Hedging Instrument [Member] | Natural Gas [Member]
Derivative [Line Items]
Derivative Assets	0	86
Derivative Liabilities	$ 0	$ (13)