Derivatives (Effects Of Derivatives On Measures Of Financial Performance) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011
Gain (Loss) Recognized in OCI (Effective Portion)	$ 2,291		$ 14,788
Gains (Losses) Reclassified from OCI into Income (Effective Portion)	201		3,257
Gain (Loss) Recognized in Income (Ineffective Portion and Amount Excluded from Effectiveness Testing)
Firm Commitments [Member]
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge
Coal [Member]
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge
Coal Purchases [Member]
Gain (Loss) Recognized in OCI (Effective Portion)	(202)	[1]	(876)	[1]
Gain (Loss) Recognized in Income (Ineffective Portion and Amount Excluded from Effectiveness Testing)
Coal Sales [Member]
Gain (Loss) Recognized in OCI (Effective Portion)	2,493	[2]	1,406	[2]
Gains (Losses) Reclassified from OCI into Income (Effective Portion)	201	[2]	87	[2]
Gain (Loss) Recognized in Income (Ineffective Portion and Amount Excluded from Effectiveness Testing)
Coal [Member]
Derivatives Not Designated as Hedging Instruments, Losses, Unrealized	7,552	[3]	(1,045)	[3]
Derivatives Not Designated as Hedging Instruments, Gain, Realized	3,158	[3]
Heating Oil-Diesel Purchases [Member]
Gain (Loss) Recognized in OCI (Effective Portion)			14,258	[1]
Gains (Losses) Reclassified from OCI into Income (Effective Portion)			3,170	[1]
Gain (Loss) Recognized in Income (Ineffective Portion and Amount Excluded from Effectiveness Testing)
Derivatives Not Designated as Hedging Instruments, Losses, Unrealized	423	[3]
Heating Oil-Fuel Surchages [Member]
Derivatives Not Designated as Hedging Instruments, Losses, Unrealized	$ 367

[1] Cost of sales

[2] Revenues

[3] Other operating income, net