Derivatives (Effects Of Derivatives On Measures Of Financial Performance) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended						15 Months Ended
	Dec. 31, 2012	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI (Effective Portion)	$ 5,250					$ 282		$ 4,208
Coal derivative settlements, non-hedging		43,990		(7)		4,542
Gains (Losses) Reclassified from OCI into Income (Effective Portion)	2,675					(2,367)		16,438
Coal Purchases [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI (Effective Portion)	(2,440)					5,145		(2,009)
Gains (Losses) Reclassified from OCI into Income (Effective Portion)	0					(1,202)		0
Coal Sales [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI (Effective Portion)	7,690					(4,714)		4,923
Gains (Losses) Reclassified from OCI into Income (Effective Portion)	2,675					(1,602)		1,572
Coal [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives		8,272	[1]	6,438	[1]	(10,991)	[1]
Coal derivative settlements, non-hedging		43,990	[1]	(7)	[1]	4,542	[1]
Heating Oil-Diesel Purchases [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI (Effective Portion)	0					(149)		1,294
Gains (Losses) Reclassified from OCI into Income (Effective Portion)	0					437		14,866
Realized Gains (Losses)		(22,281)	[1]	(2,906)	[1]		[1]
Heating Oil-Fuel Surchages [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Realized Gains (Losses)		$ (2,209)	[1]		[1]		[1]

[1] .