Derivatives (Disclosure of Fair Value of Derivatives) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Derivative [Line Items]
Derivative Assets	$ 67,681	$ 54,863
Derivative Liabilities	(47,739)	(24,285)
Effect of counterparty netting in derivative assets	(47,727)	(22,548)
Effect of counterparty netting in derivative liabilities	47,727	22,548
Net derivative assets as classified in the balance sheet	19,954	32,315
Net derivative liabilities as classified in the balance sheet	(12)	(1,737)
Net derivatives as classified in the balance sheet	19,942	30,578
Designated as Hedging Instrument [Member] | Coal [Member]
Derivative [Line Items]
Derivative Assets	909	3,277
Derivative Liabilities	(26)	(10)
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Assets	66,772	51,586
Derivative Liabilities	(47,713)	(24,275)
Not Designated as Hedging Instrument [Member] | Heating Oil-Diesel Purchases [Member]
Derivative [Line Items]
Derivative Assets	4,681	7,379
Derivative Liabilities
Not Designated as Hedging Instrument [Member] | Heating Oil-Fuel Surchages [Member]
Derivative [Line Items]
Derivative Assets	422	1,961
Derivative Liabilities
Not Designated as Hedging Instrument [Member] | Coal Held for Trading Purposes [Member]
Derivative [Line Items]
Derivative Assets	55,327	17,403
Derivative Liabilities	(45,763)	(16,933)
Not Designated as Hedging Instrument [Member] | Coal [Member]
Derivative [Line Items]
Derivative Assets	6,342	24,843
Derivative Liabilities	$ (1,950)	$ (7,342)