Derivatives (Disclosure of Fair Value of Derivatives) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Derivative Assets	$ 116,080	$ 112,243
Derivative Liabilities	(107,028)	(99,452)
Effect of counterparty netting in derivative assets	(107,028)	(98,686)
Effect of counterparty netting in derivative liabilities	107,028	98,686
Net derivative assets as classified in the balance sheet	9,052	13,557
Net derivative liabilities as classified in the balance sheet	0	(766)
Net derivatives as classified in the balance sheet	9,052	12,791
Designated as Hedging Instrument [Member] | Coal [Member]
Derivative [Line Items]
Derivative Assets	4	6,535
Derivative Liabilities	(20)	(2,492)
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Assets	116,076	105,708
Derivative Liabilities	(107,008)	(96,960)
Not Designated as Hedging Instrument [Member] | Heating Oil-Diesel Purchases [Member]
Derivative [Line Items]
Derivative Assets	1,017	300
Not Designated as Hedging Instrument [Member] | Coal Held for Trading Purposes [Member]
Derivative [Line Items]
Derivative Assets	110,653	96,898
Derivative Liabilities	(104,814)	(93,272)
Not Designated as Hedging Instrument [Member] | Coal [Member]
Derivative [Line Items]
Derivative Assets	3,912	8,510
Derivative Liabilities	(1,947)	(3,688)
Not Designated as Hedging Instrument [Member] | Natural Gas [Member]
Derivative [Line Items]
Derivative Assets	494	0
Derivative Liabilities	$ (247)	$ 0