Derivative Financial Instruments (Details) bbl in Thousands, T in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) T bbl $ / bbl $ / T	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Derivative [Line Items]
Cost of new call options	$ 5,813,000	$ 3,950,000	$ 0
Net asset position	0	17,111,000
Offsetting and Balance Sheet Presentation
Gross Amounts of Recognized Assets	7,462,000	20,861,000
Gross Amounts of Recognized Liabilities	(18,196,000)	(7,357,000)
Gross Amounts Offset in the Consolidated Balance Sheet Assets	(7,462,000)	(3,749,000)
Gross Amounts offset in the Consolidated Balance Sheets Liabilities	7,462,000	3,749,000
Net Amounts Presented in the Consolidated Balance Sheets Assets	0	17,111,000
Cash Collateral Requirement		0
(Gain) loss on derivative financial instruments	$ 30,635,000	(7,805,000)	(25,611,000)
Underlying basis Barrels
Derivative Gains and Losses
Derivative, Average Forward Price | $ / bbl	37.04
International Coal Forward Contracts
Derivative [Line Items]
Net asset position	$ 0	20,732,000
Offsetting and Balance Sheet Presentation
Gross Amounts of Recognized Assets	7,462,000	20,861,000
Gross Amounts of Recognized Liabilities	(398,000)	(129,000)
Gross Amounts Offset in the Consolidated Balance Sheet Assets	(7,462,000)	(129,000)
Gross Amounts offset in the Consolidated Balance Sheets Liabilities	7,462,000	129,000
Net Amounts Presented in the Consolidated Balance Sheets Assets	0	20,732,000
Net Amounts Presented in the Consolidated Balance Sheet Liabilities	7,064,000	0
(Gain) loss on derivative financial instruments	$ 4,382,000	(21,369,000)	(25,952,000)
International Coal Forward Contracts | 2016
Derivative [Line Items]
Notional amount (tons) | T	265
Notional amount (barrels) | T	265
Net asset position	$ 7,064,000
Offsetting and Balance Sheet Presentation
Net Amounts Presented in the Consolidated Balance Sheets Assets	$ 7,064,000
Derivative Gains and Losses
Derivative, Average Forward Price | $ / T	100.13
International coal put options
Offsetting and Balance Sheet Presentation
(Gain) loss on derivative financial instruments	$ (5,813,000)	0	0
Domestic Coal Futures Contracts
Offsetting and Balance Sheet Presentation
(Gain) loss on derivative financial instruments	$ 4,340,000	1,701,000	260,000
Domestic Coal Futures Contracts | 2016
Derivative [Line Items]
Notional amount (tons) | T	120
Notional amount (barrels) | T	120
Derivative Gains and Losses
Derivative, Average Forward Price | $ / T	14.7
WTI derivative financial instruments
Derivative [Line Items]
Net asset position	$ 0	(3,620,000)
Offsetting and Balance Sheet Presentation
Gross Amounts of Recognized Assets	0	0
Gross Amounts of Recognized Liabilities	(17,798,000)	(7,228,000)
Gross Amounts Offset in the Consolidated Balance Sheet Assets	0	(3,620,000)
Gross Amounts offset in the Consolidated Balance Sheets Liabilities	0	3,620,000
Net Amounts Presented in the Consolidated Balance Sheets Assets	0	(3,620,000)
Net Amounts Presented in the Consolidated Balance Sheet Liabilities	(17,798,000)	(3,608,000)
(Gain) loss on derivative financial instruments	$ 20,229,000	11,863,000	81,000
WTI derivative financial instruments | 2016 expected consumption
Derivative [Line Items]
Estimated Consumption Hedged	50.00%
WTI derivative financial instruments | 2017 expected consumption
Derivative [Line Items]
Estimated Consumption Hedged	100.00%
WTI derivative financial instruments | Underlying basis Barrels
Derivative [Line Items]
Notional amount (tons) - floor | bbl	342
Weighted-Average Floor price | $ / bbl	53.94
Weighted-Average Ceiling price | $ / bbl	72.88
Weighted-Average per Barrel | $ / bbl	57.93
Notional amount (barrels) - floor | bbl	342
Notional amount (barrels) - ceiling | bbl	342
Notional amount (barrels) - swaps | bbl	978
WTI derivative financial instruments | Underlying basis Barrels | 2016 expected consumption | Swap positions
Derivative [Line Items]
Weighted-Average per Barrel | $ / bbl	63.39
Notional amount (barrels) - swaps | bbl	342
WTI derivative financial instruments | Underlying basis Barrels | 2016 expected consumption | Collar positions
Derivative [Line Items]
Notional amount (tons) - floor | bbl	342
Weighted-Average Floor price | $ / bbl	53.94
Weighted-Average Ceiling price | $ / bbl	72.88
Notional amount (barrels) - floor | bbl	342
Notional amount (barrels) - ceiling | bbl	342
WTI derivative financial instruments | Underlying basis Barrels | 2017 expected consumption | Swap positions
Derivative [Line Items]
Weighted-Average per Barrel | $ / bbl	55
Notional amount (barrels) - swaps | bbl	636
US On-Highway Diesel derivative financial instruments
Offsetting and Balance Sheet Presentation
(Gain) loss on derivative financial instruments	$ 7,496,000	$ 0	$ 0