Derivative Financial Instruments (Details) t in Thousands, gal in Thousands, bbl in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) bbl gal t $ / bbl $ / gal	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) bbl gal t $ / bbl $ / gal	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Derivative [Line Items]
Net asset position	$ 13,086		$ 13,086		$ 17,111
Offsetting and Balance Sheet Presentation
Gross Amounts of Recognized Assets	14,742		14,742		20,861
Gross Amounts of Recognized Liabilities	(2,079)		(2,079)		(7,357)
Gross Amounts Offset in the Consolidated Balance Sheet Assets	(1,657)		(1,657)		(3,749)
Gross Amounts offset in the Consolidated Balance Sheets Liabilities	1,657		1,657		3,749
Net Amounts Presented in the Consolidated Balance Sheets Assets	13,086		13,086		17,111
Net Amounts Presented in the Consolidated Balance Sheet Liabilities	(422)		(422)		(3,608)
Derivative financial instruments	2,761	$ (2,803)	7,546	$ (15,537)
International Coal Contracts
Derivative [Line Items]
Net asset position	13,998		13,998		20,732
Offsetting and Balance Sheet Presentation
Gross Amounts of Recognized Assets					20,861
Gross Amounts of Recognized Liabilities					(129)
Gross Amounts Offset in the Consolidated Balance Sheet Assets					(129)
Gross Amounts offset in the Consolidated Balance Sheets Liabilities					129
Net Amounts Presented in the Consolidated Balance Sheets Assets	$ 13,998		$ 13,998		20,732
Net Amounts Presented in the Consolidated Balance Sheet Liabilities					0
International Coal Contracts | Put Option
Derivative [Line Items]
Notional amount (tons) | t	2,480		2,480
Net asset position	$ 744		$ 744
Notional amount (gallons) | t	2,480		2,480
Offsetting and Balance Sheet Presentation
Gross Amounts of Recognized Assets	$ 744		$ 744
Gross Amounts of Recognized Liabilities	0		0
Gross Amounts Offset in the Consolidated Balance Sheet Assets	0		0
Gross Amounts offset in the Consolidated Balance Sheets Liabilities	0		0
Net Amounts Presented in the Consolidated Balance Sheets Assets	744		744
Net Amounts Presented in the Consolidated Balance Sheet Liabilities	0		0
Derivative financial instruments	$ 5,053		$ 5,053
International Coal Contracts | Forward Contracts
Derivative [Line Items]
Notional amount (tons) | t	390		390
Net asset position	$ 13,998		$ 13,998
Notional amount (gallons) | t	390		390
Offsetting and Balance Sheet Presentation
Gross Amounts of Recognized Assets	$ 13,998		$ 13,998
Gross Amounts of Recognized Liabilities	0		0
Gross Amounts Offset in the Consolidated Balance Sheet Assets	0		0
Gross Amounts offset in the Consolidated Balance Sheets Liabilities	0		0
Net Amounts Presented in the Consolidated Balance Sheets Assets	13,998		13,998
Net Amounts Presented in the Consolidated Balance Sheet Liabilities	0		0
Derivative financial instruments	$ 1,263	(3,490)	$ (705)	(14,082)
International Coal Contracts | 2015 | Put Option
Derivative [Line Items]
Notional amount (tons) | t	0		0
Net asset position	$ 0		$ 0
Notional amount (gallons) | t	0		0
Offsetting and Balance Sheet Presentation
Net Amounts Presented in the Consolidated Balance Sheets Assets	$ 0		$ 0
International Coal Contracts | 2015 | Forward Contracts
Derivative [Line Items]
Notional amount (tons) | t	258		258
Net asset position	$ 7,870		$ 7,870
Notional amount (gallons) | t	258		258
Offsetting and Balance Sheet Presentation
Net Amounts Presented in the Consolidated Balance Sheets Assets	$ 7,870		$ 7,870
International Coal Contracts | 2016 | Put Option
Derivative [Line Items]
Notional amount (tons) | t	2,480		2,480
Net asset position	$ 744		$ 744
Notional amount (gallons) | t	2,480		2,480
Offsetting and Balance Sheet Presentation
Net Amounts Presented in the Consolidated Balance Sheets Assets	$ 744		$ 744
International Coal Contracts | 2016 | Forward Contracts
Derivative [Line Items]
Notional amount (tons) | t	132		132
Net asset position	$ 6,128		$ 6,128
Notional amount (gallons) | t	132		132
Offsetting and Balance Sheet Presentation
Net Amounts Presented in the Consolidated Balance Sheets Assets	$ 6,128		$ 6,128
WTI derivative financial instruments
Derivative [Line Items]
Net asset position	$ (492)		$ (492)		(3,620)
Weighted-Average Floor price | $ / bbl	60.02		60.02
Weighted-Average Ceiling price | $ / bbl	74.86		74.86
Weighted-Average per Barrel | $ / bbl	59.31		59.31
Derivative Contracts Notional Amount Swap Positions | bbl	556		556
Notional amount (barrels) - floor | bbl	474		474
Notional amount (barrels) - ceiling | bbl	474		474
Weighted-Average per Gallon | $ / bbl	59.31		59.31
Offsetting and Balance Sheet Presentation
Gross Amounts of Recognized Assets	$ 0		$ 0		0
Gross Amounts of Recognized Liabilities	(914)		(914)		(7,228)
Gross Amounts Offset in the Consolidated Balance Sheet Assets	(492)		(492)		(3,620)
Gross Amounts offset in the Consolidated Balance Sheets Liabilities	492		492		3,620
Net Amounts Presented in the Consolidated Balance Sheets Assets	(492)		(492)		(3,620)
Net Amounts Presented in the Consolidated Balance Sheet Liabilities	(422)		(422)		$ (3,608)
Derivative financial instruments	$ (5,480)	(157)	$ (2,625)	(466)
WTI derivative financial instruments | Purchases Q3 2015 Hedged [Member]
Derivative [Line Items]
Estimated Consumption Hedged	100.00%		100.00%
WTI derivative financial instruments | Purchases Q3 2015 Hedged [Member] | Swap positions
Derivative [Line Items]
Estimated Consumption Hedged	25.00%		25.00%
WTI derivative financial instruments | Purchases Q3 2015 Hedged [Member] | Collar positions
Derivative [Line Items]
Estimated Consumption Hedged	75.00%		75.00%
WTI derivative financial instruments | Purchases Q4 2015 Hedged [Member]
Derivative [Line Items]
Estimated Consumption Hedged	100.00%		100.00%
WTI derivative financial instruments | Purchases Q4 2015 Hedged [Member] | Swap positions
Derivative [Line Items]
Estimated Consumption Hedged	100.00%		100.00%
WTI derivative financial instruments | Purchases 2016 Hedged [Member]
Derivative [Line Items]
Estimated Consumption Hedged	50.00%		50.00%
WTI derivative financial instruments | 2015 | Purchases Q3 2015 Hedged [Member] | Collar positions
Derivative [Line Items]
Weighted-Average Floor price | $ / bbl	75.75		75.75
Weighted-Average Ceiling price | $ / bbl	80		80
Notional amount (barrels) - floor | bbl	132		132
Notional amount (barrels) - ceiling | bbl	132		132
WTI derivative financial instruments | 2015 | Purchases Q4 2015 Hedged [Member] | Swap positions
Derivative [Line Items]
Weighted-Average per Barrel | $ / bbl	52.8		52.8
Derivative Contracts Notional Amount Swap Positions | bbl	214		214
Weighted-Average per Gallon | $ / bbl	52.8		52.8
WTI derivative financial instruments | 2016
Derivative [Line Items]
Estimated Consumption Hedged	50.00%		50.00%
WTI derivative financial instruments | 2016 | Purchases 2016 Hedged [Member]
Derivative [Line Items]
Estimated Consumption Hedged	50.00%		50.00%
WTI derivative financial instruments | 2016 | Purchases 2016 Hedged [Member] | Swap positions
Derivative [Line Items]
Weighted-Average per Barrel | $ / bbl	63.39		63.39
Derivative Contracts Notional Amount Swap Positions | bbl	342		342
Weighted-Average per Gallon | $ / bbl	63.39		63.39
WTI derivative financial instruments | 2016 | Purchases 2016 Hedged [Member] | Collar positions
Derivative [Line Items]
Weighted-Average Floor price | $ / bbl	53.94		53.94
Weighted-Average Ceiling price | $ / bbl	72.88		72.88
Notional amount (barrels) - floor | bbl	342		342
Notional amount (barrels) - ceiling | bbl	342		342
Domestic Coal Futures Contracts
Derivative [Line Items]
Notional amount (tons) | t	1,020		1,020
Notional amount (gallons) | t	1,020		1,020
Offsetting and Balance Sheet Presentation
Derivative financial instruments	$ 761	$ 845	$ 4,658	(989)
Domestic Coal Futures Contracts | Future Contracts
Derivative [Line Items]
Notional amount (tons) | t	1,020		1,020
Notional amount (gallons) | t	1,020		1,020
Domestic Coal Futures Contracts | 2015
Derivative [Line Items]
Notional amount (tons) | t	900		900
Notional amount (gallons) | t	900		900
Domestic Coal Futures Contracts | 2015 | Future Contracts
Derivative [Line Items]
Notional amount (tons) | t	900		900
Notional amount (gallons) | t	900		900
Domestic Coal Futures Contracts | 2016
Derivative [Line Items]
Notional amount (tons) | t	120		120
Notional amount (gallons) | t	120		120
Domestic Coal Futures Contracts | 2016 | Future Contracts
Derivative [Line Items]
Notional amount (tons) | t	120		120
Notional amount (gallons) | t	120		120
U.S. on-highway diesel contracts
Derivative [Line Items]
Net asset position	$ (1,165)		$ (1,165)
Offsetting and Balance Sheet Presentation
Gross Amounts of Recognized Assets	0		0
Gross Amounts of Recognized Liabilities	(1,165)		(1,165)
Gross Amounts Offset in the Consolidated Balance Sheet Assets	(1,165)		(1,165)
Gross Amounts offset in the Consolidated Balance Sheets Liabilities	1,165		1,165
Net Amounts Presented in the Consolidated Balance Sheets Assets	(1,165)		(1,165)
Net Amounts Presented in the Consolidated Balance Sheet Liabilities	0		$ 0
Derivative financial instruments	$ 1,165			$ 1,165
U.S. on-highway diesel contracts | Swap positions
Derivative [Line Items]
Notional amount (tons) | gal	11,700		11,700
Weighted-Average per Barrel | $ / gal	3.14		3.14
Notional amount (gallons) | gal	11,700		11,700
Weighted-Average per Gallon | $ / gal	3.14		3.14
U.S. on-highway diesel contracts | 2015 | Swap positions
Derivative [Line Items]
Notional amount (tons) | gal	5,300		5,300
Weighted-Average per Barrel | $ / gal	3.1		3.1
Notional amount (gallons) | gal	5,300		5,300
Weighted-Average per Gallon | $ / gal	3.1		3.1
U.S. on-highway diesel contracts | 2016 | Swap positions
Derivative [Line Items]
Notional amount (tons) | gal	6,400		6,400
Weighted-Average per Barrel | $ / gal	3.18		3.18
Notional amount (gallons) | gal	6,400		6,400
Weighted-Average per Gallon | $ / gal	3.18		3.18