Derivatives (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Risk management derivatives
Net asset position	$ 37,252		$ 37,252		$ 13,785
Offsetting and Balance Sheet Presentation
Gross Amounts of Recognized Assets	38,750		38,750		13,815
Gross Amounts of Recognized Liabilities	1,558		1,558		30
Gross Amounts Offset in the Consolidated Balance Sheet Assets	(1,498)		(1,498)		(30)
Gross Amounts offset in the Consolidated Balance Sheets Liabilities	(1,498)		(1,498)		(30)
Net Amounts Presented in the Consolidated Balance Sheets Assets	37,252		37,252		13,785
Net Amounts Presented in the Consolidated Balance Sheet Liabilities	60		60
Cash Collateral Requirement	0		0		0
Derivative Gains and Losses
Derivative mark-to-market (gains) and losses	(12,284)	(20,183)	(25,936)	(18,127)
International Coal Forward Contracts
Risk management derivatives
Net asset position	37,185		37,185		13,647
Offsetting and Balance Sheet Presentation
Gross Amounts of Recognized Assets	38,677		38,677		13,677
Gross Amounts of Recognized Liabilities	1,492		1,492		30
Gross Amounts Offset in the Consolidated Balance Sheet Assets	(1,492)		(1,492)		(30)
Gross Amounts offset in the Consolidated Balance Sheets Liabilities	(1,492)		(1,492)		(30)
Net Amounts Presented in the Consolidated Balance Sheets Assets	37,185		37,185		13,647
Derivative Gains and Losses
Derivative mark-to-market (gains) and losses	(12,402)	(20,119)	(26,184)	(18,063)
International Coal Forward Contracts | 2013
Risk management derivatives
Net asset position	10,981		10,981
Offsetting and Balance Sheet Presentation
Net Amounts Presented in the Consolidated Balance Sheets Assets	10,981		10,981
International Coal Forward Contracts | 2014
Risk management derivatives
Net asset position	17,350		17,350
Offsetting and Balance Sheet Presentation
Net Amounts Presented in the Consolidated Balance Sheets Assets	17,350		17,350
International Coal Forward Contracts | 2015
Risk management derivatives
Net asset position	6,583		6,583
Offsetting and Balance Sheet Presentation
Net Amounts Presented in the Consolidated Balance Sheets Assets	6,583		6,583
International Coal Forward Contracts | 2016
Risk management derivatives
Net asset position	2,272		2,272
Offsetting and Balance Sheet Presentation
Net Amounts Presented in the Consolidated Balance Sheets Assets	2,272		2,272
International Coal Forward Contracts | Underlying basis Tons
Risk management derivatives
Notional amount (tons)	2,224,000		2,224,000
International Coal Forward Contracts | Underlying basis Tons | 2013
Risk management derivatives
Notional amount (tons)	558,000		558,000
International Coal Forward Contracts | Underlying basis Tons | 2014
Risk management derivatives
Notional amount (tons)	1,190,000		1,190,000
International Coal Forward Contracts | Underlying basis Tons | 2015
Risk management derivatives
Notional amount (tons)	344,000		344,000
International Coal Forward Contracts | Underlying basis Tons | 2016
Risk management derivatives
Notional amount (tons)	132,000		132,000
WTI Collars
Risk management derivatives
Net asset position	66		66		138
Offsetting and Balance Sheet Presentation
Gross Amounts of Recognized Assets	73		73		138
Gross Amounts of Recognized Liabilities	67		67
Gross Amounts Offset in the Consolidated Balance Sheet Assets	(7)		(7)
Gross Amounts offset in the Consolidated Balance Sheets Liabilities	(7)		(7)
Net Amounts Presented in the Consolidated Balance Sheets Assets	66		66		138
Net Amounts Presented in the Consolidated Balance Sheet Liabilities	60		60
Derivative Gains and Losses
Derivative mark-to-market (gains) and losses	25	(64)	132	(64)
WTI Collars | Underlying basis Barrels
Risk management derivatives
Notional amount (tons)	552,000		552,000
Weighted-Average Floor price	71.12		71.12
Weighted-Average Ceiling price	111.62		111.62
WTI Collars | Underlying basis Barrels | 2013
Risk management derivatives
Notional amount (tons)	264,000		264,000
Weighted-Average Floor price	71.97		71.97
Weighted-Average Ceiling price	112.01		112.01
WTI Collars | Underlying basis Barrels | 2014
Risk management derivatives
Notional amount (tons)	288,000		288,000
Weighted-Average Floor price	70.34		70.34
Weighted-Average Ceiling price	111.26		111.26
Domestic Coal Futures Contracts
Derivative Gains and Losses
Derivative mark-to-market (gains) and losses	$ 93		$ 117
Domestic Coal Futures Contracts | Underlying basis Tons
Risk management derivatives
Notional amount (tons)	1,260,000		1,260,000
Domestic Coal Futures Contracts | Underlying basis Tons | 2014
Risk management derivatives
Notional amount (tons)	1,080,000		1,080,000
Domestic Coal Futures Contracts | Underlying basis Tons | 2015
Risk management derivatives
Notional amount (tons)	180,000		180,000