Segment Information (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
Segment information
Consolidated revenue	$ 329,996	$ 343,183	$ 668,048	$ 716,086
Adjusted EBITDA	37,277	65,632	85,529	141,375
Interest expense, net	(10,252)	(7,624)	(20,611)	(13,028)
Depreciation, depletion and accretion	(29,585)	(25,707)	(56,924)	(51,745)
Income tax expense	(2,462)	(18,806)	(11,297)	(33,908)
Derivative financial instruments
Exclusion of fair value mark-to-market gains	12,284	20,183	25,936	18,127
Inclusion of cash amounts received	(2,553)		(2,529)	(524)
Total derivative financial instruments	9,731	20,183	23,407	17,603
Net income	4,709	33,678	20,104	60,296
Total Assets	2,401,689		2,401,689		2,351,323
Capital Expenditures			36,163	21,875
Total revenue from external customers by geographic location
United States	264,568	267,602	545,068	571,630
Asia	62,522	74,491	120,074	143,153
Other	2,906	1,090	2,906	1,303
Total revenue from external customers	329,996	343,183	668,048	716,086
Owned and Operated Mines
Segment information
Consolidated revenue	264,975	265,462	543,749	568,034
Adjusted EBITDA	34,230	51,564	79,892	118,804
Derivative financial instruments
Total Assets	1,793,508		1,793,508		1,826,165
Capital Expenditures			34,325	18,090
Logistics and Related Activities
Segment information
Consolidated revenue	67,069	83,491	132,939	163,045
Adjusted EBITDA	2,752	12,481	4,117	22,850
Derivative financial instruments
Total Assets	71,943		71,943		46,426
Capital Expenditures			337
Corporate and Other
Segment information
Consolidated revenue	11,811	10,769	17,640	16,466
Adjusted EBITDA	252	1,629	1,743	(98)
Derivative financial instruments
Total Assets	536,357		536,357		478,536
Capital Expenditures			1,501	3,785
Eliminations of intersegment sales
Segment information
Consolidated revenue	(13,860)	(16,539)	(26,280)	(31,459)
Adjusted EBITDA	44	(41)	(224)	(181)
Derivative financial instruments
Total Assets	$ (119)		$ (119)		$ 196