BUSINESS SEGMENT INFORMATION - Customer Concentration Disclosures (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Revenue, Major Customer [Line Items]
Revenues	$ 310,004	$ 337,830	$ 287,956	$ 180,202	$ 173,947	$ 176,792	$ 162,499	$ 161,448	$ 1,115,992		$ 674,686		$ 600,437
Total Revenue [Member] | Customer Concentration Risk [Member]
Revenue, Major Customer [Line Items]
Concentration Risk, Percentage									61.00%	[1]	77.00%		80.00%
Customer A [Member] | Coal Canada Segment [Member]
Revenue, Major Customer [Line Items]
Revenues									144,863	[1]	0		0
Customer B [Member] | Coal Segment [Member]
Revenue, Major Customer [Line Items]
Revenues									128,104	[1]	117,545		126,982
Customer C [Member] | Coal Segment [Member]
Revenue, Major Customer [Line Items]
Revenues									101,778	[1]	112,061		96,718
Customer D [Member] | Coal Segment [Member]
Revenue, Major Customer [Line Items]
Revenues									100,234	[1]	89,266		81,981
Customer E [Member] | Power [Member]
Revenue, Major Customer [Line Items]
Revenues									85,254	[1]	86,390		80,109
Customer F [Member] | Coal Segment [Member]
Revenue, Major Customer [Line Items]
Revenues									79,505	[1]	85,929		66,128
Customer G [Member] | Coal Segment [Member]
Revenue, Major Customer [Line Items]
Revenues									$ 44,234	[1],[2]	$ 25,958	[2]	$ 26,525	[2]

[1] The revenue from Customers B, C, D, E, F and G did not exceed 10% in 2014.

[2] The revenue from Customer G did not exceed 10% in 2013 or 2012.