Assets and Liabilities Held-for-Sale (Details) (Disposal Group, Held-for-sale, Not Discontinued Operations, Gauley Eagle, USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Disposal Group, Held-for-sale, Not Discontinued Operations | Gauley Eagle
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Mineral interests, net	$ 1,654	$ 16,464
Property, plant and equipment, net	1,527	20,233
Advance mining royalties	3,368	2,863
Other current assets	6,549	39,560
Asset retirement obligations	4,049	7,365
Other current liabilities	$ 4,049	$ 7,365