Other Transactions (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset retirement obligation charge			$ 2,361	$ 1,562
Severance and other employee benefit costs			600
Assets held for sale		$ 1,319	17,497	1,319
Asset impairment	$ 7,500
Cash in escrow from investment			$ 0	0	$ 5,000
NACoal
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash in escrow from investment				5,000
NACoal | Land
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of property				2,200
NACoal | Assets Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of property				3,500
NACoal | Reed Minerals
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset impairment		$ 105,100
HBB | Weston Products, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Preliminary purchase price				25,400
Cash paid at closing				25,000
Net working capital and EBITDA adjustment				$ 400