Losses from disposed operations resulting from Patriot Coal Bankruptcy (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
	Nov. 22, 2016	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Losses related to previously disposed off operations		$ 0	$ (21,297)
Discontinued Operations, Held-for-sale or Disposed of by Sale | Magnum
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Losses related to previously disposed off operations				$ 116,300
Surety Bond | Magnum
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Decease in collateral held by bonding company	$ 20,000