Losses from disposed operations resulting from Patriot Coal bankruptcy (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Losses from disposed operations resulting from Patriot Coal bankruptcy	$ 0	$ 149,314	$ 0	$ 149,314
Losses from disposed operations resulting from Patriot Coal bankruptcy			$ 0	$ 149,314
Patriot Coal [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Losses from disposed operations resulting from Patriot Coal bankruptcy		$ 149,300