Losses from disposed operations resulting from Patriot Coal Bankruptcy (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	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			$ (17.8)
Discontinued Operations, Held-for-sale or Disposed of by Sale [Member] | Magnum [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Losses related to previously disposed off operations		$ 149.3	$ 116.3
Adjustment to prior period gain (loss) on disposal	$ 33.0