Quarterly Financial Data Items affecting quarter results (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016
After-tax restructuring and related costs	$ 0.9	$ 2.2	$ 2.2	$ 3.6	$ 1.4	$ 0.1	$ 4.0	$ 0.8
After-tax corporate development and related costs	0.7	1.4	2.8	$ 3.2	15.9	3.0	1.8	0.3
After-tax gain on sale of investment			$ 1.0
Unrecognized Tax Benefits, Period Increase (Decrease)		$ (3.3)
After tax reduction to other expense	8.9
After tax U.K. coal restructuring and related costs	1.2
After-tax write off of debt issuance costs					0.5
Australian Operations [Member]
After tax asset impairment and related charges	1.8							16.8
Internal Revenue Service (IRS) [Member]
Tax Cuts And Jobs Act of 2017 Incomplete Accounting Provisional Income Tax Expense (Benefit)	$ (371.9)
Tax benefit associated with the United States Short Line Tax Credit					7.5	7.8	$ 7.2	$ 6.3
Her Majesty's Revenue and Customs (HMRC) [Member]
Income tax (benefit) associated with reduction in U.K. income tax rate						$ (4.3)
U.K Coal [Member] | U.K./European Operations [Member]
After tax asset impairment and related charges					8.6
ERS Railways [Member] | U.K./European Operations [Member]
After tax asset impairment and related charges					$ 21.5