Contingent Liability Contingent Liability (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2016	Dec. 31, 2012	Dec. 31, 2016
Loss Contingencies [Line Items]
Environmental Exit Costs, Reasonably Possible Additional Losses, Low Estimate		$ 0.9
Environmental Exit Costs, Liability for Remediation	$ 1.0			$ 0.3
Minimum [Member]
Loss Contingencies [Line Items]
Environmental Exit Costs, Anticipated Cost	0.9		$ 0.3
Maximum [Member]
Loss Contingencies [Line Items]
Environmental Exit Costs, Anticipated Cost	$ 2.0		$ 1.4