Contingent Liability Contingent Liability (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2015	Dec. 31, 2012
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