Dispositions and Special Charges - Special Charges Liabilities (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
ETG – Lease liabilities and other costs | Lease Liabilities And Other Costs
Restructuring Reserve [Roll Forward]
Beginning Balance	$ 1.2
Charged to expense (income)	0.0
Paid or otherwise settled	0.0
Ending Balance	1.2
NATG – Lease liabilities and other exit costs | Lease Liabilities and Other Exit Costs
Restructuring Reserve [Roll Forward]
Beginning Balance	19.0
Charged to expense (income)	(0.2)
Paid or otherwise settled	(5.1)
Ending Balance	13.7
Total
Restructuring Reserve [Roll Forward]
Beginning Balance	20.2
Charged to expense (income)	(0.2)
Paid or otherwise settled	(5.1)
Ending Balance	$ 14.9