Special Charges - Table of Special Charge Liabilities (Details) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($)
Restructuring Reserve [Roll Forward]
Beginning Balance	$ 20.5
Charged to expense	4.1
Paid or otherwise settled	(6.2)
Ending Balance	18.4
ETG | Severance and Other Costs
Restructuring Reserve [Roll Forward]
Beginning Balance	0.0
Charged to expense	0.3
Paid or otherwise settled	(0.3)
Ending Balance	0.0
ETG | Lease Liabilities And Other Costs
Restructuring Reserve [Roll Forward]
Beginning Balance	1.2
Charged to expense	0.0
Paid or otherwise settled	0.0
Ending Balance	1.2
NATG | Lease Liabilities and Other Exit Costs
Restructuring Reserve [Roll Forward]
Beginning Balance	19.3
Charged to expense	3.8
Paid or otherwise settled	(5.9)
Ending Balance	$ 17.2