Employee Retirement Plans (Pension Plans) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net pension income	$ 3.1	$ 2.6	$ 9.3	$ 7.8	$ 9.6	$ 5.1
Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Interest cost	(4.0)	(4.3)	(12.0)	(12.9)
Expected return on plan assets	7.1	6.9	21.3	20.7
Net pension income	$ 3.1	$ 2.6	9.3	$ 7.8
Pension Plans | Non-qualified
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Benefit payments on non-qualified pension plans			0.8
Pension Plans | Qualified
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Contributions on qualified pension plans			$ 6.0