Employee Retirement Benefits - Net Periodic Costs (Benefits) (Details) - Pension - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0.0	$ 0.1	$ 0.1
Interest cost	6.1	5.8	5.6
Expected return on plan assets	(6.2)	(8.0)	(8.7)
Net amortization of actuarial net loss	1.4	1.4	0.3
Settlement loss	0.9	0.0	0.2
Net periodic pension cost (benefit)	$ 2.2	$ (0.7)	$ (2.5)
Weighted‑Average Assumptions Used to Determine Net Periodic Pension Cost (Benefit)
Discount rate	4.10%	4.30%	3.90%
Expected return on assets	6.50%	7.20%	7.40%
Average rate of increase in compensation	0.00%	4.30%	4.00%