Retirement Plans - Defined Benefit Plans (Details) - Defined Benefit Plans - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Components of net periodic benefit cost
Service cost	$ 0.7	$ 0.8	$ 2.0	$ 2.5
Interest cost	1.3	1.3	3.9	4.0
Expected return on plan assets	(2.5)	(2.5)	(7.6)	(7.6)
Amortization of unrecognized loss	0.6	0.6	1.9	1.8
Net periodic benefit cost	$ 0.1	$ 0.2	$ 0.2	$ 0.7