Retirement Plans - Defined Benefit Plans (Details) - Defined Benefit Plans - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Components of net periodic benefit cost
Service cost	$ 0.6	$ 0.9	$ 1.3	$ 1.7
Interest cost	1.3	1.4	2.6	2.7
Expected return on plan assets	(2.6)	(2.6)	(5.1)	(5.1)
Amortization of unrecognized loss	0.7	0.6	1.3	1.2
Net periodic benefit cost	$ 0.0	$ 0.3	$ 0.1	$ 0.5