Employee Retirement Plans - Components of net retirement cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 0.0	$ 0.1	$ 0.0	$ 0.1
Interest	3.7	4.9	7.4	9.8
Expected return on plan assets	(5.2)	(5.8)	(10.4)	(11.5)
Amortization of actuarial loss	1.5	1.1	3.0	2.2
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	0.3	0.0	0.6	0.0
Net periodic benefit cost	0.3	0.3	0.6	0.6
Profit sharing	1.9	3.0	4.6	5.2
Net expense	2.2	3.3	5.2	5.8
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 0.2	$ 0.0	$ 0.5	$ 0.2