Employee Retirement Plans - Components of net retirement cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Interest	0.1	3.8	0.3	11.2
Expected return on plan assets	0.0	(5.3)	0.0	(15.7)
Amortization of actuarial loss	0.0	1.5	0.2	4.5
Amortization of prior service cost	0.0	0.3	0.0	0.9
Net periodic benefit cost	0.1	0.3	0.5	0.9
Profit sharing	3.1	1.9	8.3	6.5
Net expense	$ 3.2	$ 2.2	$ 8.8	$ 7.4