Pension and Retirement Plans - Net Periodic Benefit Costs (Details) - Pension Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of net periodic benefit (income) cost:
Interest cost	$ 0.5	$ 0.6	$ 0.6
Expected return on plan assets	(1.0)	(0.8)	(0.8)
Amortization of actuarial loss	0.4	0.5	0.5
Net periodic benefit (income) cost	(0.1)	0.3	0.3
Other changes in plan assets and benefit obligations recognized in other comprehensive income (loss):
Net actuarial loss (gain) for the year	0.4	(0.3)	0.7
Amortization of net loss	(0.4)	(0.5)	(0.5)
Total recognized in other comprehensive income (loss)	0.0	(0.8)	0.2
Total recognized in net periodic benefit cost and other comprehensive income (loss)	$ (0.1)	$ (0.5)	$ 0.5
Weighted average assumptions used to determine net periodic benefit cost for years ended December 31:
Discount rate	3.10%	4.10%	3.50%
Expected return on plan assets	6.00%	6.00%	6.25%