Pensions and Postretirement Benefits (Summary of Change in Defined Benefit and Postretirement Benefit Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in plan assets
Fair value of plan assets at beginning of year	$ 144.4
Fair value of plan assets at end of year	161.3	$ 144.4
Funded (underfunded) status of the plans
Pension assets	74.8	65.0
Pension Benefits
Change in benefit obligation
Benefit obligation at beginning of year	79.4	71.2
Service costs	4.1	3.8	$ 3.7
Interest costs	2.1	2.6	2.2
Actuarial losses	6.6	7.4
Benefits and expenses paid	(5.7)	(5.6)
Benefit obligation at end of year	86.5	79.4	71.2
Change in plan assets
Fair value of plan assets at beginning of year	144.4	128.2
Actual return on plan assets	23.2	22.7
Company contributions	0.0	0.0
Cash transfer to fund postretirement benefit payments	(0.6)	(0.9)
Benefits and expenses paid	(5.7)	(5.6)
Fair value of plan assets at end of year	161.3	144.4	128.2
Funded (underfunded) status of the plans	74.8	65.0
Funded (underfunded) status of the plans
Pension assets	74.8	65.0
Other current liabilities	0.0	0.0
Other long-term liabilities	0.0	0.0
Asset, defined benefit plan	74.8	65.0
Amounts recognized in Accumulated other comprehensive loss
Net actuarial loss	21.3	28.1
Net prior service cost	0.2	0.2
Accumulated other comprehensive loss	21.5	28.3
Postretirement Benefits
Change in benefit obligation
Benefit obligation at beginning of year	8.0	8.5
Service costs	0.0	0.0	0.0
Interest costs	0.2	0.3	0.3
Actuarial losses	0.6	0.5
Benefits and expenses paid	(0.9)	(1.3)
Benefit obligation at end of year	7.9	8.0	8.5
Change in plan assets
Fair value of plan assets at beginning of year	0.0	0.0
Actual return on plan assets	0.0	0.0
Company contributions	0.9	1.3
Cash transfer to fund postretirement benefit payments	0.0	0.0
Benefits and expenses paid	(0.9)	(1.3)
Fair value of plan assets at end of year	0.0	0.0	$ 0.0
Funded (underfunded) status of the plans	(7.9)	(8.0)
Funded (underfunded) status of the plans
Other current liabilities	0.8	0.9
Other long-term liabilities	7.1	7.1
Liability, defined benefit plan	7.9	8.0
Amounts recognized in Accumulated other comprehensive loss
Net actuarial loss	3.0	2.6
Net prior service cost	0.0	0.0
Accumulated other comprehensive loss	$ 3.0	$ 2.6