Pension and Other Benefits Amounts Recognized in Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Settlement charge	$ 31.3
Pension Plan
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Settlement charge			$ (31.3)
Other comprehensive income (loss), defined benefit plan, gain (loss) arising during period, before tax		$ 4.5	8.5	$ (7.8)
Amortization of net actuarial loss		(2.0)	(2.6)	(4.3)
Other comprehensive (income) loss, defined benefit plan, prior service cost (credit), reclassification adjustment from AOCI, before tax		(0.1)	(0.1)	(0.1)
Other comprehensive (income) loss, defined benefit plan, after reclassification adjustment, before tax		2.4	(25.5)	(12.2)
Defined benefit plan, net periodic benefit cost (credit)		4.4	36.7	5.7
Defined benefit plan, amount recognized in net periodic benefit cost (credit) and other comprehensive (income) loss, before tax		6.8	11.2	(6.5)
Other Postretirement Benefits Plan
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Settlement charge			0.0
Other comprehensive income (loss), defined benefit plan, gain (loss) arising during period, before tax		(0.1)	0.1	(0.1)
Amortization of net actuarial loss		(0.1)	(0.1)	(0.1)
Other comprehensive (income) loss, defined benefit plan, prior service cost (credit), reclassification adjustment from AOCI, before tax		0.0	0.0	0.0
Other comprehensive (income) loss, defined benefit plan, after reclassification adjustment, before tax		(0.2)	0.0	(0.2)
Defined benefit plan, net periodic benefit cost (credit)		0.1	0.1	0.1
Defined benefit plan, amount recognized in net periodic benefit cost (credit) and other comprehensive (income) loss, before tax		$ (0.1)	$ 0.1	$ (0.1)