Pension and Postretirement Benefits - Changes in Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Feb. 07, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Feb. 08, 2019
Change in Plan Assets:
Fair value of plan assets at the beginning of the period		$ 1,572.9
Fair value of plan assets at the end of the period		1,604.2	$ 1,572.9
Amounts recorded in the consolidated balance sheets:
Prepaid pension costs		4.3	9.9
Long-term pension and postretirement benefits		(293.5)	(206.6)
Pension Plans
Change in Benefit Obligation:
Benefit obligation at the beginning of the period	$ (1,897.2)	(1,762.4)	(1,897.2)
Take-Private Transaction assumed benefit obligation	0.0	0.0	(1,790.8)
Service cost	(0.3)	(1.7)	(1.4)	$ (3.7)
Interest cost	(6.8)	(42.2)	(46.7)	(57.2)
Benefits paid	198.9	86.6	187.6
Plan amendment	0.0	0.0	0.0
Settlement	0.0	7.5	0.0
Plan participant contributions	0.0	(0.1)	(0.2)
Actuarial (loss) gain	(80.5)	(162.5)	(111.2)
Effect of changes in foreign currency exchange rates	(4.9)	(11.1)	0.3
Benefit obligation at the end of the period	(1,790.8)	(1,885.9)	(1,762.4)	(1,897.2)
Change in Plan Assets:
Fair value of plan assets at the beginning of the period	1,413.1	1,572.9	1,413.1
Take-Private Transaction acquired plan assets	0.0	0.0	1,477.3
Actual return on plan assets	67.3	109.6	169.5
Employer contributions	191.0	5.1	113.5
Plan participant contributions	0.0	0.1	0.2
Benefits paid	(198.9)	(86.6)	(187.6)
Settlement	0.0	(7.5)	0.0
Effect of changes in foreign currency exchange rates	4.8	10.5	0.0
Fair value of plan assets at the end of the period	1,477.3	1,604.1	1,572.9	1,413.1
Net funded status of plan	(313.5)	(281.8)	(189.5)
Amounts recorded in the consolidated balance sheets:
Prepaid pension costs		4.3	9.9
Short-term pension and postretirement benefits		(0.4)	(0.3)
Long-term pension and postretirement benefits		(285.7)	(199.1)
Net amount recognized		(281.8)	(189.5)
Accumulated benefit obligation		1,878.7	1,755.8
Amount recognized in accumulated other comprehensive loss consists of:
Actuarial loss (gain)		164.2	24.3
Prior service cost (credit)		0.5	0.0
Total amount recognized - pretax		164.7	24.3		$ 0.0
Postretirement Benefit Obligations
Change in Benefit Obligation:
Benefit obligation at the beginning of the period	(5.3)	(2.0)	(5.3)
Take-Private Transaction assumed benefit obligation	0.0	0.0	(5.6)
Service cost	0.0	0.0	0.0	(0.5)
Interest cost	0.0	0.0	(0.1)	(0.3)
Benefits paid	0.1	0.8	0.7
Plan amendment	0.0	0.0	3.0
Settlement	0.0	0.0	0.0
Plan participant contributions	0.0	(0.1)	(0.1)
Actuarial (loss) gain	(0.4)	(0.3)	0.1
Effect of changes in foreign currency exchange rates	0.0	0.0	0.0
Benefit obligation at the end of the period	(5.6)	(1.6)	(2.0)	(5.3)
Change in Plan Assets:
Fair value of plan assets at the beginning of the period	0.0	0.0	0.0
Take-Private Transaction acquired plan assets	0.0	0.0	0.0
Actual return on plan assets	0.0	0.0	0.0
Employer contributions	0.1	0.7	0.6
Plan participant contributions	0.0	0.1	0.1
Benefits paid	(0.1)	(0.8)	(0.7)
Settlement	0.0	0.0	0.0
Effect of changes in foreign currency exchange rates	0.0	0.0	0.0
Fair value of plan assets at the end of the period	0.0	0.0	0.0	$ 0.0
Net funded status of plan	$ (5.6)	(1.6)	(2.0)
Amounts recorded in the consolidated balance sheets:
Prepaid pension costs		0.0	0.0
Short-term pension and postretirement benefits		(0.2)	(0.4)
Long-term pension and postretirement benefits		(1.4)	(1.6)
Net amount recognized		(1.6)	(2.0)
Amount recognized in accumulated other comprehensive loss consists of:
Actuarial loss (gain)		0.2	(0.2)
Prior service cost (credit)		(2.6)	(3.1)
Total amount recognized - pretax		$ (2.4)	$ (3.3)