Pension and Postretirement Benefits - Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Details) - USD ($) $ in Millions	1 Months Ended	11 Months Ended	12 Months Ended
	Feb. 07, 2019	Dec. 31, 2019	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Other changes in plan assets and benefit obligations recognized in other comprehensive income (loss)
Net actuarial gain (loss), tax expense (benefit)		$ 8.1	$ (38.3)	$ 32.2
Prior service credit (cost), tax benefit (expense)		$ (0.8)	(0.1)	0.1	$ (0.8)
Amortization of actuarial (loss) gain, tax expense	$ (22.2)		0.6
Amortization of prior service (cost) credit, tax expense			(0.1)	(0.1)
Pension plans
Other changes in plan assets and benefit obligations recognized in other comprehensive income (loss)
Actuarial (loss) gain arising during the year, before tax benefit (expense)	0.0		145.1	(127.3)	(34.6)
Prior service credit (cost) arising during the year, before tax benefit (expense)	0.0		0.3	(0.5)	0.0
Amortization of actuarial (loss) gain, before tax benefit (expense)	(87.7)		(2.3)	0.0	0.0
Amortization of prior service (cost) credit, before tax benefit (expense)	0.0		0.0	0.0	0.0
Postretirement benefit obligations
Other changes in plan assets and benefit obligations recognized in other comprehensive income (loss)
Actuarial (loss) gain arising during the year, before tax benefit (expense)	0.0		0.1	(0.4)	0.2
Prior service credit (cost) arising during the year, before tax benefit (expense)	0.0		0.0	(0.1)	3.1
Amortization of actuarial (loss) gain, before tax benefit (expense)	0.1		0.0	0.0	0.0
Amortization of prior service (cost) credit, before tax benefit (expense)	$ 0.1		$ 0.4	$ 0.4	$ 0.0