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, 2020	Dec. 31, 2019	Dec. 31, 2018
Other changes in plan assets and benefit obligations recognized in other comprehensive income (loss)
Net actuarial gain (loss), tax benefit (expense)	$ (22.2)	$ 6.1	$ 34.6	$ 6.1	$ 15.7
Prior service credit (cost), tax benefit (expense)		$ (0.8)	0.1	(0.9)	(1.2)
Amortization of actuarial (loss) gain, tax expense	(22.2)				(9.0)
Amortization of prior service (cost) credit, tax expense			(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		(139.9)	(24.3)	(71.7)
Prior service credit (cost) arising during the year, before tax (benefit) expense	0.0		(0.5)	0.0	(4.0)
Amortization of actuarial (loss) gain, before tax (benefit) expense	(87.7)		0.0	0.0	(42.5)
Amortization of prior service (cost) credit, before tax benefit (expense)	0.0		0.0	0.0	(0.2)
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		(0.4)	0.2	(0.1)
Prior service credit (cost) arising during the year, before tax (benefit) expense	0.0		(0.1)	3.1	9.8
Amortization of actuarial (loss) gain, before tax (benefit) expense	0.1		0.0	0.0	1.5
Amortization of prior service (cost) credit, before tax benefit (expense)	$ 0.1		$ 0.4	$ 0.0	$ 0.3