Pension and Postretirement Benefit Costs (Details) - USD ($) $ in Millions		3 Months Ended			6 Months Ended
	Apr. 01, 2017	Jun. 30, 2018	Jun. 30, 2017	Apr. 01, 2017	Jun. 30, 2018
Successor
Components of net periodic benefit cost
Net periodic pension (benefit) cost		$ 4.6	$ 6.6		$ 9.1
Successor | Defined benefit pension plans
Components of net periodic benefit cost
Service cost for benefits earned		0.5	0.6		1.1
Interest cost on projected benefit obligation		7.9	9.3		15.7
Expected return on plan assets		(10.7)	(11.2)		(21.4)
Amortization of prior service cost and net actuarial loss		0.0	0.0		0.0
Net periodic pension (benefit) cost		$ (2.3)	(1.3)		$ (4.6)
Other Defined Benefit Plan Information [Abstract]
Funding threshold		80.00%			80.00%
Defined Contribution Plan, Employer Discretionary Contribution Amount		$ 12.0			$ 42.0
Successor | Postretirement benefit plans
Components of net periodic benefit cost
Service cost for benefits earned		2.1	2.3		4.1
Interest cost on projected benefit obligation		7.1	8.3		14.2
Amortization of prior service cost and net actuarial loss		0.0	0.0		0.0
Net periodic pension (benefit) cost		$ 9.2	$ 10.6		$ 18.3
Predecessor
Components of net periodic benefit cost
Net periodic pension (benefit) cost	$ 0.0			$ 14.4
Predecessor | Defined benefit pension plans
Components of net periodic benefit cost
Service cost for benefits earned				0.6
Interest cost on projected benefit obligation				9.7
Expected return on plan assets				(11.0)
Amortization of prior service cost and net actuarial loss				6.4
Net periodic pension (benefit) cost				5.7
Predecessor | Postretirement benefit plans
Components of net periodic benefit cost
Service cost for benefits earned				2.3
Interest cost on projected benefit obligation				8.4
Amortization of prior service cost and net actuarial loss				3.2
Net periodic pension (benefit) cost				$ 13.9