Pension and Postretirement Benefit Costs (Details) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2017 USD ($)	Apr. 01, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Apr. 02, 2017 plan
Defined benefit pension plans
Components of net periodic benefit cost
Funding threshold	80.00%
Number of non-qualified pension plans prior to reorganization | plan					2
Contributions by employer	$ 0.8
Estimated contribution in 2017 to pension plan	5.9
Successor | Defined benefit pension plans
Components of net periodic benefit cost
Service cost for benefits earned	0.6
Interest cost on projected or accumulated postretirement benefit obligation	9.3
Expected return on plan assets	(11.2)
Amortization of prior service cost and net actuarial loss	0.0
Net periodic pension or postretirement benefit cost	(1.3)
Successor | Postretirement benefit plans
Components of net periodic benefit cost
Service cost for benefits earned	2.3
Interest cost on projected or accumulated postretirement benefit obligation	8.3
Amortization of prior service cost and net actuarial loss	0.0
Net periodic pension or postretirement benefit cost	$ 10.6
Predecessor | Defined benefit pension plans
Components of net periodic benefit cost
Service cost for benefits earned		$ 0.6	$ 0.7	$ 1.3
Interest cost on projected or accumulated postretirement benefit obligation		9.7	10.3	20.7
Expected return on plan assets		(11.0)	(11.3)	(22.6)
Amortization of prior service cost and net actuarial loss		6.4	6.3	12.5
Net periodic pension or postretirement benefit cost		5.7	6.0	11.9
Predecessor | Postretirement benefit plans
Components of net periodic benefit cost
Service cost for benefits earned		2.3	2.6	5.2
Interest cost on projected or accumulated postretirement benefit obligation		8.4	8.8	17.6
Amortization of prior service cost and net actuarial loss		3.2	2.3	4.7
Net periodic pension or postretirement benefit cost		$ 13.9	$ 13.7	$ 27.5