Retirement Plans - Net Annual Benefit Costs Assumptions (Details) - USD ($) $ in Millions	8 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Pensions
Components of net annual benefit cost:
Expected return on plan assets (percentage)		4.50%	4.50%
Service cost	$ 0.7	$ 1.2	$ 0.7
Interest cost	0.6	1.0	0.6
Expected return on plan assets		(0.9)	(0.6)
Amortization of prior service cost		0.0	0.0
Amortization of net actuarial and other (gain) loss		0.0	0.0
Net annual benefit cost		1.3	0.7
Other Benefits
Components of net annual benefit cost:
Service cost	0.0	0.0	0.0
Interest cost	$ 0.0	0.0	0.0
Expected return on plan assets		0.0	0.0
Amortization of prior service cost		0.0	0.0
Amortization of net actuarial and other (gain) loss		0.0	0.0
Net annual benefit cost		$ 0.0	$ 0.0
Qualified Plan | Pensions
Components of net annual benefit cost:
Discount rate - qualified benefit plans		4.10%	4.00%
Qualified Plan | Other Benefits
Components of net annual benefit cost:
Discount rate - qualified benefit plans		0.00%	0.00%
Nonqualified Plan | Pensions
Components of net annual benefit cost:
Discount rate - qualified benefit plans		4.15%	3.75%
Nonqualified Plan | Other Benefits
Components of net annual benefit cost:
Discount rate - qualified benefit plans		3.95%	3.75%