Retirement Plans - Components of Defined Benefit Pension and Post-retirement Benefit Plans (Details) - USD ($) $ in Millions	8 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Change in plan assets
Company contributions		$ 1.2
Pensions
Change in projected benefit obligation
Beginning balance, value	$ 24.2	24.4
Service cost	0.7	1.2	$ 0.7
Interest cost	0.6	1.0	0.6
Actuarial loss (gain)	(1.1)	2.0
Plan amendments	0.0	0.6
Benefit payments	0.0	(0.4)
Ending balance, value	24.4	28.8	24.4
Change in plan assets
Beginning balance, value	19.8	19.2
Actual return on plan assets	(1.6)	2.4
Company contributions	1.0	1.4
Benefit payments	0.0	(0.4)
Ending balance, value	19.2	22.6	19.2
Net Funded Status of the Plan (Liability)	(5.2)	(6.2)	(5.2)
Pension and other postretirement benefit asset	0.0	0.0	0.0
Pension and other postretirement benefit (liability)	5.2	6.2	5.2
Other Benefits
Change in projected benefit obligation
Beginning balance, value	0.8	0.7
Service cost	0.0	0.0	0.0
Interest cost	0.0	0.0	0.0
Actuarial loss (gain)	(0.1)	0.1
Plan amendments	0.0	0.0
Benefit payments	0.0	0.0
Ending balance, value	0.7	0.8	0.7
Change in plan assets
Beginning balance, value	0.0	0.0
Actual return on plan assets	0.0	0.0
Company contributions	0.0	0.0
Benefit payments	0.0	0.0
Ending balance, value	0.0	0.0	0.0
Net Funded Status of the Plan (Liability)	(0.7)	(0.8)	(0.7)
Pension and other postretirement benefit asset	0.0	0.0	0.0
Pension and other postretirement benefit (liability)	$ 0.7	$ 0.8	$ 0.7
Qualified Plan | Pensions
Defined Benefit Plan Disclosure
Discount rate - qualified benefit plans	4.10%	3.55%	4.10%
Qualified Plan | Other Benefits
Defined Benefit Plan Disclosure
Discount rate - qualified benefit plans	0.00%	0.00%	0.00%
Nonqualified Plan | Pensions
Defined Benefit Plan Disclosure
Discount rate - qualified benefit plans	4.15%	3.55%	4.15%
Nonqualified Plan | Other Benefits
Defined Benefit Plan Disclosure
Discount rate - qualified benefit plans	3.95%	3.45%	3.95%