Defined Benefit Plans - Net Periodic Pension Cost (Benefit) (Details) - USD ($) $ in Millions	7 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Retirement Benefits [Abstract]
Service cost	$ 0.9	$ 1.2	$ 0.9
Interest cost	$ 42.9	58.8	42.9
Expected return on plan assets		(73.0)	(46.7)
Other		(0.3)	0.0
Net periodic pension benefit		$ (13.3)	$ (2.9)