Stock-Based Compensation, Restricted Stock Unit Awards and Deferred Stock Units (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended		9 Months Ended
	May 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Weighted-Average Grant-Date Fair Value [Abstract]
Stock-based compensation expense		$ 0.9	$ 7.8	$ 6.1	$ 69.2
Restricted Stock Unit [Member]
Shares [Roll Forward]
Non-vested, beginning balance (in shares)				0
Granted (in shares)				365,000
Vested (in shares)				0
Forfeited (in shares)				(10,000)
Non-vested, ending balance (in shares)		355,000		355,000
Weighted-Average Grant-Date Fair Value [Abstract]
Non-vested, beginning balance (in dollars per share)				$ 0
Granted (in dollars per share)				31.84
Vested (in dollars per share)				0
Forfeited (in dollars per share)				32.06
Non-vested, ending balance (in dollars per share)		$ 31.83		$ 31.83
Deferred Stock Units [Member]
Weighted-Average Grant-Date Fair Value [Abstract]
Deferred stock units authorized (in shares)	5.5
Stock-based compensation expense			$ 2.0		$ 96.8
Fair value of deferred stock units (in dollars per share)	$ 17.20
Initial public offering share price (in dollars per share)	$ 20.00
Holding period restriction	1 year 5 months 1 day
Assumed volatility	51.50%