Background and Basis of Presentation - Summary of Earnings (Loss), Weighted Average Common Shares and Potential Common Shares Outstanding (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net income (loss)	$ (2.3)	$ 33.7
Denominator:
Weighted average common shares outstanding - basic	192.8	191.4
Dilutive effect of equity-based awards		4.1
Weighted average common shares outstanding - diluted	192.8	195.5
Earnings (loss) per share:
Basic	$ (0.01)	$ 0.18
Diluted	$ (0.01)	$ 0.17