Equity and Stock-based Compensation - Summary of Stock Option and SAR Activity (Detail)	3 Months Ended
	Mar. 31, 2018 shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares, Outstanding Opening	31,339,515
Granted	347,500
Cancelled/forfeited	(218,882)
Exercised	(1,413,744)
Number of Shares, Outstanding Closing	30,054,389