Share-Based Compensation - Schedule of Weighted-Average Grant Date Fair Value Per Share of Stock Options, Restricted Share Awards, Restricted Stock Units and Performance Stock Units Awarded (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value of grants. Stock options (in dollars per share)	$ 9.56	$ 8.41	$ 10.85
Restricted share awards, restricted stock units and performance stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted, Weighted Average Grant Date Fair Value (in dollars per share)	$ 38.88	$ 37.64	$ 39.01