Share-Based Awards - Fair Value of Options Granted, Excluding Options Grant to Executives Estimated at Date of Grant Using Black Scholes Merton Option Pricing Model (Detail) - Option Awards - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, minimum	42.00%	45.00%	49.00%
Expected volatility, maximum	45.00%	49.00%	51.00%
Expected dividend yield	0.00%	0.00%	0.00%
Risk-free interest rate, minimum	2.52%	1.67%	0.89%
Risk-free interest rate, maximum	2.84%	2.06%	1.89%
Weighted-average fair value of options granted (usd per share)	$ 19.11	$ 14.51	$ 9.42
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average expected life	4 years 6 months	4 years 3 months	3 years 9 months
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average expected life	5 years	4 years 9 months	4 years 3 months