Stock-based compensation - Summary of stock based compensation award granted (Detail) - Employees $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) shares
RSUs
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Number of shares granted, RSUs | shares	931	[1]
Fair value of shares granted | $	$ 54,100	[1]
Nonqualified Stock Options
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Number of shares granted, options | shares	1,446
Fair value of shares granted | $	$ 26,000

[1] Includes approximately 0.1 million RSUs which were granted in connection with the election by certain employees to receive 50% of their 2018 target incentive compensation under the Company's Corporate Incentive Compensation Plan in the form of RSUs instead of cash. Stock-based compensation of approximately $8.2 million associated with this RSU grant is expected to be recognized over a one-year period beginning on the grant date.