Defined Contribution Plan - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) yr	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Deferred Compensation Arrangement with Individual, Contributions by Employer | $	$ 0	$ 0	$ 0
Minimum age to be eligible for participation | yr	18
Period of service to be eligible for participation	3 months
Percentage of earnings that can be deferred	60.00%