Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Summary of Significant Accounting Policies [Line Items]
Shares withheld for taxes of financing activities	$ 1,211	$ 650
Accounting Standards Update 2016-09 [Member]
Summary of Significant Accounting Policies [Line Items]
Excess tax benefits related to share-based payments recorded as offset to income tax expense	1,100
Excess tax benefits related to share-based payments classified as operating activities	1,100
Shares withheld for taxes of financing activities	$ 1,200	$ 700
Minimum [Member]
Summary of Significant Accounting Policies [Line Items]
Percentage of revenue derived from transporting	99.00%