Operating Segment and Enterprise Wide Reporting - Schedule of Net Long-lived Tangible Assets by Location (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Revenues From External Customers And Long Lived Assets [Line Items]
Net long-lived tangible assets	$ 25,249	$ 23,894
United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net long-lived tangible assets	23,943	22,150
United Kingdom [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net long-lived tangible assets	727	782
Europe (Excluding United Kingdom) [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net long-lived tangible assets	163	259
Other [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net long-lived tangible assets	$ 416	$ 703