Segment Reporting - Information on Long-Lived Assets and Net Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Property and equipment, net of accumulated depreciation	$ 75,075	$ 61,856
United States
Segment Reporting Information [Line Items]
Property and equipment, net of accumulated depreciation	52,709	39,584
Net assets	654,010	709,634
United Kingdom
Segment Reporting Information [Line Items]
Property and equipment, net of accumulated depreciation	14,761	15,312
Net assets	207,885	193,276
All other foreign countries
Segment Reporting Information [Line Items]
Property and equipment, net of accumulated depreciation	7,605	6,960
Net assets	$ 330,076	$ 304,448