Operating Segment and Enterprise Wide Reporting - Schedule of Revenues by Geographic Destination and as a Percentage of Total Revenues (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues From External Customers And Long Lived Assets [Line Items]
Revenue	$ 90,377	$ 80,369	$ 89,733	$ 77,194	$ 83,446	$ 77,427	$ 87,821	$ 77,401	$ 337,673	$ 326,095	$ 313,726
Revenues by geographical area percentage									100.00%	100.00%	100.00%
United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenue									$ 260,077	$ 252,222	$ 241,025
Revenues by geographical area percentage									77.00%	77.00%	77.00%
Europe (Excluding United Kingdom) [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenue									$ 28,525	$ 27,061	$ 24,607
Revenues by geographical area percentage									9.00%	8.00%	8.00%
United Kingdom [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenue									$ 13,651	$ 14,808	$ 16,815
Revenues by geographical area percentage									4.00%	5.00%	5.00%
Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenue									$ 14,523	$ 13,806	$ 14,424
Revenues by geographical area percentage									4.00%	4.00%	5.00%
Other [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenue									$ 20,897	$ 18,198	$ 16,855
Revenues by geographical area percentage									6.00%	6.00%	5.00%