Segment and Geographic Information - Schedule of Generated Revenue (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
Segment Reporting Information [Line Items]
Revenue	$ 39,988	$ 39,248	$ 39,935	$ 40,449	$ 42,200	$ 43,027	$ 44,732	$ 45,633	$ 159,620	$ 175,592
United States
Segment Reporting Information [Line Items]
Revenue									146,524	163,104
Canada
Segment Reporting Information [Line Items]
Revenue									$ 13,096	$ 12,488