Goodwill and Intangible Assets - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	$ 83,164,000	$ 75,110,000	$ 74,565,000
Impairment of goodwill	0	0	0
Amortization expense	2,800,000	$ 2,400,000	$ 1,300,000
Capitalized internal-use software costs	$ 800,000