Income Taxes - Summary of Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Balance at beginning of period		$ 777	$ 124	$ 77
Additions based on tax positions related to the current year		162	118	51
Additions based on tax positions related to prior years			537
Reductions due to statute of limitations expiring	$ (200)	(173)	(2)	(4)
Revaluation due to Tax Reform		108
Balance at end of period		$ 874	$ 777	$ 124