Deferred Compensation Plans - Summary of Changes in Deferred Compensation Plan Liability (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Compensation Related Costs [Abstract]
Balance, beginning of year	$ 2,080	$ 1,226	$ 1,087
Deferrals, dividends, and changes in fair value recorded as an expense	444	936	209
Deferred compensation payments	(21)	(82)	(70)
Balance, end of year	$ 2,503	$ 2,080	$ 1,226