Summary of Treasury Stock Activity (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) shares
Treasury Stock Shares [Abstract]
Beginning Balance, Shares | shares	4,525,870
Restricted stock withholdings | shares	75,801	[1]
Restricted stock forfeitures | shares	18,643
Ending Balance, Shares | shares	4,620,314
Beginning Balance, Cost | $	$ 76,354
Restricted stock withholdings | $	2,905	[1]
Restricted stock forfeitures | $	0
Ending Balance, Cost | $	$ 79,259

[1] The Company withheld restricted shares as a result of the election by certain employees to satisfy their tax liabilities upon vesting in restricted stock and restricted stock units. The Company determined the number of shares to be withheld based upon market values ranging from $33.77 to $39.03 per share.