Employee Benefit Plans - Summary of Activity for the Stock Option Component of the Incentive Plan (Detail)	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Compensation Related Costs [Abstract]
Number of options, Outstanding at beginning of the year | shares	276,122
Number of options, Granted during the year | shares	117,550
Number of options, Exercised during the year | shares	(53,584)
Number of options, Forfeited or expired during the year | shares	(3,400)
Number of options, Outstanding at end of the year | shares	336,688
Number of options, Exercisable at year end | shares	277,913
Weighted average exercise price per share, Outstanding at beginning of the year | $ / shares	$ 29.71
Weighted average exercise price per share, Granted during the year | $ / shares	38.88
Weighted average exercise price per share, Exercised during the year | $ / shares	27.96
Weighted average exercise price per share, Forfeited or expired during the year | $ / shares	28.83
Weighted average exercise price per share, Outstanding at end of the year | $ / shares	33.20
Weighted average exercise price per share, Exercisable at year end | $ / shares	$ 31.99