Restricted Stock Units Issued (Detail) - Restricted Stock Units (RSUs)	3 Months Ended
	Mar. 31, 2018 $ / shares shares
Restricted Stock Units
Beginning Restricted Stock Units | shares	52,224
Restricted Stock Units Granted | shares	0
Dividend Equivalents Earned | shares	453
Restricted Stock Units Settled | shares	0
Ending Restricted Stock Units | shares	52,677
Weighted-Average Stock Price
Beginning Restricted Stock Units | $ / shares	$ 36.22
Restricted Stock Units Granted | $ / shares	0
Dividend Equivalents Earned | $ / shares	42.04
Restricted Stock Units Settled | $ / shares	0
Ending Restricted Stock Units | $ / shares	$ 36.27