Restricted Stock Units Issued (Detail) - Restricted Stock Units (RSUs) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Restricted Stock Units
Beginning Restricted Stock Units	43,345	33,588
Restricted Stock Units Granted	7,522	8,505
Dividend Equivalents Earned	1,357	1,252
Restricted Stock Units Settled	0	0
Ending Restricted Stock Units	52,224	43,345
Weighted-Average Stock Price
Beginning Restricted Stock Units	$ 33.40	$ 31.83
Restricted Stock Units Granted	50.23	38.51
Dividend Equivalents Earned	48.57	41.00
Restricted Stock Units Settled	0	0
Ending Restricted Stock Units	$ 36.22	$ 33.40