Restricted Stock Units Issued (Detail) - Restricted Stock Units (RSUs) - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Restricted Stock Units
Beginning Restricted Stock Units	33,588	23,576
Restricted Stock Units Granted	8,505	8,965
Dividend Equivalents Earned	1,252	1,047
Restricted Stock Units Settled	0	0
Ending Restricted Stock Units	43,345	33,588
Weighted-Average Stock Price
Beginning Restricted Stock Units	$ 31.83	$ 29.90
Restricted Stock Units Granted	38.51	36.54
Dividend Equivalents Earned	41.00	35.01
Restricted Stock Units Settled	0	0
Ending Restricted Stock Units	$ 33.40	$ 31.83