Summary of Performance-based Award Activity (Detail) - Performance Shares	9 Months Ended
	Sep. 30, 2017 USD ($) $ / shares shares
Number of shares
Beginning balance | shares	145,962	[1]
Grants | shares	87,000	[1]
Cancellations | shares	(4,070)	[1]
Ending balance | shares	228,892	[1]
Total unrecognized compensation expense remaining | $	$ 2,090,000	[1]
Weighted-average years expected to be recognized over	1 year 10 months 25 days	[1]
Weighted-average grant date fair value
Beginning balance | $ / shares	$ 15.32
Grants | $ / shares	18.45
Cancellations | $ / shares	16.52
Ending balance | $ / shares	$ 16.49

[1] Represents the target number of shares to be issued for each performance-based award.