Activity of Awarded Deferred Common Stock Units (Detail) (Deferred Common Stock Units, USD $)	12 Months Ended
	Dec. 31, 2014
Deferred Common Stock Units
Number of shares
Balance at beginning of year (in shares)	217,708
Granted (in shares)	36,171
Converted (in shares)	0
Balance at end of year (in shares)	253,879
Weighted-Average Grant Date Fair Value
Balance at beginning of year (in dollars per share)	$ 13.01
Granted (in dollars per share)	$ 11.61
Converted (in dollars per share)	$ 0.00
Balance at end of year (in dollars per share)	$ 12.81