Summary of Significant Accounting Policies - Computation of Basic and Diluted Earnings Per Share (Detail) (USD $)
In Thousands, except Share data, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Regulatory Assets [Abstract]                      
Net income available to common stockholders $ 4,015 $ 5,321 $ 3,025 $ 3,068 $ 2,109 $ 2,714 $ 5,447 $ 2,156 $ 15,429 $ 12,426 $ 11,970
Average number of common shares outstanding                 21,507,759 21,470,945 21,603,084
Effect of unvested stock awards                 129,585 158,723 202,059
Basic weighted average shares outstanding 21,628,168 21,632,828 21,649,423 21,639,122 21,617,801 21,606,540 21,630,660 21,662,471 21,637,344 21,629,668 21,805,143
Effect of dilutive stock options                 350,465 228,713 126,720
Diluted weighted average shares outstanding 22,035,664 22,000,504 21,962,628 21,952,607 21,925,933 21,871,578 21,808,507 21,826,307 21,987,809 21,858,381 21,931,863
Earnings per share:                      
Basic $ 0.19 $ 0.25 $ 0.14 $ 0.14 $ 0.10 $ 0.13 $ 0.25 $ 0.10 $ 0.71 $ 0.57 $ 0.55
Diluted $ 0.18 $ 0.24 $ 0.14 $ 0.14 $ 0.10 $ 0.12 $ 0.25 $ 0.10 $ 0.70 $ 0.57 $ 0.55