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Earnings Per Share (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Earnings Per Share [Abstract]                      
Net income $ 19,483 $ 26,575 $ 24,383 $ 23,508 $ 22,567 $ 22,906 $ 21,356 $ 20,973 $ 93,949 $ 87,802 $ 83,722
Basic weighted average common shares outstanding                 64,384,851 63,643,622 62,945,669
Dilutive shares                 194,371 208,364 169,049
Diluted weighted average common shares outstanding                 64,579,222 63,851,986 63,114,718
Earnings per share:                      
Basic (in dollars per share) $ 0.30 $ 0.41 $ 0.38 $ 0.37 $ 0.35 $ 0.36 $ 0.34 $ 0.33 $ 1.46 $ 1.38 $ 1.33
Diluted (in dollars per share) $ 0.30 $ 0.41 $ 0.38 $ 0.37 $ 0.35 $ 0.36 $ 0.34 $ 0.33 $ 1.45 $ 1.38 $ 1.33
Anti-dilutive stock options and awards excluded from computation of earnings per share (in shares)                 369,772 432,895 469,018