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Net Income per Common Share (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ 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, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Basic:                              
Net income available to common shareholders $ 15,569 $ 29,874 $ 27,772 $ 27,017 $ 25,717 $ 24,523 $ 24,145 $ 23,322 $ 22,286 $ 21,283 $ 20,759 $ 19,238 $ 100,232 $ 97,707 $ 83,566
Average common shares outstanding (in shares)                         34,139 33,587 32,836
Basic net income per common share (in dollars per share) $ 0.46 [1] $ 0.87 [1] $ 0.81 [1] $ 0.79 [1] $ 0.76 [1] $ 0.73 [1] $ 0.72 [1] $ 0.70 [1] $ 0.67 [1] $ 0.64 [1] $ 0.62 [1] $ 0.62 [1] $ 2.94 $ 2.91 $ 2.54
Diluted:                              
Net income available to common shareholders $ 15,569 $ 29,874 $ 27,772 $ 27,017 $ 25,717 $ 24,523 $ 24,145 $ 23,322 $ 22,286 $ 21,283 $ 20,759 $ 19,238 $ 100,232 $ 97,707 $ 83,566
Average common shares outstanding (in shares)                         34,139 33,587 32,836
Adjustment for common share equivalents (in shares)                         182 594 643
Average common shares outstanding-diluted (in shares)                         34,321 34,181 33,479
Diluted net income per common share (in dollars per share) $ 0.45 [1] $ 0.87 [1] $ 0.81 [1] $ 0.79 [1] $ 0.75 [1] $ 0.72 [1] $ 0.71 [1] $ 0.68 [1] $ 0.65 [1] $ 0.63 [1] $ 0.61 [1] $ 0.61 [1] $ 2.92 $ 2.86 $ 2.50
Anti-dilutive shares (in shares)                           7 5
[1] Earnings per common share are calculated on a quarterly basis and may not be additive to the year to date amount.