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Summary of Significant Accounting Policies - Net income per common share (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Computation of Basic and Diluted Net Income Per Common Share [Abstract]                      
Net income $ 20,165 $ 33,647 $ 15,870 $ 12,209 $ 9,895 $ 8,499 $ 7,193 $ 7,630 $ 81,891 $ 33,217 $ 32,933
Net income per common share - basic (in dollars per share) $ 0.42 $ 0.71 $ 0.33 $ 0.26 $ 0.21 $ 0.18 $ 0.15 $ 0.16 $ 1.72 $ 0.71 $ 0.72
Net income per common share - diluted (in dollars per share) $ 0.41 $ 0.69 $ 0.32 $ 0.25 $ 0.20 $ 0.18 $ 0.15 $ 0.16 $ 1.68 $ 0.69 $ 0.69
Weighted average common shares outstanding - basic                 47,525 46,504 45,963
Plus dilutive effect of potential common shares                 1,318 1,582 1,824
Weighted average common shares outstanding - diluted                 48,843 48,086 47,787
Stock Compensation Plan                      
Class of Stock [Line Items]                      
Common stock excluded from calculation of EPS (shares)                 9 298 308