Annual report pursuant to Section 13 and 15(d)

Shareholders' Equity, Stock Options and Net Income per Share - Net Income per Share (Details)

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Shareholders' Equity, Stock Options and Net Income per Share - Net Income per Share (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]                      
Net income and comprehensive income $ 24,168 $ 22,195 $ 22,330 $ 18,407 $ 27,684 $ 22,329 $ 24,298 $ 17,741 $ 87,099 $ 92,051 $ 87,255
Undistributed Earnings (Loss) Allocated to Participating Securities, Basic                 (945) (881) (700)
Numerator for net income and comprehensive income to common shareholders                 $ 86,154 $ 91,170 $ 86,555
Denominator for basic income per share - weighted-average shares                 28,195 29,076 29,867
Denominator for diluted income per share - adjusted weighted-average shares                 28,308 29,190 29,964
Basic net income per share $ 0.86 $ 0.78 $ 0.78 $ 0.64 $ 0.95 $ 0.76 $ 0.83 $ 0.60 $ 3.06 $ 3.14 $ 2.90
Diluted net income per share $ 0.85 $ 0.78 $ 0.78 $ 0.64 $ 0.95 $ 0.76 $ 0.82 $ 0.60 $ 3.04 $ 3.12 $ 2.89
Total number anti-dilutive options and non-vested shares excluded from income per diluted share computation                 183 151 172
Equity Option [Member]                      
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]                      
Weighted Average Number Diluted Shares Outstanding Adjustment                 82 80 64
Total number anti-dilutive options and non-vested shares excluded from income per diluted share computation                 183 126 172
Key Employee Performance Share Based Plan [Member]                      
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]                      
Weighted Average Number Diluted Shares Outstanding Adjustment                 31 34 33
Total number anti-dilutive options and non-vested shares excluded from income per diluted share computation                   16  
Employee Non-vested Shares [Member]                      
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]                      
Total number anti-dilutive options and non-vested shares excluded from income per diluted share computation                   9