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, 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
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]                      
Net income and comprehensive income $ 23,228 $ 15,687 $ 11,824 $ 4,836 $ 17,045 $ 16,744 $ 17,178 $ 10,202 $ 55,575 $ 61,169 $ 54,467
Undistributed Earnings (Loss) Allocated to Participating Securities, Basic                 (369) (404)  
Numerator for net income and comprehensive income to common shareholders                 $ 55,206 $ 60,765 $ 54,467
Denominator for basic income per share - weighted-average shares                 30,728 30,599 30,135
Denominator for diluted income per share - adjusted weighted-average shares                 31,040 31,072 30,762
Basic net income per share $ 0.75 $ 0.51 $ 0.38 $ 0.16 $ 0.56 $ 0.55 $ 0.56 $ 0.33 $ 1.80 $ 1.99 $ 1.81
Diluted net income per share $ 0.75 $ 0.50 $ 0.38 $ 0.16 $ 0.55 $ 0.54 $ 0.55 $ 0.33 $ 1.78 $ 1.96 $ 1.77
Total number anti-dilutive options and non-vested shares excluded from income per diluted share computation                 208 99 192
Equity Option [Member]                      
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]                      
Weighted Average Number Diluted Shares Outstanding Adjustment                 277 431 615
Total number anti-dilutive options and non-vested shares excluded from income per diluted share computation                 184 99 192
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                 35 42 12
Total number anti-dilutive options and non-vested shares excluded from income per diluted share computation                 24