Annual report pursuant to Section 13 and 15(d)

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

v3.8.0.1
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, 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, Basic, by Common Class, Including Two Class Method [Line Items]                      
Net income and comprehensive income $ 35,374 $ 18,155 $ 19,550 $ 14,243 $ 12,706 $ 11,931 $ (10,066) $ 13,099 $ 87,321 $ 27,670 $ 55,575
Undistributed Earnings (Loss) Allocated to Participating Securities, Basic                 (700) (212) (369)
Numerator for net income and comprehensive income to common shareholders                 $ 86,621 $ 27,458 $ 55,206
Denominator for basic income per share - weighted-average shares                 29,867 30,283 30,728
Denominator for diluted income per share - adjusted weighted-average shares                 29,964 30,444 31,040
Basic net income per share $ 1.19 $ 0.60 $ 0.65 $ 0.47 $ 0.42 $ 0.39 $ (0.33) $ 0.43 $ 2.90 $ 0.91 $ 1.80
Diluted net income per share $ 1.18 $ 0.60 $ 0.64 $ 0.47 $ 0.42 $ 0.39 $ (0.33) $ 0.43 $ 2.89 $ 0.90 $ 1.78
Total number anti-dilutive options and non-vested shares excluded from income per diluted share computation                 172 310 208
Equity Option [Member]                      
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]                      
Weighted Average Number Diluted Shares Outstanding Adjustment                 64 130 277
Total number anti-dilutive options and non-vested shares excluded from income per diluted share computation                 172 310 184
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                 33 31 35
Total number anti-dilutive options and non-vested shares excluded from income per diluted share computation                     24