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Operations and Summary of Significant Accounting Policies - Shares and EPS (Details)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2017
$ / shares
Sep. 30, 2017
$ / shares
Jun. 30, 2017
$ / shares
Mar. 31, 2017
$ / shares
Dec. 31, 2016
$ / shares
Sep. 30, 2016
$ / shares
Jun. 30, 2016
$ / shares
Mar. 31, 2016
$ / shares
Dec. 31, 2017
USD ($)
vote / shares
series
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Dec. 31, 2015
USD ($)
$ / shares
shares
Common Stock                      
Voting rights per common share | vote / shares                 1    
Preferred Stock                      
Number of series by which the Board may issue authorized and unissued preferred stock, minimum | series                 1    
Reconciliation of basic earnings per share ("EPS") to diluted EPS                      
Net income | $                 $ 92,617 $ 89,734 $ 67,888
Basic weighted-average shares outstanding                 47,486 48,084 48,952
Dilutive effect of potential common stock equivalents - stock options (in shares)                 288 211 229
Diluted weighted-average shares outstanding                 47,774 48,295 49,181
Net earnings per share:                      
Basic (in USD per share) | $ / shares $ 0.28 $ 0.60 $ 0.59 $ 0.49 $ 0.37 $ 0.62 $ 0.54 $ 0.34 $ 1.95 $ 1.87 $ 1.39
Diluted (in USD per share) | $ / shares $ 0.27 $ 0.59 $ 0.59 $ 0.48 $ 0.36 $ 0.62 $ 0.54 $ 0.34 $ 1.94 $ 1.86 $ 1.38
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount                 0 0 0