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Net Income Per Share of Class A and Class B Common Stock (Earnings Per Share Reconciliation) (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]    
Weighted average common shares outstanding - basic (in shares) 25,050 25,180
Weighted average common shares outstanding - diluted (in shares) 25,158 25,250
Net income per common share - basic (in dollars per share) $ 2.08 $ 2.01
Net income per common share - diluted (in dollars per share) $ 2.07 $ 2.01
Class A common stock    
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]    
Net income applicable to common stockholders - basic $ 49,983 $ 47,826
Reallocation of net income as a result of conversion of dilutive stock options 1 1
Reallocation of net income due to conversion of Class B to Class A common shares outstanding 269 359
Conversion of Class B common shares into Class A common shares 1,801 2,534
Effect of dilutive stock options on net income 7 7
Net income applicable to common stockholders - diluted $ 52,061 $ 50,727
Weighted average common shares outstanding - basic (in shares) 24,050 23,740
Conversion of Class B common shares into Class A common shares (in shares) 1,000 1,440
Effect of dilutive stock options on weighted average common shares (in shares) 108 70
Weighted average common shares outstanding - diluted (in shares) 25,158 25,250
Net income per common share - basic (in dollars per share) $ 2.08 $ 2.01
Net income per common share - diluted (in dollars per share) $ 2.07 $ 2.01
Class B common stock    
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]    
Net income applicable to common stockholders - basic $ 2,078 $ 2,901
Reallocation of net income as a result of conversion of dilutive stock options (1) (1)
Reallocation of net income due to conversion of Class B to Class A common shares outstanding 0 0
Conversion of Class B common shares into Class A common shares 0 0
Effect of dilutive stock options on net income (7) (7)
Net income applicable to common stockholders - diluted $ 2,070 $ 2,893
Weighted average common shares outstanding - basic (in shares) 1,000 1,440
Conversion of Class B common shares into Class A common shares (in shares) 0 0
Effect of dilutive stock options on weighted average common shares (in shares) 0 0
Weighted average common shares outstanding - diluted (in shares) 1,000 1,440
Net income per common share - basic (in dollars per share) $ 2.08 $ 2.01
Net income per common share - diluted (in dollars per share) $ 2.07 $ 2.01
Employee Stock Option | Class A common stock    
Antidilutive Securities    
Shares issuable pursuant to stock options not included since they were antidilutive (in shares) 26 0
Employee Stock Option | Class B common stock    
Antidilutive Securities    
Shares issuable pursuant to stock options not included since they were antidilutive (in shares) 0 0