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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 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Weighted average common shares outstanding - basic (in shares) 24,793 25,053 24,930 25,116
Weighted average common shares outstanding - diluted (in shares) 24,882 25,106 25,028 25,177
Net income per common share - basic (in dollars per share) $ 2.45 $ 2.12 $ 4.52 $ 4.14
Net income per common share - diluted (in dollars per share) $ 2.44 $ 2.12 $ 4.50 $ 4.13
Class A common stock        
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Net income applicable to common stockholders - basic $ 58,213 $ 50,520 $ 108,199 $ 98,337
Reallocation of net income as a result of conversion of dilutive stock options 1 1 2 2
Reallocation of net income due to conversion of Class B to Class A common shares outstanding 289 340 557 700
Conversion of Class B common shares into Class A common shares 2,149 2,334 3,947 4,876
Effect of dilutive stock options on net income 8 5 16 12
Net income applicable to common stockholders - diluted $ 60,660 $ 53,200 $ 112,721 $ 103,927
Weighted average common shares outstanding - basic (in shares) 23,793 23,791 23,930 23,765
Conversion of Class B common shares into Class A common shares (in shares) 1,000 1,262 1,000 1,351
Effect of dilutive stock options on weighted average common shares (in shares) 89 53 98 61
Weighted average common shares outstanding - diluted (in shares) 24,882 25,106 25,028 25,177
Net income per common share - basic (in dollars per share) $ 2.45 $ 2.12 $ 4.52 $ 4.14
Net income per common share - diluted (in dollars per share) $ 2.44 $ 2.12 $ 4.50 $ 4.13
Class B common stock        
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Net income applicable to common stockholders - basic $ 2,447 $ 2,680 $ 4,522 $ 5,590
Reallocation of net income as a result of conversion of dilutive stock options (1) (1) (2) (2)
Reallocation of net income due to conversion of Class B to Class A common shares outstanding 0 0 0 0
Conversion of Class B common shares into Class A common shares 0 0 0 0
Effect of dilutive stock options on net income (8) (5) (16) (12)
Net income applicable to common stockholders - diluted $ 2,438 $ 2,674 $ 4,504 $ 5,576
Weighted average common shares outstanding - basic (in shares) 1,000 1,262 1,000 1,351
Conversion of Class B common shares into Class A common shares (in shares) 0 0 0 0
Effect of dilutive stock options on weighted average common shares (in shares) 0 0 0 0
Weighted average common shares outstanding - diluted (in shares) 1,000 1,262 1,000 1,351
Net income per common share - basic (in dollars per share) $ 2.45 $ 2.12 $ 4.52 $ 4.14
Net income per common share - diluted (in dollars per share) $ 2.44 $ 2.12 $ 4.50 $ 4.13
Employee Stock Option | Class A common stock        
Antidilutive Securities        
Shares issuable pursuant to stock options not included since they were antidilutive (in shares) 30 22 56 11
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 0 0