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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, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Weighted average common shares outstanding - basic (in shares) 25,053 25,462 25,116 25,639
Weighted average common shares outstanding - diluted (in shares) 25,106 25,534 25,177 25,754
Net income per common share - basic (in dollars per share) $ 2.12 $ 2.02 $ 4.14 $ 3.58
Net income per common share - diluted (in dollars per share) $ 2.12 $ 2.01 $ 4.13 $ 3.56
Class A common stock        
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Net income applicable to common stockholders - basic $ 50,520 $ 47,869 $ 98,337 $ 84,445
Reallocation of net income as a result of conversion of dilutive stock options 1 1 2 5
Reallocation of net income due to conversion of Class B to Class A common shares outstanding 340 440 700 907
Conversion of Class B common shares into Class A common shares 2,334 3,109 4,876 6,313
Effect of dilutive stock options on net income (in shares) 5 9 12 28
Net income applicable to common stockholders - diluted (in shares) $ 53,200 $ 51,428 $ 103,927 $ 91,698
Weighted average common shares outstanding - basic (in shares) 23,791 23,700 23,765 23,611
Conversion of Class B common shares into Class A common shares 1,262 1,762 1,351 2,028
Effect of dilutive stock options on weighted average common shares (in shares) 53 72 61 115
Weighted average common shares outstanding - diluted (in shares) 25,106 25,534 25,177 25,754
Net income per common share - basic (in dollars per share) $ 2.12 $ 2.02 $ 4.14 $ 3.58
Net income per common share - diluted (in dollars per share) $ 2.12 $ 2.01 $ 4.13 $ 3.56
Antidilutive Securities        
Shares issuable pursuant to stock options not included since they were antidilutive (in shares) 22 0 11 10
Class B common stock        
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Net income applicable to common stockholders - basic $ 2,680 $ 3,559 $ 5,590 $ 7,253
Reallocation of net income as a result of conversion of dilutive stock options (1) (1) (2) (5)
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 (in shares) (5) (9) (12) (28)
Net income applicable to common stockholders - diluted (in shares) $ 2,674 $ 3,549 $ 5,576 $ 7,220
Weighted average common shares outstanding - basic (in shares) 1,262 1,762 1,351 2,028
Conversion of Class B common shares into Class A common 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) 1,262 1,762 1,351 2,028
Net income per common share - basic (in dollars per share) $ 2.12 $ 2.02 $ 4.14 $ 3.58
Net income per common share - diluted (in dollars per share) $ 2.12 $ 2.01 $ 4.13 $ 3.56
Antidilutive Securities        
Shares issuable pursuant to stock options not included since they were antidilutive (in shares) 0 0 0 0