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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 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Weighted average common shares outstanding - basic (in shares) 25,008 25,194 25,090 25,490
Weighted average common shares outstanding - diluted (in shares) 25,076 25,290 25,158 25,598
Net income per common share - basic (in dollars per share) $ 2.07 $ 2.15 $ 6.21 $ 5.72
Net income per common share - diluted (in dollars per share) $ 2.07 $ 2.14 $ 6.19 $ 5.69
Class A common stock        
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Net income applicable to common stockholders - basic $ 49,687 $ 50,262 $ 147,876 $ 134,533
Reallocation of net income as a result of conversion of dilutive stock options 1 1 3 5
Reallocation of net income due to conversion of Class B to Class A common shares outstanding 285 439 1,006 1,365
Conversion of Class B common shares into Class A common shares 1,908 3,326 6,909 9,794
Effect of dilutive stock options on net income 5 13 19 42
Net income applicable to common stockholders - diluted $ 51,886 $ 54,041 $ 155,813 $ 145,739
Weighted average common shares outstanding - basic (in shares) 23,948 23,432 23,812 23,530
Conversion of Class B common shares into Class A common shares (in shares) 1,060 1,762 1,278 1,960
Effect of dilutive stock options on weighted average common shares (in shares) 68 96 68 108
Weighted average common shares outstanding - diluted (in shares) 25,076 25,290 25,158 25,598
Antidilutive Securities        
Shares issuable pursuant to stock options not included since they were antidilutive (in shares) 9 0 10 0
Class B common stock        
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Net income applicable to common stockholders - basic $ 2,199 $ 3,779 $ 7,937 $ 11,206
Reallocation of net income as a result of conversion of dilutive stock options (1) (1) (3) (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 (5) (13) (19) (42)
Net income applicable to common stockholders - diluted $ 2,193 $ 3,765 $ 7,915 $ 11,159
Weighted average common shares outstanding - basic (in shares) 1,060 1,762 1,278 1,960
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) 1,060 1,762 1,278 1,960
Antidilutive Securities        
Shares issuable pursuant to stock options not included since they were antidilutive (in shares) 0 0 0 0