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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, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Weighted average common shares outstanding - basic (in shares) 24,164 25,008 24,670 25,090
Weighted average common shares outstanding - diluted (in shares) 24,258 25,076 24,767 25,158
Net income per common share - basic (in dollars per share) $ 3.85 $ 2.07 $ 8.34 $ 6.21
Net income per common share - diluted (in dollars per share) $ 3.84 $ 2.07 $ 8.31 $ 6.19
Class A common stock        
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Net income applicable to common stockholders - basic $ 89,254 $ 49,687 $ 197,485 $ 147,876
Reallocation of net income as a result of conversion of dilutive stock options 1 1 4 3
Reallocation of net income due to conversion of Class B to Class A common shares outstanding 288 285 844 1,006
Conversion of Class B common shares into Class A common shares 3,551 1,908 7,466 6,909
Effect of dilutive stock options on net income 13 5 29 19
Net income applicable to common stockholders - diluted $ 93,107 $ 51,886 $ 205,828 $ 155,813
Weighted average common shares outstanding - basic (in shares) 23,164 23,948 23,670 23,812
Conversion of Class B common shares into Class A common shares (in shares) 1,000 1,060 1,000 1,278
Effect of dilutive stock options on weighted average common shares (in shares) 94 68 97 68
Weighted average common shares outstanding - diluted (in shares) 24,258 25,076 24,767 25,158
Net income per common share - basic (in dollars per share) $ 3.85 $ 2.07 $ 8.34 $ 6.21
Net income per common share - diluted (in dollars per share) $ 3.84 $ 2.07 $ 8.31 $ 6.19
Class B common stock        
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Net income applicable to common stockholders - basic $ 3,853 $ 2,199 $ 8,343 $ 7,937
Reallocation of net income as a result of conversion of dilutive stock options (1) (1) (4) (3)
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 (13) (5) (29) (19)
Net income applicable to common stockholders - diluted $ 3,839 $ 2,193 $ 8,310 $ 7,915
Weighted average common shares outstanding - basic (in shares) 1,000 1,060 1,000 1,278
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 1,060 1,000 1,278
Net income per common share - basic (in dollars per share) $ 3.85 $ 2.07 $ 8.34 $ 6.21
Net income per common share - diluted (in dollars per share) $ 3.84 $ 2.07 $ 8.31 $ 6.19
Employee Stock Option | Class A common stock        
Antidilutive Securities        
Shares issuable pursuant to stock options not included since they were antidilutive (in shares) 33 9 30 10
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