v3.8.0.1
Net Income Per Share of Class A and Class B Common Stock (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]      
Weighted average common shares outstanding - basic (in shares) 25,065 25,409 26,290
Weighted average common shares outstanding - diluted (in shares) 25,145 25,521 26,490
Net income per share - basic (in dollars per share) $ 9.78 $ 7.76 $ 6.96
Net income per share - diluted (in dollars per share) $ 9.75 $ 7.72 $ 6.91
Class A common stock      
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]      
Net income applicable to common stockholders - basic $ 233,399 $ 182,369 $ 165,172
Reallocation of distributed net income as a result of conversion of dilutive stock options 4 8 15
Reallocation of distributed net income due to conversion of Class B to Class A common shares outstanding 1,275 1,791 1,932
Conversion of Class B common shares into Class A common shares 10,505 12,833 15,760
Effect of dilutive stock options on net income 34 57 120
Net income applicable to common stockholders - diluted $ 245,217 $ 197,058 $ 182,999
Weighted average common shares outstanding - basic (in shares) 23,857 23,515 23,729
Conversion of Class B common shares into Class A common shares (in shares) 1,208 1,894 2,561
Effect employee stock purchases and restricted stock units on weighted average common shares (in shares) 80 112 200
Weighted average common shares outstanding - diluted (in shares) 25,145 25,521 26,490
Net income per share - basic (in dollars per share) $ 9.78 $ 7.76 $ 6.96
Net income per share - diluted (in dollars per share) $ 9.75 $ 7.72 $ 6.91
Class A common stock | Employee Stock Option      
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]      
Shares issuable pursuant to employee stock purchases and restricted stock units not included since they were antidilutive (in shares) 12 0 16
Class B common stock      
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]      
Net income applicable to common stockholders - basic $ 11,818 $ 14,689 $ 17,827
Reallocation of distributed net income as a result of conversion of dilutive stock options (4) (8) (15)
Reallocation of distributed net income due to conversion of Class B to Class A common shares outstanding 0 0 0
Conversion of Class B common shares into Class A common shares 0 0 0
Effect of dilutive stock options on net income (34) (57) (120)
Net income applicable to common stockholders - diluted $ 11,780 $ 14,624 $ 17,692
Weighted average common shares outstanding - basic (in shares) 1,208 1,894 2,561
Conversion of Class B common shares into Class A common shares (in shares) 0 0 0
Effect employee stock purchases and restricted stock units on weighted average common shares (in shares) 0 0 0
Weighted average common shares outstanding - diluted (in shares) 1,208 1,894 2,561
Net income per share - basic (in dollars per share) $ 9.78 $ 7.76 $ 6.96
Net income per share - diluted (in dollars per share) $ 9.75 $ 7.72 $ 6.91
Class B common stock | Employee Stock Option      
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]      
Shares issuable pursuant to employee stock purchases and restricted stock units not included since they were antidilutive (in shares) 0 0 0