Earnings Per Share (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Class A common stock
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) | $	$ 290,902	$ 314,227	$ 285,864
Weighted shares (denominator) (in shares) | shares	46,188,659	46,188,836	46,188,637
Per- share amount (in dollars per share) | $ / shares	$ 6.30	$ 6.80	$ 6.19
Dilutive effect of stock-based awards
Allocated net income (numerator) | $	$ 0	$ 0	$ 0
Weighted shares (denominator) (in shares) | shares	23,901	30,224	25,776
Assumed conversion of Class B shares
Allocated net income (numerator) | $	$ 2,402	$ 2,594	$ 2,360
Weighted shares (denominator) (in shares) | shares	6,100,800	6,100,800	6,100,800
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) | $	$ 293,304	$ 316,821	$ 288,224
Weighted shares (denominator) (in shares) | shares	52,313,360	52,319,860	52,315,213
Per- share amount (in dollars per share) | $ / shares	$ 5.61	$ 6.06	$ 5.51
Class B common stock
Reconciliation of the numerators and denominators used in the basic and diluted per-share computations
Ratio for converting shares of Class B common stock into shares of Class A common stock	2,400	2,400
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) | $	$ 2,402	$ 2,594	$ 2,360
Weighted shares (denominator) (in shares) | shares	2,542	2,542	2,542
Per- share amount (in dollars per share) | $ / shares	$ 945	$ 1,020	$ 928
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) | $	$ 2,401	$ 2,593	$ 2,359
Weighted shares (denominator) (in shares) | shares	2,542	2,542	2,542
Per- share amount (in dollars per share) | $ / shares	$ 945	$ 1,020	$ 928