Earnings Per Share (Details) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) $ / shares shares	Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2020 USD ($) $ / shares shares	Sep. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2019
Class A common stock
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) | $	$ 88,486	$ 93,398	$ 228,632	$ 255,128
Weighted shares (denominator) (in shares) | shares	46,189,030	46,189,006	46,188,544	46,188,767
Per-share amount (in dollars per share) | $ / shares	$ 1.92	$ 2.02	$ 4.95	$ 5.52
Dilutive effect of stock-based awards
Allocated net income (numerator) | $	$ 0	$ 0	$ 0	$ 0
Weighted shares (denominator) (in shares) | shares	20,599	35,319	23,244	27,708
Assumed conversion of Class B shares
Allocated net income (numerator) | $	$ 730	$ 771	$ 1,887	$ 2,106
Weighted shares (denominator) (in shares) | shares	6,100,800	6,100,800	6,100,800	6,100,800
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) | $	$ 89,216	$ 94,169	$ 230,519	$ 257,234
Weighted shares (denominator) (in shares) | shares	52,310,429	52,325,125	52,312,588	52,317,275
Per-share amount (in dollars per share) | $ / shares	$ 1.71	$ 1.80	$ 4.41	$ 4.92
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		2,400
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) | $	$ 730	$ 771	$ 1,887	$ 2,106
Weighted shares (denominator) (in shares) | shares	2,542	2,542	2,542	2,542
Per-share amount (in dollars per share) | $ / shares	$ 287	$ 303	$ 742	$ 829
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) | $	$ 730	$ 770	$ 1,887	$ 2,105
Weighted shares (denominator) (in shares) | shares	2,542	2,542	2,542	2,542
Per-share amount (in dollars per share) | $ / shares	$ 287	$ 303	$ 742	$ 828