Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Class A common stock
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) (in dollars)	$ 93,398	$ 79,787	$ 255,128	$ 224,060
Weighted shares (denominator) (in shares)	46,189,006	46,188,941	46,188,767	46,188,522
Per-share amount (in dollars per share)	$ 2.02	$ 1.73	$ 5.52	$ 4.85
Dilutive effect of stock-based awards
Allocated net income (numerator) (in dollars)	$ 0	$ 0	$ 0	$ 0
Weighted shares (denominator) (in shares)	35,319	27,697	27,708	24,320
Assumed conversion of Class B shares
Allocated net income (numerator) (in dollars)	$ 771	$ 659	$ 2,106	$ 1,850
Weighted shares (denominator) (in 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) (in dollars)	$ 94,169	$ 80,446	$ 257,234	$ 225,910
Weighted shares (denominator) (in shares)	52,325,125	52,317,438	52,317,275	52,313,642
Per-share amount (in dollars per share)	$ 1.80	$ 1.54	$ 4.92	$ 4.32
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 (as a percent)	2400.00%		2400.00%		2400.00%
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) (in dollars)	$ 771	$ 659	$ 2,106	$ 1,850
Weighted shares (denominator) (in shares)	2,542	2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 303	$ 259	$ 829	$ 728
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) (in dollars)	$ 770	$ 658	$ 2,105	$ 1,849
Weighted shares (denominator) (in shares)	2,542	2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 303	$ 259	$ 828	$ 727