Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Class A common stock
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) (in dollars)	$ 87,036	$ 79,053	$ 161,730	$ 144,273
Weighted shares (denominator) (in shares)	46,188,994	46,188,705	46,188,668	46,188,309
Per-share amount (in dollars per share)	$ 1.88	$ 1.71	$ 3.50	$ 3.12
Dilutive effect of stock-based awards
Allocated net income (numerator) (in dollars)	$ 0	$ 0	$ 0	$ 0
Weighted shares (denominator) (in shares)	24,906	23,344	23,903	22,632
Assumed conversion of Class B shares
Allocated net income (numerator) (in dollars)	$ 718	$ 653	$ 1,335	$ 1,191
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)	$ 87,754	$ 79,706	$ 163,065	$ 145,464
Weighted shares (denominator) (in shares)	52,314,700	52,312,849	52,313,371	52,311,741
Per-share amount (in dollars per share)	$ 1.68	$ 1.52	$ 3.12	$ 2.78
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)	$ 718	$ 653	$ 1,335	$ 1,191
Weighted shares (denominator) (in shares)	2,542	2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 283	$ 257	$ 525	$ 469
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) (in dollars)	$ 718	$ 653	$ 1,335	$ 1,191
Weighted shares (denominator) (in shares)	2,542	2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 283	$ 257	$ 525	$ 468