Earnings Per Share (Details) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) $ / shares shares	Jun. 30, 2019 USD ($) $ / shares shares	Jun. 30, 2020 USD ($) $ / shares shares	Jun. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2019
Class A common stock
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) | $	$ 81,306	$ 87,036	$ 140,146	$ 161,730
Weighted shares (denominator) (in shares)	46,187,808	46,188,994	46,188,299	46,188,668
Per-share amount (in dollars per share) | $ / shares	$ 1.76	$ 1.88	$ 3.03	$ 3.50
Dilutive effect of stock-based awards
Allocated net income (numerator) | $	$ 0	$ 0	$ 0	$ 0
Weighted shares (denominator) (in shares)	14,373	24,906	24,568	23,903
Assumed conversion of Class B shares
Allocated net income (numerator) | $	$ 671	$ 718	$ 1,157	$ 1,335
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) | $	$ 81,977	$ 87,754	$ 141,303	$ 163,065
Weighted shares (denominator) (in shares)	52,302,981	52,314,700	52,313,667	52,313,371
Per-share amount (in dollars per share) | $ / shares	$ 1.57	$ 1.68	$ 2.70	$ 3.12
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)	2,400		2,400		2,400
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) | $	$ 671	$ 718	$ 1,157	$ 1,335
Weighted shares (denominator) (in shares)	2,542	2,542	2,542	2,542
Income available to stockholders on equivalent shares (Diluted EPS:)
Weighted shares (denominator) (in shares)	2,542	2,542	2,542	2,542
Class B – Basic and diluted EPS:
Weighted shares (denominator) (in shares)	2,542	2,542	2,542	2,542
Per share amount (in dollars per share) | $ / shares	$ 264	$ 283	$ 455	$ 525