Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Class A
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) (in dollars)	$ 79,053	$ 58,048	$ 144,273	$ 105,532
Weighted shares (denominator) (in shares)	46,188,705	46,180,852	46,188,309	46,184,666
Per-share amount (in dollars per share)	$ 1.71	$ 1.26	$ 3.12	$ 2.28
Dilutive effect of stock-based awards
Allocated net income (numerator) (in dollars)	$ 0	$ 0	$ 0	$ 0
Weighted shares (denominator) (in shares)	23,344	17,743	22,632	69,748
Assumed conversion of Class B shares
Allocated net income (numerator) (in dollars)	$ 653	$ 479	$ 1,191	$ 871
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)	$ 79,706	$ 58,527	$ 145,464	$ 106,403
Weighted shares (denominator) (in shares)	52,312,849	52,299,395	52,311,741	52,355,214
Per-share amount (in dollars per share)	$ 1.52	$ 1.12	$ 2.78	$ 2.03
Class B
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)	$ 653	$ 479	$ 1,191	$ 871
Weighted shares (denominator) (in shares)	2,542	2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 257	$ 189	$ 469	$ 343
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) (in dollars)	$ 653	$ 479	$ 1,191	$ 871
Weighted shares (denominator) (in shares)	2,542	2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 257	$ 188	$ 468	$ 343