Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Class A
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) (in dollars)	$ 65,220	$ 47,484
Weighted shares (denominator) (in shares)	46,187,908	46,188,522
Per-share amount (in dollars per share)	$ 1.41	$ 1.03
Dilutive effect of stock-based awards
Allocated net income (numerator) (in dollars)	$ 0	$ 0
Weighted shares (denominator) (in shares)	21,920	119,238
Assumed conversion of Class B shares
Allocated net income (numerator) (in dollars)	$ 538	$ 392
Weighted shares (denominator) (in shares)	6,100,800	6,100,800
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) (in dollars)	$ 65,758	$ 47,876
Weighted shares (denominator) (in shares)	52,310,628	52,408,560
Per-share amount (in dollars per share)	$ 1.26	$ 0.91
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%
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) (in dollars)	$ 538	$ 392
Weighted shares (denominator) (in shares)	2,542	2,542
Per-share amount (in dollars per share)	$ 212	$ 154
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) (in dollars)	$ 538	$ 392
Weighted shares (denominator) (in shares)	2,542	2,542
Per-share amount (in dollars per share)	$ 212	$ 154