Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Class A
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) (in dollars)	$ 47,484	$ 45,520
Weighted shares (denominator) (in shares)	46,188,522	46,189,068
Per-share amount (in dollars per share)	$ 1.03	$ 0.99
Dilutive effect of stock-based awards
Allocated net income (numerator) (in dollars)	$ 0	$ 0
Weighted shares (denominator) (in shares)	119,238	234,059
Assumed conversion of Class B shares
Allocated net income (numerator) (in dollars)	$ 392	$ 375
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)	$ 47,876	$ 45,895
Weighted shares (denominator) (in shares)	52,408,560	52,523,927
Per-share amount (in dollars per share)	$ 0.91	$ 0.87
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)	$ 392	$ 375
Weighted shares (denominator) (in shares)	2,542	2,542
Per-share amount (in dollars per share)	$ 154	$ 148
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) (in dollars)	$ 392	$ 375
Weighted shares (denominator) (in shares)	2,542	2,542
Per-share amount (in dollars per share)	$ 154	$ 148