Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Class A
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) (in dollars)	$ 58,048	$ 60,807	$ 105,532	$ 106,327
Weighted shares (denominator) (in shares)	46,180,852	46,188,867	46,184,666	46,188,967
Per-share amount (in dollars per share)	$ 1.26	$ 1.32	$ 2.28	$ 2.30
Dilutive effect of stock-based awards
Allocated net income (numerator) (in dollars)	$ 0	$ 0	$ 0	$ 0
Weighted shares (denominator) (in shares)	17,743	103,195	69,748	168,627
Assumed conversion of Class B shares
Allocated net income (numerator) (in dollars)	$ 479	$ 502	$ 871	$ 877
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)	$ 58,527	$ 61,309	$ 106,403	$ 107,204
Weighted shares (denominator) (in shares)	52,299,395	52,392,862	52,355,214	52,458,394
Per-share amount (in dollars per share)	$ 1.12	$ 1.17	$ 2.03	$ 2.04
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)	$ 479	$ 502	$ 871	$ 877
Weighted shares (denominator) (in shares)	2,542	2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 189	$ 197	$ 343	$ 345
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) (in dollars)	$ 479	$ 502	$ 871	$ 877
Weighted shares (denominator) (in shares)	2,542	2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 188	$ 197	$ 343	$ 345