Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Class A
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) (in dollars)	$ 60,807	$ 55,690	$ 106,327	$ 94,205
Weighted shares (denominator) (in shares)	46,188,867	46,189,068	46,188,967	46,189,068
Per-share amount (in dollars per share)	$ 1.32	$ 1.21	$ 2.30	$ 2.04
Dilutive effect of stock-based awards
Allocated net income (numerator) (in dollars)	$ 0	$ 0	$ 0	$ 0
Weighted shares (denominator) (in shares)	103,195	272,646	168,627	308,765
Assumed conversion of Class B shares
Allocated net income (numerator) (in dollars)	$ 502	$ 460	$ 877	$ 778
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)	$ 61,309	$ 56,150	$ 107,204	$ 94,983
Weighted shares (denominator) (in shares)	52,392,862	52,562,514	52,458,394	52,598,633
Per-share amount (in dollars per share)	$ 1.17	$ 1.07	$ 2.04	$ 1.81
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)	$ 502	$ 460	$ 877	$ 778
Weighted shares (denominator) (in shares)	2,542	2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 197	$ 181	$ 345	$ 306
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) (in dollars)	$ 502	$ 458	$ 877	$ 776
Weighted shares (denominator) (in shares)	2,542	2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 197	$ 180	$ 345	$ 305