Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Class A
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) (in dollars)	$ 45,520	$ 38,515
Weighted shares (denominator) (in shares)	46,189,068	46,189,068
Per-share amount (in dollars per share)	$ 0.99	$ 0.83
Dilutive effect of stock-based awards
Allocated net income (numerator) (in dollars)	$ 0	$ 0
Weighted shares (denominator) (in shares)	234,059	344,884
Assumed conversion of Class B shares
Allocated net income (numerator) (in dollars)	$ 375	$ 318
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)	$ 45,895	$ 38,833
Weighted shares (denominator) (in shares)	52,523,927	52,634,752
Per-share amount (in dollars per share)	$ 0.87	$ 0.74
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)	$ 375	$ 318
Weighted shares (denominator) (in shares)	2,542	2,542
Per-share amount (in dollars per share)	$ 148	$ 125
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) (in dollars)	$ 375	$ 318
Weighted shares (denominator) (in shares)	2,542	2,542
Per-share amount (in dollars per share)	$ 148	$ 125