Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Class A
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) (in dollars)									$ 173,248	$ 166,134	$ 161,290
Weighted shares (denominator) (in shares)									46,186,671	46,247,876	46,660,651
Per-share amount (in dollars per share)	$ 0.65	$ 1.06	$ 1.21	$ 0.83	$ 0.54	$ 1.01	$ 1.05	$ 0.99	$ 3.75	$ 3.59	$ 3.46
Dilutive effect of stock-based awards
Allocated net income (numerator) (in dollars)									$ 0	$ 0	$ 0
Weighted shares (denominator) (in shares)									211,340	267,558	94,306
Assumed conversion of Class B shares
Allocated net income (numerator) (in dollars)									$ 1,430	$ 1,371	$ 1,321
Weighted shares (denominator) (in shares)									6,100,800	6,100,800	6,100,800
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) (in dollars)									$ 174,678	$ 167,505	$ 162,611
Weighted shares (denominator) (in shares)									52,498,811	52,616,234	52,855,757
Per-share amount (in dollars per share)	$ 0.57	0.94	1.07	0.74	$ 0.48	0.90	0.94	0.88	$ 3.33	$ 3.18	$ 3.08
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%	2400.00%
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) (in dollars)									$ 1,430	$ 1,371	$ 1,321
Weighted shares (denominator) (in shares)									2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 97	160	181	125	$ 81	151	158	149	$ 563	$ 539	$ 520
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) (in dollars)									$ 1,429	$ 1,369	$ 1,320
Weighted shares (denominator) (in shares)									2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 97	$ 159	$ 180	$ 125	$ 81	$ 151	$ 158	$ 149	$ 562	$ 538	$ 519