Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Class A
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) (in dollars)									$ 208,644	$ 173,248	$ 166,134
Weighted shares (denominator) (in shares)									46,188,952	46,186,671	46,247,876
Per-share amount (in dollars per share)	$ 0.98	$ 1.23	$ 1.32	$ 0.99	$ 0.65	$ 1.06	$ 1.21	$ 0.83	$ 4.52	$ 3.75	$ 3.59
Dilutive effect of stock-based awards
Allocated net income (numerator) (in dollars)									$ 0	$ 0	$ 0
Weighted shares (denominator) (in shares)									145,551	211,340	267,558
Assumed conversion of Class B shares
Allocated net income (numerator) (in dollars)									$ 1,722	$ 1,430	$ 1,371
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)									$ 210,366	$ 174,678	$ 167,505
Weighted shares (denominator) (in shares)									52,435,303	52,498,811	52,616,234
Per-share amount (in dollars per share)	$ 0.87	1.09	1.17	0.87	$ 0.57	0.94	1.07	0.74	$ 4.01	$ 3.33	$ 3.18
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)									$ 1,722	$ 1,430	$ 1,371
Weighted shares (denominator) (in shares)									2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 147	185	197	148	$ 97	160	181	125	$ 678	$ 563	$ 539
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) (in dollars)									$ 1,721	$ 1,429	$ 1,369
Weighted shares (denominator) (in shares)									2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 147	$ 185	$ 197	$ 148	$ 97	$ 159	$ 180	$ 125	$ 677	$ 562	$ 538