Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effect of Tax Cuts and Jobs Act of 2017
Increase in income tax expense									$ 10,095
Class A
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) (in dollars)									$ 195,386	$ 208,644	$ 173,248
Weighted shares (denominator) (in shares)									46,186,831	46,188,952	46,186,671
Per-share amount (in dollars per share)	$ 0.69	$ 1.26	$ 1.26	$ 1.03	$ 0.98	$ 1.23	$ 1.32	$ 0.99	$ 4.23	$ 4.52	$ 3.75
Dilutive effect of stock-based awards
Allocated net income (numerator) (in dollars)									$ 0	$ 0	$ 0
Weighted shares (denominator) (in shares)									49,832	145,551	211,340
Assumed conversion of Class B shares
Allocated net income (numerator) (in dollars)									$ 1,613	$ 1,722	$ 1,430
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)									$ 196,999	$ 210,366	$ 174,678
Weighted shares (denominator) (in shares)									52,337,463	52,435,303	52,498,811
Per-share amount (in dollars per share)	$ 0.61	1.12	1.12	0.91	$ 0.87	1.09	1.17	0.87	$ 3.76	$ 4.01	$ 3.33
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)									$ 1,613	$ 1,722	$ 1,430
Weighted shares (denominator) (in shares)									2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 103	189	189	154	$ 147	185	197	148	$ 635	$ 678	$ 563
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) (in dollars)									$ 1,613	$ 1,721	$ 1,429
Weighted shares (denominator) (in shares)									2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 103	$ 189	$ 188	$ 154	$ 147	$ 185	$ 197	$ 148	$ 634	$ 677	$ 562
Tax Cuts and Jobs Act of 2017
Effect of Tax Cuts and Jobs Act of 2017
Increase in income tax expense	$ 10,100								$ 10,100
Tax Cuts and Jobs Act of 2017 | Class A
Effect of Tax Cuts and Jobs Act of 2017
Impact on basic earnings per share									$ (0.22)
Impact on diluted earnings per share	$ (0.19)								(0.19)
Tax Cuts and Jobs Act of 2017 | Class B
Effect of Tax Cuts and Jobs Act of 2017
Impact on basic earnings per share									(33)
Impact on diluted earnings per share									$ (32)