Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effect of Tax Cuts and Jobs Act of 2017
Increase in income tax expense									$ 0	$ 10,095	$ 0
Class A Common Stock
Income available to stockholders (Basic EPS:)
Allocated net income (numerator)									$ 285,864	$ 195,386	$ 208,644
Weighted shares (denominator)									46,188,637	46,186,831	46,188,952
Per- share amount	$ 1.34	$ 1.73	$ 1.71	$ 1.41	$ 0.69	$ 1.26	$ 1.26	$ 1.03	$ 6.19	$ 4.23	$ 4.52
Dilutive effect of stock-based awards
Allocated net income (numerator)									$ 0	$ 0	$ 0
Weighted shares (denominator)									25,776	49,832	145,551
Assumed conversion of Class B shares
Allocated net income (numerator)									$ 2,360	$ 1,613	$ 1,722
Weighted shares (denominator)									6,100,800	6,100,800	6,100,800
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator)									$ 288,224	$ 196,999	$ 210,366
Weighted shares (denominator)									52,315,213	52,337,463	52,435,303
Per- share amount	$ 1.19	1.54	1.52	1.26	$ 0.61	1.12	1.12	0.91	$ 5.51	$ 3.76	$ 4.01
Class B Common Stock
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)									$ 2,360	$ 1,613	$ 1,722
Weighted shares (denominator)									2,542	2,542	2,542
Per- share amount	$ 201	259	257	212	$ 103	189	189	154	$ 928	$ 635	$ 678
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator)									$ 2,359	$ 1,613	$ 1,721
Weighted shares (denominator)									2,542	2,542	2,542
Per- share amount	$ 201	$ 259	$ 257	$ 212	$ 103	$ 189	$ 188	$ 154	$ 928	$ 634	$ 677
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 Common Stock
Effect of Tax Cuts and Jobs Act of 2017
Impact on basic EPS (per share)					$ (0.22)
Impact on diluted EPS (per share)					(0.19)					$ (0.19)
Tax Cuts and Jobs Act of 2017 | Class B Common Stock
Effect of Tax Cuts and Jobs Act of 2017
Impact on basic EPS (per share)					(33)
Impact on diluted EPS (per share)					$ (32)