Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effect of Tax Cuts and Jobs Act of 2017
Increase in income tax expense									$ 0	$ 0	$ 10,095
Class A Common Stock
Income available to stockholders (Basic EPS:)
Allocated net income (numerator)									$ 314,227	$ 285,864	$ 195,386
Weighted shares (denominator) (in shares)									46,188,836	46,188,637	46,186,831
Per- share amount (in dollars per share)	$ 1.28	$ 2.02	$ 1.88	$ 1.62	$ 1.34	$ 1.73	$ 1.71	$ 1.41	$ 6.80	$ 6.19	$ 4.23
Dilutive effect of stock-based awards
Allocated net income (numerator)									$ 0	$ 0	$ 0
Weighted shares (denominator) (in shares)									30,224	25,776	49,832
Assumed conversion of Class B shares
Allocated net income (numerator)									$ 2,594	$ 2,360	$ 1,613
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)									$ 316,821	$ 288,224	$ 196,999
Weighted shares (denominator) (in shares)									52,319,860	52,315,213	52,337,463
Per- share amount (in dollars per share)	$ 1.14	1.80	1.68	1.44	$ 1.19	1.54	1.52	1.26	$ 6.06	$ 5.51	$ 3.76
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)									$ 2,594	$ 2,360	$ 1,613
Weighted shares (denominator) (in shares)									2,542	2,542	2,542
Per- share amount (in dollars per share)	$ 192	303	283	243	$ 201	259	257	212	$ 1,020	$ 928	$ 635
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator)									$ 2,593	$ 2,359	$ 1,613
Weighted shares (denominator) (in shares)									2,542	2,542	2,542
Per- share amount (in dollars per share)	$ 192	$ 303	$ 283	$ 243	$ 201	$ 259	$ 257	$ 212	$ 1,020	$ 928	$ 634
Tax Cuts and Jobs Act of 2017
Effect of Tax Cuts and Jobs Act of 2017
Increase in income tax expense											$ 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 (in dollars per share)											$ 0.22
Impact on diluted EPS (in dollars per share)											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 (in dollars per share)											33
Impact on diluted EPS (in dollars per share)											$ 32