Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Class A
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) (in dollars)	$ 58,064	$ 56,906	$ 163,596	$ 163,233
Weighted shares (denominator) (in shares)	46,188,949	46,188,980	46,186,109	46,188,971
Per-share amount (in dollars per share)	$ 1.26	$ 1.23	$ 3.54	$ 3.53
Dilutive effect of stock-based awards
Allocated net income (numerator) (in dollars)	$ 0	$ 0	$ 0	$ 0
Weighted shares (denominator) (in shares)	27,127	121,523	55,541	152,926
Assumed conversion of Class B shares
Allocated net income (numerator) (in dollars)	$ 479	$ 470	$ 1,350	$ 1,347
Weighted shares (denominator) (in shares)	6,100,800	6,100,800	6,100,800	6,100,800
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) (in dollars)	$ 58,543	$ 57,376	$ 164,946	$ 164,580
Weighted shares (denominator) (in shares)	52,316,876	52,411,303	52,342,450	52,442,697
Per-share amount (in dollars per share)	$ 1.12	$ 1.09	$ 3.15	$ 3.14
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%
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) (in dollars)	$ 479	$ 470	$ 1,350	$ 1,347
Weighted shares (denominator) (in shares)	2,542	2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 189	$ 185	$ 531	$ 530
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) (in dollars)	$ 479	$ 469	$ 1,350	$ 1,346
Weighted shares (denominator) (in shares)	2,542	2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 189	$ 185	$ 531	$ 529