Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Class A
Income available to stockholders (Basic EPS:)
Allocated net income (numerator) (in dollars)	$ 56,906	$ 49,156	$ 163,233	$ 143,361
Weighted shares (denominator) (in shares)	46,188,980	46,189,068	46,188,971	46,189,068
Per-share amount (in dollars per share)	$ 1.23	$ 1.06	$ 3.53	$ 3.10
Dilutive effect of stock-based awards
Allocated net income (numerator) (in dollars)	$ 0	$ 0	$ 0	$ 0
Weighted shares (denominator) (in shares)	121,523	312,215	152,926	309,915
Assumed conversion of Class B shares
Allocated net income (numerator) (in dollars)	$ 470	$ 406	$ 1,347	$ 1,184
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)	$ 57,376	$ 49,562	$ 164,580	$ 144,545
Weighted shares (denominator) (in shares)	52,411,303	52,602,083	52,442,697	52,599,783
Per-share amount (in dollars per share)	$ 1.09	$ 0.94	$ 3.14	$ 2.75
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)	$ 470	$ 406	$ 1,347	$ 1,184
Weighted shares (denominator) (in shares)	2,542	2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 185	$ 160	$ 530	$ 466
Income available to stockholders on equivalent shares (Diluted EPS:)
Allocated net income (numerator) (in dollars)	$ 469	$ 405	$ 1,346	$ 1,181
Weighted shares (denominator) (in shares)	2,542	2,542	2,542	2,542
Per-share amount (in dollars per share)	$ 185	$ 159	$ 529	$ 465