Quarterly Results of Operations (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Results of Operations (unaudited)
Operating revenue	$ 379,613	$ 418,406	$ 423,884	$ 374,728	$ 356,380	$ 396,637	$ 401,660	$ 350,831	$ 1,596,631	$ 1,505,508	$ 1,407,119
Operating expenses	322,928	336,151	338,125	307,063	314,541	328,348	331,677	298,401	1,304,267	1,272,967	1,184,272
Investment income, net of interest expense	13,438	4,326	7,404	2,559	4,244	7,220	15,705	6,539	27,727	33,708
Income before income taxes	70,123	86,581	93,163	70,224	46,083	75,509	85,688	58,969	320,091	266,249	251,264
Net income	$ 45,786	$ 57,376	$ 61,309	$ 45,895	$ 30,133	$ 49,562	$ 56,150	$ 38,833	$ 210,366	$ 174,678	$ 167,505
Class A
Net income per share
Common stock - basic (in dollars per share)	$ 0.98	$ 1.23	$ 1.32	$ 0.99	$ 0.65	$ 1.06	$ 1.21	$ 0.83	$ 4.52	$ 3.75	$ 3.59
Common stock - diluted (in dollars per share)	0.87	1.09	1.17	0.87	0.57	0.94	1.07	0.74	4.01	3.33	3.18
Class B
Net income per share
Common stock - basic (in dollars per share)	147	185	197	148	97	160	181	125	678	563	539
Common stock - diluted (in dollars per share)	$ 147	$ 185	$ 197	$ 148	$ 97	$ 159	$ 180	$ 125	$ 677	$ 562	$ 538