Quarterly Results of Operations (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Results of Operations (unaudited)
Operating revenue	$ 356,380	$ 396,637	$ 401,660	$ 350,831	$ 337,256	$ 369,638	$ 373,978	$ 326,247	$ 1,505,508	$ 1,407,119	$ 1,297,331
Operating expenses	314,541	328,348	331,677	298,401	301,843	308,550	306,060	267,819	1,272,967	1,184,272	1,087,995
Investment income	4,244	7,220	15,705	6,539	2,342	7,604	6,872	11,599	33,708	28,417	37,278
Income before income taxes	46,083	75,509	85,688	58,969	37,755	68,692	74,790	70,027	266,249	251,264	246,614
Net income	$ 30,133	$ 49,562	$ 56,150	$ 38,833	$ 25,296	$ 46,900	$ 49,047	$ 46,262	$ 174,678	$ 167,505	$ 162,611
Class A
Net income per share
Common stock - basic (in dollars per share)	$ 0.65	$ 1.06	$ 1.21	$ 0.83	$ 0.54	$ 1.01	$ 1.05	$ 0.99	$ 3.75	$ 3.59	$ 3.46
Common stock - diluted (in dollars per share)	0.57	0.94	1.07	0.74	0.48	0.90	0.94	0.88	3.33	3.18	3.08
Class B
Net income per share
Common stock - basic (in dollars per share)	97	160	181	125	81	151	158	149	563	539	520
Common stock - diluted (in dollars per share)	$ 97	$ 159	$ 180	$ 125	$ 81	$ 151	$ 158	$ 149	$ 562	$ 538	$ 519