Quarterly Results of Operations (unaudited) (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
Quarterly Results of Operations (unaudited)
Operating revenue	$ 596,740	$ 638,742	$ 647,710	$ 594,106	$ 576,468	$ 612,126	$ 621,458	$ 572,160	$ 2,477,298	$ 2,382,212	$ 1,691,774
Operating expenses	528,040	532,835	551,100	507,984	501,710	515,431	526,135	494,593	2,119,959	2,037,869	1,401,522
Investment income	6,914	13,606	9,652	9,795	4,995	8,431	6,207	6,163	39,967	25,796	28,592
Interest expense and other (income), net	(36)	11	224	402	(2,889)	655	544	509	601	(1,181)
Income before income taxes	75,650	119,502	106,038	95,515	82,642	104,471	100,986	83,221	396,705	371,320	315,695
Net income	$ 59,587	$ 94,169	$ 87,754	$ 75,311	62,314	$ 80,446	$ 79,706	$ 65,758	$ 316,821	288,224	$ 196,999
Net income per share
Gain on sale of field offices					$ 3,400					$ 3,400
Class A Common Stock
Net income per share
Common stock - basic (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
Common stock - diluted (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
Net income per share
Common stock - basic (in dollars per share)	192	303	283	243	201	259	257	212	1,020	928	635
Common stock - diluted (in dollars per share)	$ 192	$ 303	$ 283	$ 243	$ 201	$ 259	$ 257	$ 212	$ 1,020	$ 928	$ 634