Quarterly Results of Operations (unaudited) (Details) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares	Sep. 30, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($) $ / shares	Mar. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares	Sep. 30, 2017 USD ($) $ / shares	Jun. 30, 2017 USD ($) $ / shares	Mar. 31, 2017 USD ($) $ / shares	Dec. 31, 2018 USD ($) Obligations $ / shares	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares
Quarterly Results of Operations (unaudited)
Operating revenue	$ 576,468	$ 612,126	$ 621,458	$ 572,160	$ 401,402	$ 442,492	$ 448,564	$ 399,316	$ 2,382,212	$ 1,691,774	$ 1,596,631
Operating expenses	501,710	515,431	526,135	494,593	342,777	361,253	365,116	332,376	2,037,869	1,401,522	1,303,114
Investment income	4,995	8,431	6,207	6,163	7,134	8,418	6,451	6,589	25,796	28,592	27,839
Interest expense and other (income), net	(2,889)	655	544	509	1,118	792	664	575	(1,181)	3,149
Income before income taxes	82,642	104,471	100,986	83,221	64,641	88,865	89,235	72,954	371,320	315,695	320,091
Net income	$ 62,314	$ 80,446	$ 79,706	$ 65,758	$ 32,053	$ 58,543	$ 58,527	$ 47,876	288,224	196,999	210,366
Gain on sale of field offices									$ 3,400
Revenue, Performance Obligation [Abstract]
Performance obligations under subscriber's agreement | Obligations									2
Effect of Tax Cuts and Jobs Act of 2017
Increase in income tax expense									$ 0	$ 10,095	$ 0
Class A Common Stock
Net income per share
Common stock - basic (in dollars per share) | $ / shares	$ 1.34	$ 1.73	$ 1.71	$ 1.41	$ 0.69	$ 1.26	$ 1.26	$ 1.03	$ 6.19	$ 4.23	$ 4.52
Common stock - diluted (in dollars per share) | $ / shares	1.19	1.54	1.52	1.26	0.61	1.12	1.12	0.91	5.51	3.76	4.01
Class B Common Stock
Net income per share
Common stock - basic (in dollars per share) | $ / shares	201	259	257	212	103	189	189	154	928	635	678
Common stock - diluted (in dollars per share) | $ / shares	$ 201	$ 259	$ 257	$ 212	$ 103	$ 189	$ 188	$ 154	$ 928	$ 634	$ 677
Tax Cuts and Jobs Act of 2017
Effect of Tax Cuts and Jobs Act of 2017
Increase in income tax expense					$ 10,100					$ 10,100
Tax Cuts and Jobs Act of 2017 | Class A Common Stock
Effect of Tax Cuts and Jobs Act of 2017
Impact on diluted EPS (per share) | $ / shares					$ (0.19)					$ (0.19)
Tax Cuts and Jobs Act of 2017 | Class B Common Stock
Effect of Tax Cuts and Jobs Act of 2017
Impact on diluted EPS (per share) | $ / shares					$ (32)