Quarterly Results of Operations (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Results of Operations (unaudited)
Operating revenue	$ 401,402	$ 442,492	$ 448,564	$ 399,316	$ 379,613	$ 418,406	$ 423,884	$ 374,728	$ 1,691,774	$ 1,596,631	$ 1,505,508
Operating expenses	343,444	361,656	365,520	332,782	322,928	336,151	338,125	307,063	1,403,402	1,304,267	1,272,967
Investment income	7,121	8,406	6,448	6,586	13,539	4,326	7,404	2,559	28,561	27,828	33,708
Interest expense, net	438	377	257	166	101				1,238	101
Income before income taxes	64,641	88,865	89,235	72,954	70,123	86,581	93,163	70,224	315,695	320,091	266,249
Net income	$ 32,053	$ 58,543	$ 58,527	$ 47,876	$ 45,786	$ 57,376	$ 61,309	$ 45,895	196,999	$ 210,366	$ 174,678
Effect of Tax Cuts and Jobs Act of 2017
Increase in income tax expense									$ 10,095
Class A
Net income per share
Common stock - basic (in dollars per share)	$ 0.69	$ 1.26	$ 1.26	$ 1.03	$ 0.98	$ 1.23	$ 1.32	$ 0.99	$ 4.23	$ 4.52	$ 3.75
Common stock - diluted (in dollars per share)	0.61	1.12	1.12	0.91	0.87	1.09	1.17	0.87	3.76	4.01	3.33
Class B
Net income per share
Common stock - basic (in dollars per share)	103	189	189	154	147	185	197	148	635	678	563
Common stock - diluted (in dollars per share)	$ 103	$ 189	$ 188	$ 154	$ 147	$ 185	$ 197	$ 148	$ 634	$ 677	$ 562
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
Effect of Tax Cuts and Jobs Act of 2017
Impact on diluted earnings per share	$ (0.19)								$ (0.19)
Tax Cuts and Jobs Act of 2017 | Class B
Effect of Tax Cuts and Jobs Act of 2017
Impact on diluted earnings per share									$ (32)