Quarterly Data (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 1,385,160	$ 1,587,289	$ 1,362,928	$ 1,197,373	$ 1,378,213	$ 1,563,799	$ 1,303,573	$ 1,091,191	$ 5,532,750	$ 5,336,776	$ 4,531,552
Operating expenses	1,239,270	1,383,578	1,224,674	1,140,303	1,247,992	1,374,329	1,206,262	1,026,973	4,987,825	4,855,556	4,129,829
Operating income	145,890	203,711	138,254	57,070	130,221	189,470	97,311	64,218	544,925	481,220	401,723
Income from continuing operations	112,131	153,015	99,842	25,539	94,804	136,128	63,145	41,089	390,527	335,166	269,386
Income (loss) from discontinued operations, net of tax	163	63	(139)	(409)	261	1,509	(1,320)	(163)	(322)	287	2,932
Net income	$ 112,294	$ 153,078	$ 99,703	$ 25,130	$ 95,065	$ 137,637	$ 61,825	$ 40,926	$ 390,205	$ 335,453	$ 272,318
Income from continuing operations	$ 0.56	$ 0.76	$ 0.50	$ 0.13	$ 0.47	$ 0.68	$ 0.32	$ 0.21	$ 1.95	$ 1.69	$ 1.38
Discontinued operations, net of tax	0	0	0	0	0	0.01	(0.01)	0	0	0	0.01
Earnings per share - basic	0.56	0.76	0.50	0.13	0.47	0.69	0.31	0.21	1.95	1.69	1.39
Income from continuing operations	0.56	0.76	0.50	0.13	0.47	0.68	0.32	0.21	1.95	1.69	1.38
Discontinued operations, net of tax	0	0	0	0	0	0.01	(0.01)	0	0	0	0.01
Earnings per share - diluted	$ 0.56	$ 0.76	$ 0.50	$ 0.13	$ 0.47	$ 0.69	$ 0.31	$ 0.21	$ 1.95	$ 1.69	$ 1.39
Weighted average common shares outstanding - basic	200,522	200,522	200,522	200,440	200,383	199,343	198,270	196,401	200,502	198,612	195,720
Weighted average common shares outstanding - diluted	200,923	200,619	200,539	200,456	200,478	199,383	198,287	196,414	200,571	198,626	196,150