Quarterly financial information (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Operating revenues	$ 1,378,213	$ 1,563,799	$ 1,303,573	$ 1,091,191	$ 1,209,875	$ 1,280,787	$ 1,064,597	$ 976,293	$ 5,336,776	$ 4,531,552	$ 4,443,351
Operating expenses	1,247,992	1,374,329	1,206,262	1,026,973	1,091,524	1,140,783	990,605	906,917	4,855,556	4,129,829	4,019,303
Operating income	130,221	189,470	97,311	64,218	118,351	140,004	73,992	69,376	481,220	401,723	424,048
Income from continuing operations	94,804	136,128	63,145	41,089	75,982	107,369	44,075	41,960	335,166	269,386	284,986
Income (loss) from discontinued operations, net of tax	261	1,509	(1,320)	(163)	2,846	(118)	(273)	477	287	2,932	(3,783)
Net income	$ 95,065	$ 137,637	$ 61,825	$ 40,926	$ 78,828	$ 107,251	$ 43,802	$ 42,437	$ 335,453	$ 272,318	$ 281,203
Earnings per common share - basic:
Income from continuing operations	$ 0.47	$ 0.68	$ 0.32	$ 0.21	$ 0.39	$ 0.55	$ 0.22	$ 0.22	$ 1.69	$ 1.38	$ 1.46
Discontinued operations, net of tax	0	0.01	(0.01)	0	0.01	0	0	0	0	0.01	(0.02)
Earnings per share - basic	$ 0.47	$ 0.69	$ 0.31	$ 0.21	$ 0.40	$ 0.55	$ 0.22	$ 0.22	$ 1.69	$ 1.39	$ 1.44
Weighted average common shares outstanding - basic	200,383	199,343	198,270	196,401	196,023	196,018	195,524	195,304	198,612	195,720	195,304
Earnings per common share - diluted:
Income from continuing operations	$ 0.47	$ 0.68	$ 0.32	$ 0.21	$ 0.39	$ 0.55	$ 0.22	$ 0.22	$ 1.69	$ 1.38	$ 1.45
Discontinued operations, net of tax	0	0.01	(0.01)	0	0.01	0	0	0	0	0.01	(0.02)
Earnings per share - diluted	$ 0.47	$ 0.69	$ 0.31	$ 0.21	$ 0.40	$ 0.55	$ 0.22	$ 0.22	$ 1.69	$ 1.39	$ 1.43
Weighted average common shares outstanding - diluted	200,478	199,383	198,287	196,414	196,385	196,265	196,169	195,982	198,626	196,150	195,687