Quarterly financial information (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Operating revenues	$ 1,165,239	$ 1,272,548	$ 1,067,639	$ 937,925	$ 1,016,099	$ 1,208,567	$ 1,043,948	$ 860,214	$ 4,443,351	$ 4,128,828	$ 4,014,052
Operating expenses	1,039,695	1,116,171	987,960	870,813	904,613	1,061,883	954,983	798,229	4,014,639	3,719,708	3,694,277
Operating income	125,544	156,377	79,679	67,112	111,486	146,684	88,965	61,985	428,712	409,120	319,775
Income from continuing operations	115,394	89,549	44,405	35,638	66,547	88,386	46,298	31,865	$ 284,986	$ 233,096	$ 176,389
Loss from discontinued operations attributable to the Company, net of tax	(82)	(2,198)	(3,190)	1,687	(816)	(5,400)	(155,451)	(6,996)
Net income (loss) attributable to parent	$ 115,312	$ 87,351	$ 41,215	$ 37,325	$ 65,731	$ 82,986	$ (109,153)	$ 24,869
Earnings (loss) per common share - basic:
Earnings before discontinued operations	$ 0.59	$ 0.46	$ 0.22	$ 0.18	$ 0.34	$ 0.45	$ 0.24	$ 0.16	$ 1.46	$ 1.19	$ 0.90
Discontinued operations attributable to the Company, net of tax	0	(0.01)	(0.01)	0.01	0	(0.03)	(0.80)	(0.03)	(0.02)	(0.86)	(4.10)
Earnings (loss) per common share - basic	$ 0.59	$ 0.45	$ 0.21	$ 0.19	$ 0.34	$ 0.42	$ (0.56)	$ 0.13	$ 1.44	$ 0.33	$ (3.20)
Weighted average common shares outstanding - basic	195,304	195,304	195,304	195,304	195,304	195,304	195,304	195,284	195,304	195,299	194,928
Earnings (loss) per common share - diluted:
Earnings before discontinued operations	$ 0.59	$ 0.46	$ 0.22	$ 0.18	$ 0.33	$ 0.45	$ 0.24	$ 0.16	$ 1.45	$ 1.19	$ 0.90
Discontinued operations attributable to the Company, net of tax	0	(0.01)	(0.01)	0.01	0	(0.03)	(0.80)	(0.03)	(0.02)	(0.86)	(4.10)
Earnings (loss) per common share - diluted	$ 0.59	$ 0.45	$ 0.21	$ 0.19	$ 0.33	$ 0.42	$ (0.56)	$ 0.13	$ 1.43	$ 0.33	$ (3.20)
Weighted average common shares outstanding - diluted	195,617	195,783	195,973	196,023	195,889	195,811	195,699	195,284	195,687	195,618	194,986
Additional information [Abstract]
Income tax expense (benefit), continuing operations, adjustment of deferred tax (asset) liability	$ (39,500)								$ 7,735	$ 0	$ 0
Operating Engineers Local 800 and WY Contractors Association Pension Plan for WY [Member] | Withdrawal from multiemployer defined benefit plan [Member]
Additional information [Abstract]
Loss contingency accrual, period increase (decrease)					$ (11,100)