Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Revenues	$ 937,925	$ 860,214
Intersegment operating revenues	0	0
Earnings on common stock before income (loss) from discontinued operations	35,467	31,694
Income (loss) from discontinued operations, net of tax	1,687	(18,036)
Loss from discontinued operations attributable to noncontrolling interest	0	(11,040)
Earnings on common stock	37,154	24,698
Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	433,614	385,865
Intersegment operating revenues	21,489	21,098
Earnings on common stock before income (loss) from discontinued operations	46,751	41,648
Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	504,311	474,349
Intersegment operating revenues	1,869	2,333
Earnings on common stock before income (loss) from discontinued operations	(13,457)	(9,954)
Electric [Member] \| Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	88,225	82,923
Intersegment operating revenues	0	0
Earnings on common stock before income (loss) from discontinued operations	14,333	11,119
Natural gas distribution [Member] \| Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	342,519	299,395
Intersegment operating revenues	0	0
Earnings on common stock before income (loss) from discontinued operations	27,861	25,241
Pipeline and midstream [Member] \| Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	2,870	3,547
Intersegment operating revenues	21,489	21,098
Earnings on common stock before income (loss) from discontinued operations	4,557	5,288
Pipeline and midstream [Member] \| Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	3,643	8,697
Intersegment operating revenues	34	84
Earnings on common stock before income (loss) from discontinued operations	(628)	1
Construction materials and contracting [Member] \| Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	200,776	209,852
Intersegment operating revenues	86	118
Earnings on common stock before income (loss) from discontinued operations	(19,912)	(14,471)
Construction services [Member] \| Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	299,572	255,500
Intersegment operating revenues	6	462
Earnings on common stock before income (loss) from discontinued operations	7,362	5,974
Other [Member] \| Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	320	300
Intersegment operating revenues	1,743	1,669
Earnings on common stock before income (loss) from discontinued operations	(279)	(1,458)
Intersegment eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment operating revenues	(23,358)	(23,431)
Earnings on common stock before income (loss) from discontinued operations	$ 2,173	$ 0

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- Definition

Intersegment operating revenues.

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Amount after tax of income (loss) from a discontinued operation including the portion attributable to the noncontrolling interest. Includes, but is not limited to, the income (loss) from operations during the phase-out period, gain (loss) on disposal, gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and adjustments to a prior period gain (loss) on disposal.

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Amount after tax of income (loss) from a discontinued operation attributable to the noncontrolling interest. Includes, but is not limited to, the income (loss) from operations during the phase-out period, gain (loss) on disposal, gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and adjustments to a prior period gain (loss) on disposal.

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- Definition

Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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