Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting Information [Line Items]
Revenues	$ 986,215	$ 952,564	$ 1,848,564	$ 1,853,326
Intersegment operating revenues	0	0	0	0
Earnings (loss) on common stock	(229,775)	53,935	(535,863)	110,426
Earnings (loss) on common stock before income (loss) from discontinued operations	21,640	30,054	40,157	61,433
Income (loss) from discontinued operations, net of tax	(251,415)	23,881	(576,020)	48,993
Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	215,472	226,472	621,762	678,775
Intersegment operating revenues	6,564	6,937	27,625	24,210
Earnings (loss) on common stock before income (loss) from discontinued operations	4,864	6,943	39,997	48,236
Unregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	770,743	726,092	1,226,802	1,174,551
Intersegment operating revenues	6,650	17,300	21,270	26,972
Earnings (loss) on common stock before income (loss) from discontinued operations	17,460	24,065	1,835	14,624
Electric [Member] \| Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	64,265	65,149	136,041	138,796
Intersegment operating revenues	0	0	0	0
Earnings (loss) on common stock before income (loss) from discontinued operations	5,910	7,823	14,237	18,856
Natural gas distribution [Member] \| Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	132,965	146,077	463,538	520,311
Intersegment operating revenues	0	0	0	0
Earnings (loss) on common stock before income (loss) from discontinued operations	(5,375)	(4,494)	16,075	22,768
Pipeline and energy services [Member] \| Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	18,242	15,246	22,183	19,668
Intersegment operating revenues	6,564	6,937	27,625	24,210
Earnings (loss) on common stock before income (loss) from discontinued operations	4,329	3,614	9,685	6,612
Pipeline and energy services [Member] \| Unregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	63,131	16,044	77,834	29,859
Intersegment operating revenues	110	177	316	371
Earnings (loss) on common stock before income (loss) from discontinued operations	(5,933)	2,175	(7,271)	3,526
Construction materials and contracting [Member] \| Unregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	495,640	434,452	701,298	598,875
Intersegment operating revenues	1,257	8,106	2,205	12,123
Earnings (loss) on common stock before income (loss) from discontinued operations	20,136	10,554	5,501	(13,019)
Construction services [Member] \| Unregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	211,515	275,109	446,918	545,002
Intersegment operating revenues	3,491	7,273	15,186	11,010
Earnings (loss) on common stock before income (loss) from discontinued operations	7,003	14,307	11,763	30,875
Other [Member] \| Unregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	457	487	752	815
Intersegment operating revenues	1,792	1,744	3,563	3,468
Earnings (loss) on common stock before income (loss) from discontinued operations	(3,746)	(2,971)	(8,158)	(6,758)
Intersegment eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment operating revenues	(13,214)	(24,237)	(48,895)	(51,182)
Earnings (loss) on common stock before income (loss) from discontinued operations	$ (684)	$ (954)	$ (1,675)	$ (1,427)

Earnings loss on common stock before income (loss) from discontinued operations

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- Definition

Intersegment operating revenues.

+ References

No definition available.

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

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-Name Regulation S-X (SX)

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-Section 03

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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 260

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- Definition

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).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1)

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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