Business segment data (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ 905,153	$ 862,349
Intersegment operating revenues	0	0
Earnings (loss) on common stock before loss from discontinued operations	25,533	18,517
Loss from discontinued operations, net of tax	(835)	(324,605)
Earnings (loss) on common stock	24,698	(306,088)
Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	385,738	406,289
Intersegment operating revenues	21,225	21,261
Earnings (loss) on common stock before loss from discontinued operations	41,648	35,135
Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	519,415	456,060
Intersegment operating revenues	2,333	14,740
Earnings (loss) on common stock before loss from discontinued operations	(16,233)	(15,627)
Electric [Member] \| Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	82,923	71,776
Intersegment operating revenues	0	0
Earnings (loss) on common stock before loss from discontinued operations	11,119	8,328
Natural gas distribution [Member] \| Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	299,395	330,573
Intersegment operating revenues	0	0
Earnings (loss) on common stock before loss from discontinued operations	25,241	21,450
Pipeline and midstream [Member] \| Regulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	3,420	3,940
Intersegment operating revenues	21,225	21,261
Earnings (loss) on common stock before loss from discontinued operations	5,288	5,357
Pipeline and midstream [Member] \| Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	8,697	13,000
Intersegment operating revenues	84	325
Earnings (loss) on common stock before loss from discontinued operations	1	1,055
Construction materials and contracting [Member] \| Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	209,852	205,658
Intersegment operating revenues	118	948
Earnings (loss) on common stock before loss from discontinued operations	(14,471)	(14,635)
Construction services [Member] \| Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	255,500	235,403
Intersegment operating revenues	462	11,695
Earnings (loss) on common stock before loss from discontinued operations	5,974	4,760
Refining [Member] \| Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	45,066	1,704
Intersegment operating revenues	0	0
Earnings (loss) on common stock before loss from discontinued operations	(7,187)	(2,394)
Other [Member] \| Nonregulated operation [Member]
Segment Reporting Information [Line Items]
Revenues	300	295
Intersegment operating revenues	1,669	1,772
Earnings (loss) on common stock before loss from discontinued operations	(550)	(4,413)
Intersegment eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment operating revenues	(23,558)	(36,001)
Earnings (loss) on common stock before loss from discontinued operations	$ 118	$ (991)

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 205
-SubTopic 20
-Section 50
-Paragraph 5C
-Subparagraph (b)(2)
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721675-107760

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(12))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=65878427&loc=d3e1012-107759

Reference 4: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 225
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Reference 7: http://www.xbrl.org/2003/role/presentationRef
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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
-SubTopic 10
-Section 45
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-URI http://asc.fasb.org/extlink&oid=64863997&loc=d3e1377-109256

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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
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-Name Accounting Standards Codification
-Topic 225
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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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