Business segment data (Details) (USD $) In Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Segment Reporting Information [Line Items]
External operating revenues	$ 930,757	$ 906,444	$ 1,832,562	$ 1,741,221
Earnings on common stock	44,896	48,767	87,702	90,367
Income (loss) on discontinued operations	(168)	0	280	0
Regulated Operation [Member]
Segment Reporting Information [Line Items]
External operating revenues	274,538	272,177	752,018	732,422
Inter-segment operating revenues	12,504	14,143	37,245	41,228
Earnings on common stock	11,481	14,562	54,440	52,580
Regulated Operation [Member] \| Electric [Member]
Segment Reporting Information [Line Items]
External operating revenues	49,986	45,683	107,831	95,379
Inter-segment operating revenues	0	0	0	0
Earnings on common stock	4,807	4,947	13,331	10,832
Regulated Operation [Member] \| Natural gas distribution [Member]
Segment Reporting Information [Line Items]
External operating revenues	164,626	160,138	535,010	509,162
Inter-segment operating revenues	0	0	0	0
Earnings on common stock	1,902	74	29,418	23,416
Regulated Operation [Member] \| Pipeline and energy services [Member]
Segment Reporting Information [Line Items]
External operating revenues	59,926	66,356	109,177	127,881
Inter-segment operating revenues	12,504	14,143	37,245	41,228
Earnings on common stock	4,772	9,541	11,691	18,332
Unregulated Operation [Member]
Segment Reporting Information [Line Items]
External operating revenues	656,219	634,267	1,080,544	1,008,799
Inter-segment operating revenues	33,072	28,621	62,118	66,810
Earnings on common stock	33,415	34,205	33,262	37,787
Unregulated Operation [Member] \| Construction services [Member]
Segment Reporting Information [Line Items]
External operating revenues	192,697	188,182	394,877	341,247
Inter-segment operating revenues	5,379	8	6,596	32
Earnings on common stock	6,138	2,923	10,771	3,051
Unregulated Operation [Member] \| Natural gas and oil production [Member]
Segment Reporting Information [Line Items]
External operating revenues	87,390	84,406	165,801	156,066
Inter-segment operating revenues	25,392	26,400	50,933	62,327
Earnings on common stock	21,326	24,035	37,596	46,246
Unregulated Operation [Member] \| Construction materials and contracting [Member]
Segment Reporting Information [Line Items]
External operating revenues	375,613	361,625	519,146	511,432
Inter-segment operating revenues	0	0	0	0
Earnings on common stock	4,980	5,659	(16,423)	(14,478)
Unregulated Operation [Member] \| Other [Member]
Segment Reporting Information [Line Items]
External operating revenues	519	54	720	54
Inter-segment operating revenues	2,301	2,213	4,589	4,451
Earnings on common stock	971	1,588	1,318	2,968
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
External operating revenues	0	0	0	0
Inter-segment operating revenues	(45,576)	(42,764)	(99,363)	(108,038)
Earnings on common stock	$ 0	$ 0	$ 0	$ 0

Earnings (loss) on common stock

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- Definition

This element represents the overall income (loss) from a disposal group apportioned to the parent that is classified as a component of the entity, net of income tax, reported as a separate component of income before extraordinary items after deduction or consideration of the amount which may be allocable to noncontrolling interests, if any. Includes the following (net of tax): income (loss) from operations during the phase-out period, gain (loss) on disposal, provision (or any reversals of earlier provisions) for loss on disposal, and adjustments of a prior period gain (loss) on disposal.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 19
-URI http://asc.fasb.org/extlink&oid=6921628&loc=SL4569616-111683

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Accounting Research Bulletin (ARB)
-Number 51
-Paragraph 29
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Accounting Research Bulletin (ARB)
-Number 51
-Paragraph 28
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Accounting Research Bulletin (ARB)
-Number 51
-Paragraph 38
-Subparagraph b(2)
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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- Definition

Aggregate revenue recognized during the period (derived from goods sold, services rendered, insurance premiums, or other activities that constitute an entity's earning process). For financial services companies, also includes investment and interest income, and sales and trading gains.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 5
-Section 03
-Paragraph 1

Reference 2: 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)
-URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688

+ Details

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- Definition

Amount of revenue from transactions with other operating segments of the same entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 131
-Paragraph 27
-Subparagraph b
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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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.

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