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Business segment data (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Segment Reporting Information [Line Items]        
Revenues $ 1,370,455 $ 1,285,782 $ 3,507,354 $ 3,277,980
Intersegment operating revenues 0 0 0 0
Earnings on common stock 103,038 84,285 213,464 186,970
Regulated Operation [Member]
        
Segment Reporting Information [Line Items]        
Revenues 211,536 192,103 957,769 843,670
Intersegment operating revenues 4,334 4,906 30,497 31,623
Earnings on common stock 1,970 5,523 53,732 42,319
Unregulated Operation [Member]
        
Segment Reporting Information [Line Items]        
Revenues 1,158,919 1,093,679 2,549,585 2,434,310
Intersegment operating revenues 33,473 23,092 91,214 70,283
Earnings on common stock 102,590 79,964 162,437 147,910
Electric [Member] | Regulated Operation [Member]
        
Segment Reporting Information [Line Items]        
Revenues 68,936 68,314 207,732 189,949
Intersegment operating revenues 0 0 0 0
Earnings on common stock 9,162 11,417 28,018 25,652
Natural gas distribution [Member] | Regulated Operation [Member]
        
Segment Reporting Information [Line Items]        
Revenues 96,185 77,417 616,496 536,756
Intersegment operating revenues 0 0 0 0
Earnings on common stock (12,252) (11,204) 10,516 15,420
Pipeline and energy services [Member] | Regulated Operation [Member]
        
Segment Reporting Information [Line Items]        
Revenues 46,415 46,372 133,541 116,965
Intersegment operating revenues 4,334 4,906 30,497 31,623
Earnings on common stock 5,060 5,310 15,198 1,247
Exploration and production [Member] | Unregulated Operation [Member]
        
Segment Reporting Information [Line Items]        
Revenues 147,677 119,234 393,653 371,648
Intersegment operating revenues 8,130 10,714 39,269 33,083
Earnings on common stock 34,750 17,434 74,869 70,713
Construction materials and contracting [Member] | Unregulated Operation [Member]
        
Segment Reporting Information [Line Items]        
Revenues 740,496 706,982 1,339,371 1,287,305
Intersegment operating revenues 6,322 7,422 18,445 24,673
Earnings on common stock 55,218 49,159 42,199 38,602
Construction services [Member] | Unregulated Operation [Member]
        
Segment Reporting Information [Line Items]        
Revenues 270,313 267,038 815,313 774,103
Intersegment operating revenues 16,420 3,097 27,431 7,011
Earnings on common stock 9,876 12,154 40,751 36,733
Other [Member] | Unregulated Operation [Member]
        
Segment Reporting Information [Line Items]        
Revenues 433 425 1,248 1,254
Intersegment operating revenues 2,601 1,859 6,069 5,516
Earnings on common stock 2,746 1,217 4,618 1,862
Intersegment eliminations [Member]
        
Segment Reporting Information [Line Items]        
Revenues 0 0 0 0
Intersegment operating revenues (37,807) (27,998) (121,711) (101,906)
Earnings on common stock $ (1,522) $ (1,202) $ (2,705) $ (3,259)