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Business segment data (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Segment Reporting Information [Line Items]        
Revenues $ 1,208,567 $ 1,198,342 $ 3,112,730 $ 2,997,225
Intersegment operating revenues 0 0 0 0
Earnings (loss) on common stock before loss from discontinued operations 88,215 73,715 166,036 119,974
Loss from discontinued operations, net of tax (5,400) (223,112) (299,538) (816,517)
Loss from discontinued operations attributable to noncontrolling interest 0 (9,778) (131,691) (21,060)
Earnings (loss) on common stock 82,815 (139,619) (1,811) (675,483)
Regulated operation [Member]        
Segment Reporting Information [Line Items]        
Revenues 192,079 185,417 783,997 807,585
Intersegment operating revenues 3,278 3,740 30,969 31,365
Earnings (loss) on common stock before loss from discontinued operations 5,564 5,699 53,021 45,696
Nonregulated operation [Member]        
Segment Reporting Information [Line Items]        
Revenues 1,016,488 1,012,925 2,328,733 2,189,640
Intersegment operating revenues 2,403 4,880 6,614 26,151
Earnings (loss) on common stock before loss from discontinued operations 77,052 62,775 107,416 70,771
Electric [Member] | Regulated operation [Member]        
Segment Reporting Information [Line Items]        
Revenues 82,156 74,604 238,911 210,646
Intersegment operating revenues 0 0 0 0
Earnings (loss) on common stock before loss from discontinued operations 12,699 12,605 31,840 26,842
Natural gas distribution [Member] | Regulated operation [Member]        
Segment Reporting Information [Line Items]        
Revenues 87,941 89,520 500,106 553,058
Intersegment operating revenues 0 0 0 0
Earnings (loss) on common stock before loss from discontinued operations (12,524) (12,298) 4,940 3,777
Pipeline and midstream [Member] | Regulated operation [Member]        
Segment Reporting Information [Line Items]        
Revenues 21,982 21,293 44,980 43,881
Intersegment operating revenues 3,278 3,740 30,969 31,365
Earnings (loss) on common stock before loss from discontinued operations 5,389 5,392 16,241 15,077
Pipeline and midstream [Member] | Nonregulated operation [Member]        
Segment Reporting Information [Line Items]        
Revenues 10,732 14,545 29,697 42,294
Intersegment operating revenues 41 145 161 460
Earnings (loss) on common stock before loss from discontinued operations 1,304 (8,587) 2,043 (8,498)
Construction materials and contracting [Member] | Nonregulated operation [Member]        
Segment Reporting Information [Line Items]        
Revenues 724,535 774,288 1,475,643 1,475,585
Intersegment operating revenues 155 244 370 2,450
Earnings (loss) on common stock before loss from discontinued operations 69,523 68,823 88,747 74,324
Construction services [Member] | Nonregulated operation [Member]        
Segment Reporting Information [Line Items]        
Revenues 280,801 223,676 822,226 670,594
Intersegment operating revenues 3 2,112 541 17,298
Earnings (loss) on common stock before loss from discontinued operations 7,234 4,742 20,198 16,505
Other [Member] | Nonregulated operation [Member]        
Segment Reporting Information [Line Items]        
Revenues 420 416 1,167 1,167
Intersegment operating revenues 2,204 2,379 5,542 5,943
Earnings (loss) on common stock before loss from discontinued operations (1,009) (2,203) (3,572) (11,560)
Intersegment eliminations [Member]        
Segment Reporting Information [Line Items]        
Intersegment operating revenues (5,681) (8,620) (37,583) (57,516)
Earnings (loss) on common stock before loss from discontinued operations $ 5,599 $ 5,241 $ 5,599 $ 3,507