Business segment data (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Revenues	$ 918,510	$ 1,042,853
Intersegment operating revenues	0	0
Earnings on common stock	(306,088)	56,491
Regulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	427,444	491,541
Intersegment operating revenues	21,341	18,276
Earnings on common stock	33,796	42,645
Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	491,066	551,312
Intersegment operating revenues	19,641	30,346
Earnings on common stock	(339,034)	14,197
Electric [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	71,776	73,647
Intersegment operating revenues	0	0
Earnings on common stock	8,328	11,033
Natural gas distribution [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	330,573	374,233
Intersegment operating revenues	0	0
Earnings on common stock	21,450	27,263
Pipeline and energy services [Member] | Regulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	25,095	43,661
Intersegment operating revenues	21,341	18,276
Earnings on common stock	4,018	4,349
Exploration and production [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	49,710	116,669
Intersegment operating revenues	5,226	20,867
Earnings on common stock	(328,904)	20,939
Construction materials and contracting [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	205,658	164,423
Intersegment operating revenues	948	4,017
Earnings on common stock	(14,635)	(23,574)
Construction services [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	235,403	269,892
Intersegment operating revenues	11,695	3,738
Earnings on common stock	4,760	16,568
Other [Member] | Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Revenues	295	328
Intersegment operating revenues	1,772	1,724
Earnings on common stock	(255)	264
Intersegment eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	0	0
Intersegment operating revenues	(40,982)	(48,622)
Earnings on common stock	$ (850)	$ (351)