Property, plant and equipment (Details 4) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property, plant and equipment [Line Items]
Interest capitalized		$ 0	$ 4,381	$ 7,046
AFUDC - borrowed		914	4,907	3,023
AFUDC - equity		565	7,971	5,803
Property, plant and equipment		6,510,229	6,387,702
Less accumulated depreciation, depletion and amortization		2,578,902	2,489,322	2,385,202
Net property, plant and equipment		3,931,327	3,898,380	3,520,320
Electric:
Property, plant and equipment [Line Items]
Property, plant and equipment	[1]	1,888,613	1,786,148	1,457,101
Natural gas distribution:
Property, plant and equipment [Line Items]
Property, plant and equipment	[1]	2,179,413	2,076,581	1,904,759
Pipeline and midstream:
Property, plant and equipment [Line Items]
Property, plant and equipment		672,199	758,729	818,388
Construction materials and contracting:
Property, plant and equipment [Line Items]
Property, plant and equipment		1,549,375	1,553,428	1,529,942
Construction services:
Property, plant and equipment [Line Items]
Property, plant and equipment		171,361	163,279	144,395
Other:
Property, plant and equipment [Line Items]
Property, plant and equipment		49,268	49,537	$ 50,937
Regulated: | Electric: | Generation
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 1,036,373	1,003,173
Weighted average depreciable life in years		39 years
Regulated: | Electric: | Distribution
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 398,382	375,612
Weighted average depreciable life in years		44 years
Regulated: | Electric: | Transmission
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 284,048	255,842
Weighted average depreciable life in years		57 years
Regulated: | Electric: | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 62,212	42,436
Regulated: | Electric: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 107,598	109,085
Weighted average depreciable life in years		14 years
Regulated: | Natural gas distribution: | Distribution
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 1,718,633	1,624,645
Weighted average depreciable life in years		46 years
Regulated: | Natural gas distribution: | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 19,934	20,530
Regulated: | Natural gas distribution: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 440,846	431,406
Weighted average depreciable life in years		18 years
Regulated: | Pipeline and midstream: | Transmission
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 490,143	460,305
Weighted average depreciable life in years		54 years
Regulated: | Pipeline and midstream: | Gathering
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 37,831	37,831
Weighted average depreciable life in years		20 years
Regulated: | Pipeline and midstream: | Storage
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 45,350	44,011
Weighted average depreciable life in years		62 years
Regulated: | Pipeline and midstream: | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 16,507	7,549
Regulated: | Pipeline and midstream: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 40,873	40,168
Weighted average depreciable life in years		33 years
Nonregulated: | Pipeline and midstream: | Gathering and processing
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 31,682	158,949
Weighted average depreciable life in years		19 years
Nonregulated: | Pipeline and midstream: | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 13	89
Nonregulated: | Pipeline and midstream: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 9,800	9,827
Weighted average depreciable life in years		10 years
Nonregulated: | Construction materials and contracting: | Land
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 94,625	95,870
Nonregulated: | Construction materials and contracting: | Buildings and improvements
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 102,347	96,864
Weighted average depreciable life in years		19 years
Nonregulated: | Construction materials and contracting: | Machinery, vehicles and equipment
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 930,471	937,084
Weighted average depreciable life in years		12 years
Nonregulated: | Construction materials and contracting: | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 16,181	18,615
Nonregulated: | Construction materials and contracting: | Aggregate reserves
Property, plant and equipment [Line Items]
Property, plant and equipment	[2]	405,751	404,995
Nonregulated: | Construction services: | Land
Property, plant and equipment [Line Items]
Property, plant and equipment		5,346	5,025
Nonregulated: | Construction services: | Buildings and improvements
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 26,693	25,259
Weighted average depreciable life in years		26 years
Nonregulated: | Construction services: | Machinery, vehicles and equipment
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 132,217	121,940
Weighted average depreciable life in years		6 years
Nonregulated: | Construction services: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 7,105	11,055
Weighted average depreciable life in years		4 years
Nonregulated: | Other: | Land
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 2,837	2,837
Nonregulated: | Other: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 46,431	$ 46,700
Weighted average depreciable life in years		23 years

[1] Includes allocations of common utility property

[2] Depleted on the units-of-production method based on recoverable aggregate reserves.