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Property, plant and equipment (Details 3) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Property, plant and equipment [Line Items] |  |  |  |
Interest capitalized |  | $ 0 | $ 0 | $ 4,381
AFUDC - borrowed |  | 966 | 914 | 4,907
AFUDC - equity |  | 909 | 565 | 7,971
Property, plant and equipment |  | 6,770,829 | 6,510,229 |
Less accumulated depreciation, depletion and amortization |  | 2,691,641 | 2,578,902 | 2,489,322
Net property, plant and equipment |  | 4,079,188 | 3,931,327 | 3,898,380
Electric: |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment | [1] | 1,982,264 | 1,888,613 | 1,786,148
Natural gas distribution: |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment | [1] | 2,319,845 | 2,179,413 | 2,076,581
Pipeline and midstream: |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | 700,284 | 672,199 | 758,729
Construction materials and contracting: |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | 1,560,048 | 1,549,375 | 1,553,428
Construction services: |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | 177,265 | 171,361 | 163,279
Other: |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | 31,123 | 49,268 | $ 49,537
Regulated: | Electric: | Generation |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | $ 1,034,765 | 1,036,373 |
Weighted average depreciable life in years |  | 39 years |  |
Regulated: | Electric: | Distribution |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | $ 415,543 | 398,382 |
Weighted average depreciable life in years |  | 44 years |  |
Regulated: | Electric: | Transmission |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | $ 296,941 | 284,048 |
Weighted average depreciable life in years |  | 57 years |  |
Regulated: | Electric: | Construction in progress |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | $ 117,906 | 62,212 |
Regulated: | Electric: | Other |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | $ 117,109 | 107,598 |
Weighted average depreciable life in years |  | 15 years |  |
Regulated: | Natural gas distribution: | Distribution |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | $ 1,831,795 | 1,718,633 |
Weighted average depreciable life in years |  | 47 years |  |
Regulated: | Natural gas distribution: | Construction in progress |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | $ 19,823 | 19,934 |
Regulated: | Natural gas distribution: | Other |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | $ 468,227 | 440,846 |
Weighted average depreciable life in years |  | 18 years |  |
Regulated: | Pipeline and midstream: | Transmission |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | $ 516,932 | 490,143 |
Weighted average depreciable life in years |  | 53 years |  |
Regulated: | Pipeline and midstream: | Gathering |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | $ 37,837 | 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 |  | $ 45,629 | 45,350 |
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 |  | $ 17,488 | 16,507 |
Regulated: | Pipeline and midstream: | Other |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | $ 41,054 | 40,873 |
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 |  | $ 31,678 | 31,682 |
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 |  | $ 17 | 13 |
Nonregulated: | Pipeline and midstream: | Other |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | $ 9,649 | 9,800 |
Weighted average depreciable life in years |  | 10 years |  |
Nonregulated: | Construction materials and contracting: | Land |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | $ 95,745 | 94,625 |
Nonregulated: | Construction materials and contracting: | Buildings and improvements |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | $ 102,435 | 102,347 |
Weighted average depreciable life in years |  | 20 years |  |
Nonregulated: | Construction materials and contracting: | Machinery, vehicles and equipment |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | $ 947,979 | 930,471 |
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 |  | $ 7,750 | 16,181 |
Nonregulated: | Construction materials and contracting: | Aggregate reserves |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment | [2] | 406,139 | 405,751 |
Nonregulated: | Construction services: | Land |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | 5,216 | 5,346 |
Nonregulated: | Construction services: | Buildings and improvements |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | $ 27,351 | 26,693 |
Weighted average depreciable life in years |  | 25 years |  |
Nonregulated: | Construction services: | Machinery, vehicles and equipment |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | $ 137,924 | 132,217 |
Weighted average depreciable life in years |  | 6 years |  |
Nonregulated: | Construction services: | Other |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |
Property, plant and equipment |  | $ 6,774 | 7,105 |
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 |  | $ 28,286 | $ 46,431 |
Weighted average depreciable life in years |  | 19 years |  |
[1] | Includes allocations of common utility property
[2] | Depleted on the units-of-production method based on recoverable aggregate reserves.

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