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Property, plant and equipment (Details 3) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Property, plant and equipment [Line Items] |  |  |
AFUDC - borrowed | $ 2,290 | $ 966 | $ 914
AFUDC - equity | 1,897 | 909 | 565
Property, plant and equipment | 7,397,321 | 6,770,829 |
Less accumulated depreciation, depletion and amortization | 2,818,644 | 2,691,641 | 2,578,902
Net property, plant and equipment | 4,578,677 | 4,079,188 | 3,931,327
Electric |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | 2,148,569 | 1,982,264 | 1,888,613
Natural gas distribution |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | 2,499,093 | 2,319,845 | 2,179,413
Pipeline and midstream |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | 764,959 | 700,284 | 672,199
Construction materials and contracting |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | 1,768,006 | 1,560,048 | 1,549,375
Construction services |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | 188,586 | 177,265 | 171,361
Other |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | 28,108 | 31,123 | $ 49,268
Regulated: | Electric | Generation |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | $ 1,131,484 | 1,034,765 |
Weighted average depreciable life in years | 49 years |  |
Regulated: | Electric | Distribution |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | $ 430,750 | 415,543 |
Weighted average depreciable life in years | 46 years |  |
Regulated: | Electric | Transmission |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | $ 302,315 | 296,941 |
Weighted average depreciable life in years | 64 years |  |
Regulated: | Electric | Construction in progress |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | $ 161,893 | 117,906 |
Regulated: | Electric | Other |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | $ 122,127 | 117,109 |
Weighted average depreciable life in years | 13 years |  |
Regulated: | Natural gas distribution | Distribution |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | $ 1,981,356 | 1,831,795 |
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 | $ 21,028 | 19,823 |
Regulated: | Natural gas distribution | Other |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | $ 496,708 | 468,227 |
Weighted average depreciable life in years | 16 years |  |
Regulated: | Pipeline and midstream | Transmission |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | $ 585,594 | 516,932 |
Weighted average depreciable life in years | 54 years |  |
Regulated: | Pipeline and midstream | Gathering |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | $ 37,829 | 37,837 |
Weighted average depreciable life in years | 20 years |  |
Regulated: | Pipeline and midstream | Storage |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | $ 49,101 | 45,629 |
Weighted average depreciable life in years | 61 years |  |
Regulated: | Pipeline and midstream | Construction in progress |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | $ 5,915 | 17,488 |
Regulated: | Pipeline and midstream | Other |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | $ 45,763 | 41,054 |
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 | $ 31,094 | 31,678 |
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 | $ 86 | 17 |
Nonregulated: | Pipeline and midstream | Other |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | $ 9,577 | 9,649 |
Weighted average depreciable life in years | 10 years |  |
Nonregulated: | Construction materials and contracting | Land |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | $ 109,541 | 95,745 |
Nonregulated: | Construction materials and contracting | Buildings and improvements |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | $ 114,905 | 102,435 |
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 | $ 1,090,790 | 947,979 |
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 | $ 22,507 | 7,750 |
Nonregulated: | Construction materials and contracting | Aggregate reserves |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | 430,263 | 406,139 |
Nonregulated: | Construction services | Land |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | 5,216 | 5,216 |
Nonregulated: | Construction services | Buildings and improvements |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | $ 29,795 | 27,351 |
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 | $ 145,859 | 137,924 |
Weighted average depreciable life in years | 6 years |  |
Nonregulated: | Construction services | Other |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | $ 7,716 | 6,774 |
Weighted average depreciable life in years | 3 years |  |
Nonregulated: | Other | Land |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | $ 2,648 | 2,837 |
Nonregulated: | Other | Other |  |  |
Property, plant and equipment [Line Items] |  |  |
Property, plant and equipment | $ 25,461 | $ 28,286 |
Weighted average depreciable life in years | 14 years |  |

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