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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.

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