Property, Plant and Equipment (Details) - USD ($) $ in Thousands |
12 Months Ended |
Dec. 31, 2018 |
Dec. 31, 2017 |
Property, Plant and Equipment [Line Items] |
|
|
Property, plant and equipment, gross |
$ 6,000,015
|
$ 5,567,518
|
Less: accumulated depreciation, depletion and amortization |
1,145,136
|
1,026,088
|
Total property, plant and equipment, net |
4,854,879
|
4,541,430
|
Corporate, Non-Segment |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Property, plant and equipment, gross |
22,269
|
11,954
|
Less: accumulated depreciation, depletion and amortization |
670
|
309
|
Total property, plant and equipment, net |
39,544
|
25,715
|
Property, plant and equipment |
5,721
|
5,580
|
Construction in progress, gross |
16,548
|
6,374
|
Intercompany Eliminations |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Accumulated Depreciation - Capital Lease Elimination |
$ 17,945
|
$ 14,070
|
Weighted Average | Corporate, Non-Segment |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
8 years
|
8 years
|
Minimum | Corporate, Non-Segment |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
3 years
|
3 years
|
Maximum | Corporate, Non-Segment |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
30 years
|
30 years
|
Electric Utilities |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Total plant in service before construction work in progress |
$ 3,049,292
|
$ 2,976,613
|
Construction work in progress |
60,480
|
13,595
|
Property, plant and equipment, gross |
3,109,772
|
2,990,208
|
Less: accumulated depreciation, depletion and amortization |
706,869
|
644,022
|
Total property, plant and equipment, net |
$ 2,402,903
|
2,346,186
|
Depreciation, depletion and amortization, remaining amortization period |
12 years
|
|
Electric Utilities | Production, Electric |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Production, processing and storage |
$ 1,318,643
|
$ 1,315,044
|
Electric Utilities | Production, Electric | Weighted Average |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
41 years
|
39 years
|
Electric Utilities | Production, Electric | Minimum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
32 years
|
|
Electric Utilities | Production, Electric | Maximum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
46 years
|
|
Electric Utilities | Electric transmission |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Transmission |
$ 437,082
|
$ 407,203
|
Electric Utilities | Electric transmission | Weighted Average |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
51 years
|
51 years
|
Electric Utilities | Electric transmission | Minimum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
48 years
|
|
Electric Utilities | Electric transmission | Maximum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
53 years
|
|
Electric Utilities | Electric distribution |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Distribution |
$ 793,725
|
$ 755,213
|
Electric Utilities | Electric distribution | Weighted Average |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
48 years
|
48 years
|
Electric Utilities | Electric distribution | Minimum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
45 years
|
|
Electric Utilities | Electric distribution | Maximum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
50 years
|
|
Electric Utilities | Plant acquisition adjustment |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Plant acquisition adjustment |
$ 4,870
|
$ 4,870
|
Electric Utilities | Plant acquisition adjustment | Weighted Average |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
32 years
|
32 years
|
Electric Utilities | Plant acquisition adjustment | Minimum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
32 years
|
|
Electric Utilities | Plant acquisition adjustment | Maximum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
32 years
|
|
Electric Utilities | General |
|
|
Property, Plant and Equipment [Line Items] |
|
|
General |
$ 233,531
|
$ 232,842
|
Electric Utilities | General | Weighted Average |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
28 years
|
31 years
|
Electric Utilities | General | Minimum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
26 years
|
|
Electric Utilities | General | Maximum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
28 years
|
|
Electric Utilities | Capital lease - plant in service |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Capital lease - plant in service |
$ 261,441
|
$ 261,441
|
Electric Utilities | Capital lease - plant in service | Weighted Average |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
20 years
|
20 years
|
Electric Utilities | Capital lease - plant in service | Minimum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
20 years
|
|
Electric Utilities | Capital lease - plant in service | Maximum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
20 years
|
|
Gas Utilities |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Total plant in service before construction work in progress |
$ 2,468,260
|
$ 2,206,753
|
Construction work in progress |
38,271
|
44,440
|
Property, plant and equipment, gross |
2,506,531
|
2,251,193
|
Less: accumulated depreciation, depletion and amortization |
279,580
|
229,170
|
Total property, plant and equipment, net |
2,226,951
|
2,022,023
|
Gas Utilities | Production, Gas |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Production, processing and storage |
$ 13,580
|
$ 10,495
|
Gas Utilities | Production, Gas | Weighted Average |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
35 years
|
35 years
|
Gas Utilities | Production, Gas | Minimum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
24 years
|
|
Gas Utilities | Production, Gas | Maximum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
71 years
|
|
Gas Utilities | Gas transmission |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Transmission |
$ 423,873
|
$ 366,433
|
Gas Utilities | Gas transmission | Weighted Average |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
48 years
|
48 years
|
Gas Utilities | Gas transmission | Minimum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
22 years
|
|
Gas Utilities | Gas transmission | Maximum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
66 years
|
|
Gas Utilities | Gas distribution |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Distribution |
$ 1,595,644
|
$ 1,413,431
|
Gas Utilities | Gas distribution | Weighted Average |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
42 years
|
42 years
|
Gas Utilities | Gas distribution | Minimum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
33 years
|
|
Gas Utilities | Gas distribution | Maximum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
47 years
|
|
Gas Utilities | Cushion Gas - Depreciable |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Production, processing and storage |
$ 3,539
|
$ 3,539
|
Gas Utilities | Cushion Gas - Depreciable | Weighted Average |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
28 years
|
28 years
|
Gas Utilities | Cushion Gas - Depreciable | Minimum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
28 years
|
|
Gas Utilities | Cushion Gas - Depreciable | Maximum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
28 years
|
|
Gas Utilities | Cushion Gas - Not Depreciated |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Production, processing and storage |
$ 46,369
|
$ 47,466
|
Gas Utilities | Gas, Storage |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Production, processing and storage |
$ 29,335
|
$ 28,520
|
Gas Utilities | Gas, Storage | Weighted Average |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
30 years
|
31 years
|
Gas Utilities | Gas, Storage | Minimum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
28 years
|
|
Gas Utilities | Gas, Storage | Maximum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
38 years
|
|
Gas Utilities | General |
|
|
Property, Plant and Equipment [Line Items] |
|
|
General |
$ 355,920
|
$ 336,869
|
Gas Utilities | General | Weighted Average |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
19 years
|
19 years
|
Gas Utilities | General | Minimum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
10 years
|
|
Gas Utilities | General | Maximum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
24 years
|
|
Power Generation |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Property, plant and equipment, gross |
$ 185,793
|
$ 155,793
|
Less: accumulated depreciation, depletion and amortization |
64,273
|
57,813
|
Total property, plant and equipment, net |
121,520
|
97,980
|
Property, plant and equipment |
173,997
|
155,569
|
Construction in progress, gross |
$ 11,796
|
$ 224
|
Power Generation | Weighted Average |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
31 years
|
33 years
|
Power Generation | Minimum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
2 years
|
2 years
|
Power Generation | Maximum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
40 years
|
40 years
|
Mining |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Property, plant and equipment, gross |
$ 175,650
|
$ 158,370
|
Less: accumulated depreciation, depletion and amortization |
111,689
|
108,844
|
Total property, plant and equipment, net |
63,961
|
49,526
|
Property, plant and equipment |
175,650
|
158,370
|
Construction in progress, gross |
$ 0
|
$ 0
|
Mining | Weighted Average |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
13 years
|
14 years
|
Mining | Minimum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
2 years
|
2 years
|
Mining | Maximum |
|
|
Property, Plant and Equipment [Line Items] |
|
|
Weighted average useful life |
59 years
|
59 years
|