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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