CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Utility Plant [Abstract]
Public Utilities Property Plant And Equipment Electric Plant	$ 9,288,156	$ 9,200,231
Public Utilities Property Plant And Equipment Gas Plant	4,296,864	4,227,532
Public Utilities Property Plant And Equipment Common Plant	1,085,442	1,116,524
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(3,736,675)	(3,671,094)
Public Utilities, Property, Plant and Equipment, Net, Total	10,933,787	10,873,193
Other Property And Investments [Abstract]
Goodwill	1,656,513	1,656,513
Other Property And Investments	327,674	324,184
Total Other Property And Investments	1,984,187	1,980,697
Assets, Current [Abstract]
Cash and Cash Equivalents, at Carrying Value	36,507	52,307
Restricted Cash and Cash Equivalents, Current	24,443	29,544
Accounts Receivable, after Allowance for Credit Loss, Current	374,296	352,132
Unbilled Revenues	219,450	221,871
Other Inventory, Supplies, Gross	117,891	118,333
Fuel And Gas Inventory At Average Cost	38,743	48,795
Derivative Asset, Current	47,221	33,015
Prepaid Expense and Other Assets, Current	49,630	45,746
Power Contract Acquisition Adjustment Gain Current	15,694	14,874
Assets, Current, Total	923,875	916,617
Other Longterm And Regulatory Assets [Abstract]
Power Cost Adjustment Mechanism Asset, Noncurrent	71,257	82,801
purchase gas adjustment, long-term	46,787	87,655
Regulatory Assets Related To Power Contracts	11,398	11,728
Other Regulatory Assets	737,377	747,651
Derivative Asset, Noncurrent	4,925	8,805
Power Contract Acquisition Adjustment Non Current	75,601	80,900
Operating Lease, Right-of-Use Asset	194,245	172,167
Other Assets, Noncurrent	86,680	80,751
Total Longterm And Regulatory Assets	1,228,270	1,272,458
Assets, Total	15,070,119	15,042,965
Capitalization [Abstract]
Common stock	0	0
Additional Paid in Capital	3,313,532	3,313,532
Retained Earnings (Accumulated Deficit)	1,078,841	912,787
Accumulated Other Comprehensive Income (Loss), Net of Tax	(84,052)	(86,437)
Stockholders' Equity Attributable to Parent, Total	4,308,321	4,139,882
Long-term Debt, Excluding Current Maturities [Abstract]
Senior Notes, Noncurrent	4,212,000	4,212,000
Long-term Pollution Control Bond	161,860	161,860
Other Long-term Debt, Noncurrent	1,733,500	1,724,700
Debt discount and other	(203,026)	(206,120)
Long-term Debt, Excluding Current Maturities, Total	5,904,334	5,892,440
Capitalization, Long-term Debt and Equity, Total	10,212,655	10,032,322
Liabilities, Current [Abstract]
Accounts Payable, Current	324,037	342,404
Short-term Debt	191,000	373,800
Long-term Debt, Current Maturities	526,345	526,412
Accrued Expenses [Abstract]
Taxes Payable, Current	134,228	110,752
Employee-related Liabilities, Current	35,295	42,530
Interest Payable, Current	79,400	73,647
Derivative Liability, Current	19,839	31,441
Power Contract Acquisition Adjustment Loss Current	2,036	2,039
Operating Lease, Liability, Current	19,238	19,204
Other Liabilities, Current	88,335	73,385
Liabilities, Current, Total	1,419,753	1,595,614
Longterm And Regulatory Liabilities [Abstract]
Deferred Tax and Other Liabilities, Noncurrent	846,692	810,729
Derivative Liability, Noncurrent	24,332	29,833
Regulatory Liabilities, Noncurrent	735,368	732,498
Deferred Tax Liabilities, Regulatory Assets and Liabilities	924,567	953,274
Regulatory Liabilities Related To Power Contracts	91,295	95,774
Power Contract Acquisition Adjustment Loss Non Current	9,362	9,689
Operating Lease, Liability, Noncurrent	182,288	160,980
Other Deferred Credits	623,807	622,252
Total Longterm And Regulatory Liabilities	3,437,711	3,415,029
Commitments and Contingencies
Liabilities and Equity, Total	$ 15,070,119	15,042,965
Common stock, par value (in dollars per share)	$ 0.01
Common stock, shares authorized (in shares)	1,000
Common Stock, Shares, Outstanding	200
Subsidiaries [Member]
Utility Plant [Abstract]
Public Utilities Property Plant And Equipment Electric Plant	$ 11,116,750	11,035,402
Public Utilities Property Plant And Equipment Gas Plant	4,855,247	4,786,419
Public Utilities Property Plant And Equipment Common Plant	1,107,719	1,139,120
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(6,145,929)	(6,087,748)
Public Utilities, Property, Plant and Equipment, Net, Total	10,933,787	10,873,193
Other Property And Investments [Abstract]
Other Property And Investments	83,620	83,855
Total Other Property And Investments	83,620	83,855
Assets, Current [Abstract]
Cash and Cash Equivalents, at Carrying Value	36,051	51,177
Restricted Cash and Cash Equivalents, Current	24,443	29,544
Accounts Receivable, after Allowance for Credit Loss, Current	376,228	355,850
Unbilled Revenues	219,450	221,871
Other Inventory, Supplies, Gross	117,891	118,333
Fuel And Gas Inventory At Average Cost	37,479	47,531
Derivative Asset, Current	47,221	33,015
Prepaid Expense and Other Assets, Current	49,630	45,746
Assets, Current, Total	908,393	903,067
Other Longterm And Regulatory Assets [Abstract]
Power Cost Adjustment Mechanism Asset, Noncurrent	71,257	82,801
purchase gas adjustment, long-term	46,787	87,655
Other Regulatory Assets	737,377	747,651
Derivative Asset, Noncurrent	4,925	8,805
Operating Lease, Right-of-Use Asset	194,245	172,167
Other Assets, Noncurrent	85,294	79,231
Total Longterm And Regulatory Assets	1,139,885	1,178,310
Assets, Total	13,065,685	13,038,425
Capitalization [Abstract]
Common stock	859	859
Additional Paid in Capital	3,485,105	3,485,105
Retained Earnings (Accumulated Deficit)	1,023,818	876,401
Accumulated Other Comprehensive Income (Loss), Net of Tax	(176,402)	(180,956)
Stockholders' Equity Attributable to Parent, Total	4,333,380	4,181,409
Long-term Debt, Excluding Current Maturities [Abstract]
Senior Notes, Noncurrent	4,212,000	4,212,000
Long-term Pollution Control Bond	161,860	161,860
Debt discount and other	(35,337)	(35,816)
Long-term Debt, Excluding Current Maturities, Total	4,338,523	4,338,044
Capitalization, Long-term Debt and Equity, Total	8,671,903	8,519,453
Liabilities, Current [Abstract]
Accounts Payable, Current	324,152	342,504
Short-term Debt	191,000	373,800
Long-term Debt, Current Maturities	2,345	2,412
Accrued Expenses [Abstract]
Taxes Payable, Current	130,781	107,254
Employee-related Liabilities, Current	35,295	42,530
Interest Payable, Current	58,002	48,189
Derivative Liability, Current	19,839	31,441
Operating Lease, Liability, Current	19,238	19,204
Other Liabilities, Current	88,335	73,385
Liabilities, Current, Total	868,987	1,040,719
Longterm And Regulatory Liabilities [Abstract]
Deferred Tax and Other Liabilities, Noncurrent	1,038,347	987,382
Derivative Liability, Noncurrent	24,332	29,833
Regulatory Liabilities, Noncurrent	734,104	731,234
Deferred Tax Liabilities, Regulatory Assets and Liabilities	925,277	953,987
Operating Lease, Liability, Noncurrent	182,288	160,980
Other Deferred Credits	620,447	614,837
Total Longterm And Regulatory Liabilities	3,524,795	3,478,253
Commitments and Contingencies
Liabilities and Equity, Total	$ 13,065,685	$ 13,038,425
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	150,000,000	150,000,000
Common Stock, Shares, Outstanding	85,903,791	85,903,791