CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Utility Plant [Abstract]
Public Utilities Property Plant And Equipment Electric Plant	$ 9,577,534	$ 9,200,231
Public Utilities Property Plant And Equipment Gas Plant	4,433,625	4,227,532
Public Utilities Property Plant And Equipment Common Plant	1,081,770	1,116,524
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(3,930,396)	(3,671,094)
Public Utilities, Property, Plant and Equipment, Net, Total	11,162,533	10,873,193
Other Property And Investments [Abstract]
Goodwill	1,656,513	1,656,513
Other Property And Investments	318,611	324,184
Total Other Property And Investments	1,975,124	1,980,697
Assets, Current [Abstract]
Cash and Cash Equivalents, at Carrying Value	104,666	52,307
Restricted Cash and Cash Equivalents, Current	11,362	29,544
Accounts Receivable, after Allowance for Credit Loss, Current	320,830	352,132
Unbilled Revenues	151,364	221,871
Other Inventory, Supplies, Gross	119,763	118,333
Fuel And Gas Inventory At Average Cost	67,019	48,795
Derivative Asset, Current	306,685	33,015
Prepaid Expense and Other Assets, Current	68,275	45,746
Power Contract Acquisition Adjustment Gain Current	17,213	14,874
Assets, Current, Total	1,167,177	916,617
Other Longterm And Regulatory Assets [Abstract]
Power Cost Adjustment Mechanism Asset, Noncurrent	74,298	82,801
purchase gas adjustment, long-term	56,268	87,655
Regulatory Assets Related To Power Contracts	9,996	11,728
Other Regulatory Assets	814,015	747,651
Derivative Asset, Noncurrent	55,842	8,805
Power Contract Acquisition Adjustment Non Current	67,309	80,900
Operating Lease, Right-of-Use Asset	187,903	172,167
Other Assets, Noncurrent	93,756	80,751
Total Longterm And Regulatory Assets	1,359,387	1,272,458
Assets, Total	15,664,221	15,042,965
Capitalization [Abstract]
Common stock	0	0
Additional Paid in Capital	3,523,532	3,313,532
Retained Earnings (Accumulated Deficit)	1,138,524	912,787
Accumulated Other Comprehensive Income (Loss), Net of Tax	(80,058)	(86,437)
Stockholders' Equity Attributable to Parent, Total	4,581,998	4,139,882
Long-term Debt, Excluding Current Maturities [Abstract]
Senior Notes, Noncurrent	4,662,000	4,212,000
Long-term Pollution Control Bond	161,860	161,860
Other Long-term Debt, Noncurrent	1,583,000	1,724,700
Debt discount and other	(206,061)	(206,120)
Long-term Debt, Excluding Current Maturities, Total	6,200,799	5,892,440
Capitalization, Long-term Debt and Equity, Total	10,782,797	10,032,322
Liabilities, Current [Abstract]
Accounts Payable, Current	348,786	342,404
Short-term Debt	0	373,800
Long-term Debt, Current Maturities	450,000	526,412
Accrued Expenses [Abstract]
Taxes Payable, Current	115,174	110,752
Employee-related Liabilities, Current	37,288	42,530
Interest Payable, Current	80,602	73,647
Derivative Liability, Current	50,447	31,441
Power Contract Acquisition Adjustment Loss Current	1,817	2,039
Operating Lease, Liability, Current	19,646	19,204
Other Liabilities, Current	69,739	73,385
Liabilities, Current, Total	1,173,499	1,595,614
Longterm And Regulatory Liabilities [Abstract]
Deferred Tax and Other Liabilities, Noncurrent	914,749	810,729
Derivative Liability, Noncurrent	12,007	29,833
Regulatory Liabilities, Noncurrent	924,654	732,498
Deferred Tax Liabilities, Regulatory Assets and Liabilities	889,045	953,274
Regulatory Liabilities Related To Power Contracts	84,522	95,774
Power Contract Acquisition Adjustment Loss Non Current	8,179	9,689
Operating Lease, Liability, Noncurrent	176,245	160,980
Other Deferred Credits	698,524	622,252
Total Longterm And Regulatory Liabilities	3,707,925	3,415,029
Commitments and Contingencies
Liabilities and Equity, Total	$ 15,664,221	$ 15,042,965
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	1,000	1,000
Common Stock, Shares, Outstanding	200	200
Subsidiaries [Member]
Utility Plant [Abstract]
Public Utilities Property Plant And Equipment Electric Plant	$ 11,393,548	$ 11,035,402
Public Utilities Property Plant And Equipment Gas Plant	4,990,626	4,786,419
Public Utilities Property Plant And Equipment Common Plant	1,104,047	1,139,120
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(6,325,688)	(6,087,748)
Public Utilities, Property, Plant and Equipment, Net, Total	11,162,533	10,873,193
Other Property And Investments [Abstract]
Other Property And Investments	74,294	83,855
Total Other Property And Investments	74,294	83,855
Assets, Current [Abstract]
Cash and Cash Equivalents, at Carrying Value	94,286	51,177
Restricted Cash and Cash Equivalents, Current	11,362	29,544
Accounts Receivable, after Allowance for Credit Loss, Current	323,594	355,850
Unbilled Revenues	151,364	221,871
Other Inventory, Supplies, Gross	119,763	118,333
Fuel And Gas Inventory At Average Cost	65,755	47,531
Derivative Asset, Current	306,685	33,015
Prepaid Expense and Other Assets, Current	68,275	45,746
Assets, Current, Total	1,141,084	903,067
Other Longterm And Regulatory Assets [Abstract]
Power Cost Adjustment Mechanism Asset, Noncurrent	74,298	82,801
purchase gas adjustment, long-term	56,268	87,655
Other Regulatory Assets	814,015	747,651
Derivative Asset, Noncurrent	55,842	8,805
Operating Lease, Right-of-Use Asset	187,903	172,167
Other Assets, Noncurrent	92,638	79,231
Total Longterm And Regulatory Assets	1,280,964	1,178,310
Assets, Total	13,658,875	13,038,425
Capitalization [Abstract]
Common stock	859	859
Additional Paid in Capital	3,485,105	3,485,105
Retained Earnings (Accumulated Deficit)	1,057,515	876,401
Accumulated Other Comprehensive Income (Loss), Net of Tax	(167,993)	(180,956)
Stockholders' Equity Attributable to Parent, Total	4,375,486	4,181,409
Long-term Debt, Excluding Current Maturities [Abstract]
Senior Notes, Noncurrent	4,662,000	4,212,000
Long-term Pollution Control Bond	161,860	161,860
Debt discount and other	(39,268)	(35,816)
Long-term Debt, Excluding Current Maturities, Total	4,784,592	4,338,044
Capitalization, Long-term Debt and Equity, Total	9,160,078	8,519,453
Liabilities, Current [Abstract]
Accounts Payable, Current	358,809	342,504
Short-term Debt	0	373,800
Long-term Debt, Current Maturities	0	2,412
Accrued Expenses [Abstract]
Taxes Payable, Current	114,702	107,254
Employee-related Liabilities, Current	37,288	42,530
Interest Payable, Current	58,175	48,189
Derivative Liability, Current	50,447	31,441
Operating Lease, Liability, Current	19,646	19,204
Other Liabilities, Current	69,739	73,385
Liabilities, Current, Total	708,806	1,040,719
Longterm And Regulatory Liabilities [Abstract]
Deferred Tax and Other Liabilities, Noncurrent	1,093,920	987,382
Derivative Liability, Noncurrent	12,007	29,833
Regulatory Liabilities, Noncurrent	923,390	731,234
Deferred Tax Liabilities, Regulatory Assets and Liabilities	889,613	953,987
Operating Lease, Liability, Noncurrent	176,245	160,980
Other Deferred Credits	694,816	614,837
Total Longterm And Regulatory Liabilities	3,789,991	3,478,253
Commitments and Contingencies
Liabilities and Equity, Total	$ 13,658,875	$ 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