CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Utility Plant [Abstract]
Electric plant	$ 8,999,303	$ 8,811,889
Natural gas plant	4,078,221	3,916,040
Common plant	1,093,033	1,096,649
Less: Accumulated depreciation and amortization	(3,460,359)	(3,236,240)
Net utility plant	10,710,198	10,588,338
Other property and investments:
Goodwill	1,656,513	1,656,513
Other property and investments	313,557	286,975
Total other property and investments	1,970,070	1,943,488
Current assets:
Cash and cash equivalents	27,434	45,259
Restricted cash	28,777	20,887
Accounts Receivable, after Allowance for Credit Loss, Current	235,712	316,352
Unbilled revenue	116,225	224,657
Materials and supplies, at average cost	131,521	115,684
Fuel and natural gas inventory, at average cost	51,290	52,083
Unrealized gain on derivative instruments	27,419	23,626
Prepaid expense and other	27,675	27,504
Power contract acquisition adjustment gain	10,865	9,067
Total current assets	656,918	835,119
Other long-term and regulatory assets:
Power cost adjustment mechanism	62,269	41,745
Regulatory assets related to power contracts	12,678	14,146
Other regulatory assets	683,345	673,021
Unrealized gain on derivative instruments	7,124	7,682
Power contract acquisition adjustment gain	90,978	147,530
Operating lease right-of-use asset	179,658	183,048
Other	87,840	92,980
Total other long-term and regulatory assets	1,210,825	1,292,918
Total assets	14,548,011	14,659,863
Capitalization
Common stock	0	0
Additional paid-in capital	3,308,957	3,308,957
Retained earnings	802,130	775,491
Accumulated other comprehensive income (loss), net of tax	(77,349)	(84,149)
Total common shareholder’s equity	4,033,738	4,000,299
Long-term debt:
First mortgage bonds and senior notes	4,212,000	4,212,000
Pollution control bonds	161,860	161,860
Long-term debt	2,234,000	1,758,100
Debt discount issuance costs and other	(211,330)	(211,635)
Total long-term debt	6,396,530	5,920,325
Total capitalization	10,430,268	9,920,624
Current liabilities:
Accounts payable	270,947	325,913
Short-term debt	140,000	176,000
Accrued expenses:
Taxes	93,973	99,979
Salaries and wages	41,395	50,091
Interest	74,927	74,855
Unrealized loss on derivative instruments	29,769	13,428
Power contract acquisition adjustment loss	2,197	2,418
Operating lease liabilities	16,609	15,862
Other	83,714	107,809
Total current liabilities	755,943	1,318,767
Other long-term and regulatory liabilities:
Unrealized loss on derivative instruments	34,172	12,693
Regulatory liabilities	719,587	730,879
Regulatory liability for deferred income taxes	931,063	946,179
Regulatory liabilities related to power contracts	101,843	156,597
Power contract acquisition adjustment loss	10,481	11,728
Operating lease liabilities	170,250	174,327
Total long-term and regulatory liabilities	3,361,800	3,420,472
Commitments and Contingencies
Total capitalization and liabilities	14,548,011	14,659,863
purchase gas adjustment, long-term	86,933	132,766
Long-term Debt, Current Maturities	2,412	452,412
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	720,680	591,199
Allowance for doubtful accounts	$ 10,899	$ 8,294
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
Other Deferred Credits	$ 551,020	$ 563,349
Deferred Tax and Other Liabilities, Noncurrent	843,384	824,720
Subsidiaries [Member]
Utility Plant [Abstract]
Electric plant	10,846,456	10,671,328
Natural gas plant	4,638,729	4,478,048
Common plant	1,117,680	1,121,568
Less: Accumulated depreciation and amortization	(5,892,667)	(5,682,606)
Net utility plant	10,710,198	10,588,338
Other property and investments:
Other property and investments	82,129	81,112
Total other property and investments	82,129	81,112
Current assets:
Cash and cash equivalents	26,930	44,004
Restricted cash	28,777	20,887
Accounts Receivable, after Allowance for Credit Loss, Current	256,168	319,229
Unbilled revenue	116,225	224,657
Materials and supplies, at average cost	131,521	115,684
Fuel and natural gas inventory, at average cost	50,089	50,818
Unrealized gain on derivative instruments	27,419	23,626
Prepaid expense and other	27,675	27,504
Total current assets	664,804	826,409
Other long-term and regulatory assets:
Power cost adjustment mechanism	62,269	41,745
Other regulatory assets	683,345	673,021
Unrealized gain on derivative instruments	7,124	7,682
Operating lease right-of-use asset	179,658	183,048
Other	86,052	90,924
Total other long-term and regulatory assets	1,105,381	1,129,186
Total assets	12,562,512	12,625,045
Capitalization
Common stock	859	859
Additional paid-in capital	3,485,105	3,485,105
Retained earnings	777,742	751,193
Accumulated other comprehensive income (loss), net of tax	(176,715)	(188,477)
Total common shareholder’s equity	4,086,991	4,048,680
Long-term debt:
First mortgage bonds and senior notes	4,212,000	4,212,000
Pollution control bonds	161,860	161,860
Debt discount issuance costs and other	(36,770)	(37,718)
Total long-term debt	4,337,090	4,336,142
Total capitalization	8,424,081	8,384,822
Current liabilities:
Accounts payable	272,570	325,980
Short-term debt	140,000	176,000
Accrued expenses:
Taxes	97,917	99,977
Salaries and wages	41,395	50,091
Interest	48,087	48,917
Unrealized loss on derivative instruments	29,769	13,428
Operating lease liabilities	16,609	15,862
Other	83,714	107,809
Total current liabilities	732,473	840,476
Other long-term and regulatory liabilities:
Unrealized loss on derivative instruments	34,172	12,693
Regulatory liabilities	718,386	729,614
Regulatory liability for deferred income taxes	931,814	946,936
Operating lease liabilities	170,250	174,327
Total long-term and regulatory liabilities	3,405,958	3,399,747
Commitments and Contingencies
Total capitalization and liabilities	12,562,512	12,625,045
purchase gas adjustment, long-term	86,933	132,766
Long-term Debt, Current Maturities	2,412	2,412
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	720,680	591,199
Allowance for doubtful accounts	$ 10,899	$ 8,294
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
Other Deferred Credits	$ 545,657	$ 559,014
Deferred Tax and Other Liabilities, Noncurrent	$ 1,005,679	$ 977,163