CONSOLIDATED BALANCE SHEETS - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Utility Plant [Abstract]
Electric plant	$ 8,811,889,000	$ 8,515,482,000
Natural gas plant	3,916,040,000	3,598,732,000
Common plant	1,096,649,000	1,027,023,000
Less: Accumulated depreciation and amortization	(3,236,240,000)	(2,832,321,000)
Net utility plant	10,588,338,000	10,308,916,000
Other property and investments:
Goodwill	1,656,513,000	1,656,513,000
Other property and investments	286,975,000	244,444,000
Total other property and investments	1,943,488,000	1,900,957,000
Current assets:
Cash and cash equivalents	45,259,000	37,521,000
Restricted cash	20,887,000	18,041,000
Accounts receivable, net of allowance for doubtful accounts	316,352,000	338,782,000
Unbilled revenue	224,657,000	205,285,000
Purchased gas adjustment receivable	0	9,921,000
Materials and supplies, at average cost	115,684,000	116,180,000
Fuel and natural gas inventory, at average cost	52,083,000	53,351,000
Unrealized gain on derivative instruments	23,626,000	46,507,000
Prepaid expenses and other	27,504,000	25,674,000
Power contract acquisition adjustment gain	9,067,000	6,114,000
Total current assets	835,119,000	857,376,000
Other long-term and regulatory assets:
Power cost adjustment mechanism	41,745,000	4,735,000
purchase gas adjustment, long-term	132,766,000	0
Regulatory assets related to power contracts	14,146,000	16,693,000
Other regulatory assets	673,021,000	773,552,000
Unrealized gain on derivative instruments	7,682,000	2,512,000
Operating lease right-of-use asset	183,048,000	0
Power contract acquisition adjustment gain	147,530,000	156,597,000
Other	92,980,000	77,523,000
Total other long-term and regulatory assets	1,292,918,000	1,031,612,000
Total assets	14,659,863,000	14,098,861,000
Common shareholder’s equity:
Common stock	0	0
Additional paid-in capital	3,308,957,000	3,308,957,000
Retained earnings	775,491,000	629,003,000
Accumulated other comprehensive income (loss), net of tax	(84,149,000)	(77,202,000)
Total common shareholder’s equity	4,000,299,000	3,860,758,000
Long-term debt:
First mortgage bonds and senior notes	4,212,000,000	3,764,412,000
Pollution control bonds	161,860,000	161,860,000
Junior subordinated notes	0	0
Long-term debt	1,758,100,000	1,961,900,000
Debt discount, issuance costs and other	(211,635,000)	(215,681,000)
Total long-term debt	5,920,325,000	5,672,491,000
Total capitalization	9,920,624,000	9,533,249,000
Current liabilities:
Accounts payable	325,913,000	480,069,000
Short-term debt	176,000,000	379,297,000
Current maturities of long-term debt	452,412,000	0
Accrued expenses:
Taxes	99,979,000	118,112,000
Salaries and wages	50,091,000	50,785,000
Interest	74,855,000	70,099,000
Unrealized loss on derivative instruments	13,428,000	46,661,000
Power contract acquisition adjustment loss	2,418,000	2,547,000
Operating lease liabilities	15,862,000	0
Other	107,809,000	79,312,000
Total current liabilities	1,318,767,000	1,226,882,000
Other Long-term and regulatory liabilities:
Deferred income taxes	824,720,000	789,297,000
Unrealized loss on derivative instruments	12,693,000	11,095,000
Regulatory liabilities	730,879,000	747,203,000
Regulatory liability for deferred income taxes	946,179,000	975,974,000
Regulatory liabilities related to power contracts	156,597,000	162,711,000
Power contract acquisition adjustment loss	11,728,000	14,146,000
Operating lease liabilities	174,327,000	0
Other deferred credits	563,349,000	638,304,000
Total long-term and regulatory liabilities	3,420,472,000	3,338,730,000
Commitments and contingencies (Note 16)
Total capitalization and liabilities	14,659,863,000	14,098,861,000
Subsidiaries [Member]
Utility Plant [Abstract]
Electric plant	10,671,328,000	10,587,231,000
Natural gas plant	4,478,048,000	4,164,489,000
Common plant	1,121,568,000	1,052,544,000
Less: Accumulated depreciation and amortization	(5,682,606,000)	(5,495,348,000)
Net utility plant	10,588,338,000	10,308,916,000
Other property and investments:
Other property and investments	81,112,000	76,986,000
Total other property and investments	81,112,000	76,986,000
Current assets:
Cash and cash equivalents	44,004,000	35,452,000
Restricted cash	20,887,000	18,041,000
Accounts receivable, net of allowance for doubtful accounts	319,229,000	346,251,000
Unbilled revenue	224,657,000	205,285,000
Purchased gas adjustment receivable	0	9,921,000
Materials and supplies, at average cost	115,684,000	116,180,000
Fuel and natural gas inventory, at average cost	50,818,000	52,028,000
Unrealized gain on derivative instruments	23,626,000	46,507,000
Prepaid expenses and other	27,504,000	25,674,000
Total current assets	826,409,000	855,339,000
Other long-term and regulatory assets:
Power cost adjustment mechanism	41,745,000	4,735,000
purchase gas adjustment, long-term	132,766,000	0
Other regulatory assets	673,021,000	773,552,000
Unrealized gain on derivative instruments	7,682,000	2,512,000
Operating lease right-of-use asset	183,048,000	0
Other	90,924,000	75,483,000
Total other long-term and regulatory assets	1,129,186,000	856,282,000
Total assets	12,625,045,000	12,097,523,000
Common shareholder’s equity:
Common stock	859,000	859,000
Additional paid-in capital	3,485,105,000	3,275,105,000
Retained earnings	751,193,000	622,844,000
Accumulated other comprehensive income (loss), net of tax	(188,477,000)	(190,884,000)
Total common shareholder’s equity	4,048,680,000	3,707,924,000
Long-term debt:
First mortgage bonds and senior notes	4,212,000,000	3,764,417,000
Pollution control bonds	161,860,000	161,860,000
Junior subordinated notes	0	0
Debt discount, issuance costs and other	37,718,000	31,417,000
Total long-term debt	4,336,142,000	3,894,860,000
Total capitalization	8,384,822,000	7,602,784,000
Current liabilities:
Accounts payable	325,980,000	480,195,000
Short-term debt	176,000,000.0	379,297,000
Current maturities of long-term debt	2,412,000	0
Accrued expenses:
Taxes	99,977,000	117,993,000
Salaries and wages	50,091,000	50,785,000
Interest	48,917,000	43,951,000
Unrealized loss on derivative instruments	13,428,000	46,661,000
Operating lease liabilities	15,862,000	0
Other	107,809,000	79,312,000
Total current liabilities	840,476,000	1,198,194,000
Other Long-term and regulatory liabilities:
Deferred income taxes	977,163,000	926,403,000
Unrealized loss on derivative instruments	12,693,000	11,095,000
Regulatory liabilities	729,614,000	745,880,000
Regulatory liability for deferred income taxes	946,936,000	976,582,000
Operating lease liabilities	174,327,000	0
Other deferred credits	559,014,000	636,585,000
Total long-term and regulatory liabilities	3,399,747,000	3,296,545,000
Commitments and contingencies (Note 16)
Total capitalization and liabilities	$ 12,625,045,000	$ 12,097,523,000