CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Utility Plant [Abstract]
Electric plant	$ 8,515,482	$ 8,135,847
Natural gas plant	3,598,732	3,307,545
Common plant	1,027,023	811,815
Less: Accumulated depreciation and amortization	(2,832,321)	(2,428,524)
Net utility plant	10,308,916	9,826,683
Other property and investments:
Goodwill	1,656,513	1,656,513
Other property and investments	244,444	182,355
Total other property and investments	1,900,957	1,838,868
Current assets:
Cash and cash equivalents	37,521	26,616
Restricted cash	18,041	10,145
Accounts receivable, net of allowance for doubtful accounts	338,782	341,110
Unbilled revenue	205,285	222,186
Purchased gas adjustment receivable	9,921	0
Materials and supplies, at average cost	116,180	107,003
Fuel and natural gas inventory, at average cost	53,351	49,908
Unrealized gain on derivative instruments	46,507	22,247
Prepaid expense and other	25,674	21,996
Power contract acquisition adjustment gain	6,114	12,207
Total current assets	857,376	813,418
Other long-term and regulatory assets:
Power cost adjustment mechanism	4,735	4,576
Regulatory assets related to power contracts	16,693	19,454
Other regulatory assets	773,552	948,532
Unrealized gain on derivative instruments	2,512	2,158
Power contract acquisition adjustment gain	156,597	162,711
Other	77,523	74,389
Total other long-term and regulatory assets	1,031,612	1,211,820
Total assets	14,098,861	13,690,789
Common shareholder’s equity:
Common stock	0	0
Additional paid-in capital	3,308,957	3,308,957
Retained earnings	629,003	465,355
Accumulated other comprehensive income (loss), net of tax	(77,202)	(24,282)
Total common shareholder’s equity	3,860,758	3,750,030
Long-term debt:
First mortgage bonds and senior notes	3,764,412	3,164,412
Pollution control bonds	161,860	161,860
Junior subordinated notes	0	250,000
Long-term debt	1,961,900	1,902,600
Debt discount, issuance costs and other	(215,681)	(220,943)
Total long-term debt	5,672,491	5,257,929
Total capitalization	9,533,249	9,007,959
Current liabilities:
Accounts payable	480,069	359,586
Short-term debt	379,297	329,463
Current maturities of long-term debt	0	200,000
Purchased gas adjustment payable	0	16,051
Accrued expenses:
Taxes	118,112	117,948
Salaries and wages	50,785	53,220
Interest	70,099	73,564
Unrealized loss on derivative instruments	46,661	64,859
Power Contract Acquisition Adjustment Loss Current	2,547	2,762
Other	79,312	80,206
Total current liabilities	1,226,882	1,297,659
Other Long-term and regulatory liabilities:
Deferred income taxes	789,297	746,868
Unrealized loss on derivative instruments	11,095	21,235
Regulatory liabilities	747,203	731,587
Deferred Tax Liabilities, Regulatory Assets and Liabilities	975,974	1,011,626
Regulatory Liabilities Related To Power Contracts	162,711	174,918
Power Contract Acquisition Adjustment Loss Non Current	14,146	16,693
Other deferred credits	638,304	682,244
Total other long-term and regulatory liabilities	3,338,730	3,385,171
Commitments and contingencies (Note 16)
Total capitalization and liabilities	14,098,861	13,690,789
Subsidiaries [Member]
Utility Plant [Abstract]
Electric plant	10,587,231	10,232,771
Natural gas plant	4,164,489	3,882,733
Common plant	1,052,544	843,145
Less: Accumulated depreciation and amortization	(5,495,348)	(5,131,966)
Net utility plant	10,308,916	9,826,683
Other property and investments:
Other property and investments	76,986	76,350
Total other property and investments	76,986	76,350
Current assets:
Cash and cash equivalents	35,452	25,864
Restricted cash	18,041	10,145
Accounts receivable, net of allowance for doubtful accounts	346,251	343,546
Unbilled revenue	205,285	222,186
Purchased gas adjustment receivable	9,921	0
Materials and supplies, at average cost	116,180	107,003
Fuel and natural gas inventory, at average cost	52,028	48,585
Unrealized gain on derivative instruments	46,507	22,247
Prepaid expense and other	25,674	21,996
Total current assets	855,339	801,572
Other long-term and regulatory assets:
Power cost adjustment mechanism	4,735	4,576
Other regulatory assets	773,552	948,540
Unrealized gain on derivative instruments	2,512	2,158
Other	75,483	71,827
Total other long-term and regulatory assets	856,282	1,027,101
Total assets	12,097,523	11,731,706
Common shareholder’s equity:
Common stock	859	859
Additional paid-in capital	3,275,105	3,275,105
Retained earnings	622,844	452,066
Accumulated other comprehensive income (loss), net of tax	(190,884)	(126,906)
Total common shareholder’s equity	3,707,924	3,601,124
Long-term debt:
First mortgage bonds and senior notes	3,764,417	3,164,412
Pollution control bonds	161,860	161,860
Junior subordinated notes	0	250,000
Debt discount, issuance costs and other	(31,417)	(26,361)
Total long-term debt	3,894,860	3,549,911
Total capitalization	7,602,784	7,151,035
Current liabilities:
Accounts payable	480,195	359,585
Short-term debt	379,297	329,463
Current maturities of long-term debt	0	200,000
Purchased gas adjustment payable	0	16,051
Accrued expenses:
Taxes	117,993	117,063
Salaries and wages	50,785	53,220
Interest	43,951	47,837
Unrealized loss on derivative instruments	46,661	64,859
Other	79,312	80,206
Total current liabilities	1,198,194	1,268,284
Other Long-term and regulatory liabilities:
Deferred income taxes	926,403	869,473
Unrealized loss on derivative instruments	11,095	21,235
Regulatory liabilities	745,880	730,273
Deferred Tax Liabilities, Regulatory Assets and Liabilities	976,582	1,012,260
Other deferred credits	636,585	679,146
Total other long-term and regulatory liabilities	3,296,545	3,312,387
Commitments and contingencies (Note 16)
Total capitalization and liabilities	$ 12,097,523	$ 11,731,706