CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Utility plant (at original cost, including construction work in progress of $575,180 and $495,937, respectively):
Electric plant	$ 8,408,412	$ 8,135,847
Natural gas plant	3,521,224	3,307,545
Common plant	961,128	811,815
Less: Accumulated depreciation and amortization	(2,722,280)	(2,428,524)
Net utility plant	10,168,484	9,826,683
Other property and investments:
Goodwill	1,656,513	1,656,513
Other property and investments	232,616	182,355
Total other property and investments	1,889,129	1,838,868
Current assets:
Cash and cash equivalents	7,965	26,616	$ 6,768
Restricted cash	11,143	10,145	9,302
Accounts receivable, net of allowance for doubtful accounts	228,519	341,110
Unbilled revenue	119,042	222,186
Materials and supplies, at average cost	114,005	107,003
Fuel and gas inventory, at average cost	57,293	49,908
Unrealized gain on derivative instruments	16,524	22,247
Prepaid expense and other	37,759	21,996
Power contract acquisition adjustment gain	6,180	12,207
Total current assets	598,430	813,418
Other long-term and regulatory assets:
Power cost adjustment mechanism	4,691	4,576
Regulatory assets related to power contracts	17,148	19,454
Other regulatory assets	798,112	948,532
Derivative Asset, Noncurrent	1,897	2,158
Power contract acquisition adjustment gain	157,406	162,711
Other	88,488	74,389
Total other long-term and regulatory assets	1,067,742	1,211,820
Total assets	13,723,785	13,690,789
Capitalization:
Common stock	0	0
Additional paid-in capital	3,308,957	3,308,957
Earnings reinvested in the business	522,427	465,355
Accumulated other comprehensive income (loss), net of tax	(31,845)	(24,282)
Total common shareholder's equity	3,799,539	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	1,950,465	1,902,600
Debt discount, issuance costs and other	(218,582)	(220,943)
Total long-term debt	5,658,155	5,257,929
Total capitalization	9,457,694	9,007,959
Current liabilities:
Accounts payable	324,801	359,586
Short-term debt	206,000	329,463
Current maturities of long-term debt	0	200,000
Purchased gas adjustment liability	35,962	16,051
Accrued expenses:
Taxes	124,643	117,948
Salaries and wages	40,411	53,220
Interest	80,061	73,564
Unrealized loss on derivative instruments	30,558	64,859
Power contract acquisition adjustment loss	2,565	2,762
Other	89,821	80,206
Total current liabilities	934,822	1,297,659
Long-term and regulatory liabilities:
Deferred income taxes	775,378	746,868
Unrealized loss on derivative instruments	11,588	21,235
Regulatory liabilities	751,014	731,587
Regulatory liability for deferred income taxes	988,050	1,011,626
Regulatory liabilities related to power contracts	163,586	174,918
Power contract acquisition adjustment loss	14,583	16,693
Other deferred credits	627,070	682,244
Total long-term and regulatory liabilities	3,331,269	3,385,171
Commitments and contingencies (Note 8)
Total capitalization and liabilities	13,723,785	13,690,789
Subsidiaries [Member]
Utility plant (at original cost, including construction work in progress of $575,180 and $495,937, respectively):
Electric plant	10,487,372	10,232,771
Natural gas plant	4,089,635	3,882,733
Common plant	991,094	843,145
Less: Accumulated depreciation and amortization	(5,399,617)	(5,131,966)
Net utility plant	10,168,484	9,826,683
Other property and investments:
Other property and investments	77,429	76,350
Total other property and investments	77,429	76,350
Current assets:
Cash and cash equivalents	5,640	25,864	5,939
Restricted cash	11,143	10,145	$ 9,302
Accounts receivable, net of allowance for doubtful accounts	236,761	343,546
Unbilled revenue	119,042	222,186
Materials and supplies, at average cost	114,005	107,003
Fuel and gas inventory, at average cost	55,970	48,585
Unrealized gain on derivative instruments	16,524	22,247
Prepaid expense and other	37,759	21,996
Total current assets	596,844	801,572
Other long-term and regulatory assets:
Power cost adjustment mechanism	4,691	4,576
Other regulatory assets	798,112	948,540
Derivative Asset, Noncurrent	1,897	2,158
Other	86,324	71,827
Total other long-term and regulatory assets	891,024	1,027,101
Total assets	11,733,781	11,731,706
Capitalization:
Common stock	859	859
Additional paid-in capital	3,275,105	3,275,105
Earnings reinvested in the business	521,792	452,066
Accumulated other comprehensive income (loss), net of tax	(148,337)	(126,906)
Total common shareholder's equity	3,649,419	3,601,124
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
Debt discount, issuance costs and other	(31,785)	(26,361)
Total long-term debt	3,894,487	3,549,911
Total capitalization	7,543,906	7,151,035
Current liabilities:
Accounts payable	324,813	359,585
Short-term debt	206,000	329,463
Current maturities of long-term debt	0	200,000
Purchased gas adjustment liability	35,962	16,051
Accrued expenses:
Taxes	124,643	117,063
Salaries and wages	40,411	53,220
Interest	57,181	47,837
Unrealized loss on derivative instruments	30,558	64,859
Other	89,821	80,206
Total current liabilities	909,389	1,268,284
Long-term and regulatory liabilities:
Deferred income taxes	907,432	869,473
Unrealized loss on derivative instruments	11,588	21,235
Regulatory liabilities	749,691	730,273
Regulatory liability for deferred income taxes	988,671	1,012,260
Other deferred credits	623,104	679,146
Total long-term and regulatory liabilities	3,280,486	3,312,387
Commitments and contingencies (Note 8)
Total capitalization and liabilities	$ 11,733,781	$ 11,731,706