CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Utility plant (at original cost, including construction work in progress):
Electric plant	$ 7,918,877	$ 7,673,772
Natural gas plant	3,253,977	3,051,586
Common plant	738,409	594,994
Less: Accumulated depreciation and amortization	(2,409,508)	(2,161,796)
Net utility plant	9,501,755	9,158,556
Other property and investments:
Goodwill	1,656,513	1,656,513
Other property and investments	166,996	106,418
Total other property and investments	1,823,509	1,762,931
Current assets:
Cash and cash equivalents	6,768	28,878
Restricted cash	9,302	12,418
Accounts receivable, net of allowance for doubtful accounts	232,699	329,375
Unbilled revenue	126,252	234,053
Purchased gas adjustment receivable	0	2,785
Materials and supplies, at average cost	108,814	106,378
Fuel and gas inventory, at average cost	60,645	58,181
Unrealized gain on derivative instruments	16,605	54,341
Prepaid expense and other	35,655	43,046
Power contract acquisition adjustment gain	15,932	33,413
Total current assets	612,672	902,868
Other long-term and regulatory assets:
Regulatory asset for deferred income taxes	71,566	72,038
Power cost adjustment mechanism	4,540	4,531
Regulatory assets related to power contracts	19,998	22,613
Other regulatory assets	1,014,796	1,034,348
Derivative Asset, Noncurrent	2,877	8,738
Power contract acquisition adjustment gain	163,588	241,648
Other	65,138	58,109
Total other long-term and regulatory assets	1,342,503	1,442,025
Total assets	13,280,439	13,266,380
Capitalization:
Common stock	0	0
Additional paid-in capital	3,308,957	3,308,957
Earnings reinvested in the business	571,588	413,468
Accumulated other comprehensive income (loss), net of tax	(33,312)	(33,712)
Total common shareholder's equity	3,847,233	3,688,713
Long-term debt:
First mortgage bonds and senior notes	3,164,412	3,362,000
Pollution control bonds	161,860	161,860
Junior subordinated notes	250,000	250,000
Long-term debt	1,883,064	1,812,480
Debt discount, issuance costs and other	(224,336)	(234,679)
Total long-term debt	5,235,000	5,351,661
Total capitalization	9,082,233	9,040,374
Current liabilities:
Accounts payable	296,659	317,043
Short-term debt	139,000	245,763
Current maturities of long-term debt	200,000	2,412
Purchased gas adjustment liability	5,784	0
Accrued expenses:
Taxes	81,354	111,428
Salaries and wages	41,121	49,749
Interest	79,213	73,610
Unrealized loss on derivative instruments	49,820	44,310
Power contract acquisition adjustment loss	2,850	3,159
Other	81,486	71,996
Total current liabilities	977,287	919,470
Long-term and regulatory liabilities:
Deferred income taxes	1,652,573	1,570,931
Unrealized loss on derivative instruments	15,578	16,261
Regulatory liabilities	611,899	654,622
Regulatory liabilities related to power contracts	179,519	275,061
Power contract acquisition adjustment loss	17,148	19,454
Other deferred credits	744,202	770,207
Total long-term and regulatory liabilities	3,220,919	3,306,536
Commitments and contingencies (Note 8)
Total capitalization and liabilities	13,280,439	13,266,380
Subsidiaries [Member]
Utility plant (at original cost, including construction work in progress):
Electric plant	10,036,204	9,813,169
Natural gas plant	3,838,533	3,640,271
Common plant	776,116	632,718
Less: Accumulated depreciation and amortization	(5,149,098)	(4,927,602)
Net utility plant	9,501,755	9,158,556
Other property and investments:
Other property and investments	78,332	77,960
Total other property and investments	78,332	77,960
Current assets:
Cash and cash equivalents	5,939	28,481
Restricted cash	9,302	12,418
Accounts receivable, net of allowance for doubtful accounts	237,091	344,964
Unbilled revenue	126,252	234,053
Purchased gas adjustment receivable	0	2,785
Materials and supplies, at average cost	108,814	106,378
Fuel and gas inventory, at average cost	59,640	56,851
Unrealized gain on derivative instruments	16,605	54,341
Prepaid expense and other	35,655	43,046
Total current assets	599,298	883,317
Other long-term and regulatory assets:
Regulatory asset for deferred income taxes	71,057	71,517
Power cost adjustment mechanism	4,540	4,531
Other regulatory assets	1,014,804	1,034,352
Derivative Asset, Noncurrent	2,877	8,738
Other	65,138	58,109
Total other long-term and regulatory assets	1,158,416	1,177,247
Total assets	11,337,801	11,297,080
Capitalization:
Common stock	859	859
Additional paid-in capital	3,275,105	3,275,105
Earnings reinvested in the business	486,095	359,795
Accumulated other comprehensive income (loss), net of tax	(138,191)	(145,511)
Total common shareholder's equity	3,623,868	3,490,248
Long-term debt:
First mortgage bonds and senior notes	3,164,412	3,362,000
Pollution control bonds	161,860	161,860
Junior subordinated notes	250,000	250,000
Debt discount, issuance costs and other	(27,043)	(28,974)
Total long-term debt	3,549,229	3,744,886
Total capitalization	7,173,097	7,235,134
Current liabilities:
Accounts payable	296,659	317,043
Short-term debt	139,000	245,763
Current maturities of long-term debt	200,000	2,412
Purchased gas adjustment liability	5,784	0
Accrued expenses:
Taxes	81,354	111,428
Salaries and wages	41,121	49,749
Interest	56,254	48,087
Unrealized loss on derivative instruments	49,820	44,170
Other	81,486	71,996
Total current liabilities	951,478	890,648
Long-term and regulatory liabilities:
Deferred income taxes	1,844,886	1,732,390
Unrealized loss on derivative instruments	15,578	16,261
Regulatory liabilities	610,902	653,296
Other deferred credits	741,860	769,351
Total long-term and regulatory liabilities	3,213,226	3,171,298
Commitments and contingencies (Note 8)
Total capitalization and liabilities	$ 11,337,801	$ 11,297,080