CONSOLIDATED BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Utility plant (at original cost, including construction work in progress):
Electric plant	$ 7,046,429	$ 7,019,853
Gas plant	2,602,397	2,528,629
Common plant	456,383	504,036
Less: Accumulated depreciation and amortization	(1,478,202)	(1,373,178)
Net utility plant	8,627,007	8,679,340
Other property and investments:
Goodwill	1,656,513	1,656,513
Other property and investments	99,877	100,332
Total other property and investments	1,756,390	1,756,845
Current assets:
Cash and cash equivalents	84,707	44,302
Restricted cash	54,964	7,171
Accounts receivable, net of allowance for doubtful accounts	287,374	408,512
Unbilled revenue	112,304	219,884
Purchased Gas Adjustment Receivable	12,539	0
Materials and supplies, at average cost	82,658	88,140
Fuel and gas inventory, at average cost	64,334	66,717
Unrealized gain on derivative instruments	44,405	18,867
Income taxes	0	297
Prepaid expense and other	20,043	18,787
Power contract acquisition adjustment gain	49,020	48,509
Deferred income taxes	73,902	86,004
Total current assets	886,250	1,007,190
Other long-term and regulatory assets:
Regulatory asset for deferred income taxes	150,083	146,867
Regulatory assets related to power contracts	31,311	33,753
Other regulatory assets	733,281	749,382
Unrealized gain on derivative instruments	7,678	7,733
Power contract acquisition adjustment gain	370,468	394,556
Other	137,336	130,909
Total other long-term and regulatory assets	1,430,157	1,463,200
Total assets	12,699,804	12,906,575
Capitalization:
Common stock	0	0
Additional paid-in capital	3,308,957	3,308,957
Earnings reinvested in the business	333,933	323,007
Accumulated Other Comprehensive Income (Loss), Net of Tax	47,164	47,715
Total common shareholder's equity	3,690,054	3,679,679
Long-term debt:
First mortgage bonds and senior notes	3,351,412	3,351,412
Pollution control bonds	161,860	161,860
Junior subordinated notes	250,000	250,000
Long-term debt	1,699,000	1,699,000
Debt discount	(224,230)	(229,796)
Total long-term debt	5,238,042	5,232,476
Total capitalization	8,928,096	8,912,155
Current liabilities:
Accounts payable	245,760	296,681
Short-term debt	0	162,000
Purchased gas adjustment liability	0	5,938
Accrued expenses:
Taxes	92,023	109,559
Salaries and wages	32,256	38,491
Interest	78,759	79,303
Unrealized loss on derivative instruments	23,690	48,049
Power contract acquisition adjustment loss	3,720	3,937
Other	68,671	60,335
Total current liabilities	544,879	804,293
Long-term and regulatory liabilities:
Deferred income taxes	1,534,501	1,487,005
Unrealized loss on derivative instruments	21,761	38,162
Power Cost Adjustment Mechanism	5,445	5,345
Regulatory liabilities	694,486	689,060
Regulatory liabilities related to power contracts	419,488	443,065
Power contract acquisition adjustment loss	27,592	29,816
Other deferred credits	523,556	497,674
Total long-term and regulatory liabilities	3,226,829	3,190,127
Commitments and contingencies
Total capitalization and liabilities	12,699,804	12,906,575
Subsidiaries [Member]
Utility plant (at original cost, including construction work in progress):
Electric plant	9,296,198	9,276,400
Gas plant	3,207,261	3,134,880
Common plant	497,495	565,072
Less: Accumulated depreciation and amortization	(4,373,947)	(4,297,012)
Net utility plant	8,627,007	8,679,340
Other property and investments:
Other property and investments	91,465	91,919
Total other property and investments	91,465	91,919
Current assets:
Cash and cash equivalents	69,160	44,111
Restricted cash	54,964	7,171
Accounts receivable, net of allowance for doubtful accounts	287,848	408,827
Unbilled revenue	112,304	219,884
Purchased Gas Adjustment Receivable	12,539
Materials and supplies, at average cost	82,658	88,140
Fuel and gas inventory, at average cost	61,555	63,914
Unrealized gain on derivative instruments	44,405	18,867
Income taxes	0	297
Prepaid expense and other	20,043	18,770
Deferred income taxes	129,122	141,058
Total current assets	874,598	1,011,039
Other long-term and regulatory assets:
Regulatory asset for deferred income taxes	149,573	146,350
Other regulatory assets	733,215	749,382
Unrealized gain on derivative instruments	7,678	7,733
Other	130,131	123,125
Total other long-term and regulatory assets	1,020,597	1,026,590
Total assets	10,613,667	10,808,888
Capitalization:
Common stock	859	859
Additional paid-in capital	3,246,205	3,246,205
Earnings reinvested in the business	264,720	289,432
Accumulated Other Comprehensive Income (Loss), Net of Tax	(91,438)	(95,739)
Total common shareholder's equity	3,420,346	3,440,757
Long-term debt:
First mortgage bonds and senior notes	3,351,412	3,351,412
Pollution control bonds	161,860	161,860
Junior subordinated notes	250,000	250,000
Debt discount	(14)	(14)
Total long-term debt	3,763,258	3,763,258
Total capitalization	7,183,604	7,204,015
Current liabilities:
Accounts payable	245,753	296,675
Short-term debt	0	162,000
Short-term note owed to parent	28,933	29,598
Purchased gas adjustment liability	0	5,938
Accrued expenses:
Taxes	92,023	109,559
Salaries and wages	32,256	38,491
Interest	55,216	55,262
Unrealized loss on derivative instruments	16,895	41,465
Other	68,671	60,334
Total current liabilities	539,747	799,322
Long-term and regulatory liabilities:
Deferred income taxes	1,653,337	1,584,850
Unrealized loss on derivative instruments	16,208	31,523
Power Cost Adjustment Mechanism	5,445	5,345
Regulatory liabilities	691,642	686,176
Other deferred credits	523,684	497,657
Total long-term and regulatory liabilities	2,890,316	2,805,551
Commitments and contingencies
Total capitalization and liabilities	$ 10,613,667	$ 10,808,888