CONSOLIDATED BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Utility plant (at original cost, including construction work in progress):
Electric plant	$ 7,079,136	$ 7,019,853
Gas plant	2,557,017	2,528,629
Common plant	457,071	504,036
Less: Accumulated depreciation and amortization	(1,401,652)	(1,373,178)
Net utility plant	8,691,572	8,679,340
Other property and investments:
Goodwill	1,656,513	1,656,513
Other property and investments	101,572	100,332
Total other property and investments	1,758,085	1,756,845
Current assets:
Cash and cash equivalents	23,581	44,302
Restricted cash	25,437	7,171
Accounts receivable, net of allowance for doubtful accounts	432,641	408,512
Unbilled revenue	167,105	219,884
Materials and supplies, at average cost	87,926	88,140
Fuel and gas inventory, at average cost	31,245	66,717
Unrealized gain on derivative instruments	44,889	18,867
Income taxes	149	297
Prepaid expense and other	18,232	18,787
Power contract acquisition adjustment gain	48,243	48,509
Deferred income taxes	77,343	86,004
Total current assets	956,791	1,007,190
Other long-term and regulatory assets:
Regulatory asset for deferred income taxes	144,166	146,867
Regulatory assets related to power contracts	32,985	33,753
Other regulatory assets	709,094	749,382
Unrealized gain on derivative instruments	6,820	7,733
Power contract acquisition adjustment gain	380,204	394,556
Other	134,186	130,909
Total other long-term and regulatory assets	1,407,455	1,463,200
Total assets	12,813,903	12,906,575
Capitalization:
Common stock	0	0
Additional paid-in capital	3,308,957	3,308,957
Earnings reinvested in the business	388,518	323,007
Accumulated Other Comprehensive Income (Loss), Net of Tax	47,947	47,715
Total common shareholder's equity	3,745,422	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	(227,013)	(229,796)
Total long-term debt	5,235,259	5,232,476
Total capitalization	8,980,681	8,912,155
Current liabilities:
Accounts payable	288,420	296,681
Short-term debt	0	162,000
Purchased gas adjustment liability	2,859	5,938
Accrued expenses:
Taxes	121,035	109,559
Salaries and wages	28,245	38,491
Interest	74,173	79,303
Unrealized loss on derivative instruments	33,213	48,049
Power contract acquisition adjustment loss	3,889	3,937
Other	84,199	60,335
Total current liabilities	636,033	804,293
Long-term and regulatory liabilities:
Deferred income taxes	1,517,287	1,487,005
Unrealized loss on derivative instruments	27,444	38,162
Power Cost Adjustment Mechanism	5,394	5,345
Regulatory liabilities	676,750	689,060
Regulatory liabilities related to power contracts	428,447	443,065
Power contract acquisition adjustment loss	29,095	29,816
Other deferred credits	512,772	497,674
Total long-term and regulatory liabilities	3,197,189	3,190,127
Commitments and contingencies
Total capitalization and liabilities	12,813,903	12,906,575
Subsidiaries [Member]
Utility plant (at original cost, including construction work in progress):
Electric plant	9,331,849	9,276,400
Gas plant	3,162,703	3,134,880
Common plant	498,184	565,072
Less: Accumulated depreciation and amortization	(4,301,164)	(4,297,012)
Net utility plant	8,691,572	8,679,340
Other property and investments:
Other property and investments	93,159	91,919
Total other property and investments	93,159	91,919
Current assets:
Cash and cash equivalents	23,556	44,111
Restricted cash	25,437	7,171
Accounts receivable, net of allowance for doubtful accounts	432,915	408,827
Unbilled revenue	167,105	219,884
Materials and supplies, at average cost	87,926	88,140
Fuel and gas inventory, at average cost	28,463	63,914
Unrealized gain on derivative instruments	44,889	18,867
Income taxes	149	297
Prepaid expense and other	18,232	18,770
Deferred income taxes	132,248	141,058
Total current assets	960,920	1,011,039
Other long-term and regulatory assets:
Regulatory asset for deferred income taxes	143,628	146,350
Other regulatory assets	709,094	749,382
Unrealized gain on derivative instruments	6,820	7,733
Other	127,202	123,125
Total other long-term and regulatory assets	986,744	1,026,590
Total assets	10,732,395	10,808,888
Capitalization:
Common stock	859	859
Additional paid-in capital	3,246,205	3,246,205
Earnings reinvested in the business	336,401	289,432
Accumulated Other Comprehensive Income (Loss), Net of Tax	(92,941)	(95,739)
Total common shareholder's equity	3,490,524	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	7,253,782	7,204,015
Current liabilities:
Accounts payable	288,415	296,675
Short-term debt	0	162,000
Short-term note owed to parent	29,598	29,598
Purchased gas adjustment liability	2,859	5,938
Accrued expenses:
Taxes	121,035	109,559
Salaries and wages	28,245	38,491
Interest	57,094	55,262
Unrealized loss on derivative instruments	26,461	41,465
Other	84,199	60,334
Total current liabilities	637,906	799,322
Long-term and regulatory liabilities:
Deferred income taxes	1,626,368	1,584,850
Unrealized loss on derivative instruments	22,070	31,523
Power Cost Adjustment Mechanism	5,394	5,345
Regulatory liabilities	674,103	686,176
Other deferred credits	512,772	497,657
Total long-term and regulatory liabilities	2,840,707	2,805,551
Commitments and contingencies
Total capitalization and liabilities	$ 10,732,395	$ 10,808,888