CONSOLIDATED BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Utility plant (at original cost, including construction work in progress):
Electric plant	$ 6,970,492	$ 6,750,400
Gas plant	2,496,438	2,385,784
Common plant	501,006	487,931
Less: Accumulated depreciation and amortization	(1,285,287)	(1,067,424)
Net utility plant	8,682,649	8,556,691
Other property and investments:
Goodwill	1,656,513	1,656,513
Other property and investments	101,754	112,367
Total other property and investments	1,758,267	1,768,880
Current assets:
Cash and cash equivalents	14,033	135,542
Restricted cash	7,765	3,700
Accounts receivable, net of allowance for doubtful accounts	223,707	321,480
Unbilled revenue	127,759	204,359
Materials and supplies, at average cost	88,778	82,353
Fuel and gas inventory, at average cost	85,163	88,953
Unrealized gain on derivative instruments	6,643	6,869
Income taxes	0	4,796
Prepaid expense and other	38,223	13,571
Power contract acquisition adjustment gain	48,649	50,785
Deferred income taxes	26,838	53,437
Total current assets	667,558	965,845
Other long-term and regulatory assets:
Regulatory asset for deferred income taxes	139,087	119,844
Power cost adjustment mechanism	0	3,773
Regulatory assets related to power contracts	34,450	37,655
Other regulatory assets	806,747	815,785
Unrealized gain on derivative instruments	6,644	14,814
Power contract acquisition adjustment gain	405,599	456,225
Other	86,820	95,763
Total other long-term and regulatory assets	1,479,347	1,543,859
Total assets	12,587,821	12,835,275
Capitalization:
Common stock	0	0
Additional paid-in capital	3,308,957	3,308,957
Earnings reinvested in the business	257,204	208,100
Accumulated other comprehensive income (loss), net of tax	(30,777)	(32,829)
Total common shareholder's equity	3,535,384	3,484,228
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	1,834,000
Debt discount	(232,579)	(264,072)
Total long-term debt	5,229,693	5,333,200
Total capitalization	8,765,077	8,817,428
Current liabilities:
Accounts payable	238,224	321,755
Short-term debt	129,000	181,000
Current Maturities of long-term debt	10,000	13,000
Purchased gas adjustment liability	4,651	32,587
Accrued expenses:
Taxes	75,146	95,623
Salaries and wages	36,850	38,438
Interest	74,314	82,262
Unrealized loss on derivative instruments	102,847	177,519
Power contract acquisition adjustment loss	3,938	3,902
Other	66,966	72,799
Total current liabilities	741,936	1,018,885
Long-term and regulatory liabilities:
Deferred income taxes	1,330,957	1,261,636
Unrealized loss on derivative instruments	54,950	83,276
Power Cost Adjustment Mechanism	16,489	0
Regulatory liabilities	658,698	600,697
Regulatory liabilities related to power contracts	454,248	507,009
Power contract acquisition adjustment loss	30,512	33,753
Other deferred credits	534,954	512,591
Total long-term and regulatory liabilities	3,080,808	2,998,962
Commitments and contingencies
Total capitalization and liabilities	12,587,821	12,835,275
Subsidiaries [Member]
Utility plant (at original cost, including construction work in progress):
Electric plant	9,232,702	9,048,356
Gas plant	3,104,751	2,998,188
Common plant	562,067	555,549
Less: Accumulated depreciation and amortization	(4,216,871)	(4,045,402)
Net utility plant	8,682,649	8,556,691
Other property and investments:
Other property and investments	92,923	103,646
Total other property and investments	92,923	103,646
Current assets:
Cash and cash equivalents	13,863	135,530
Restricted cash	7,765	3,700
Accounts receivable, net of allowance for doubtful accounts	223,766	321,685
Unbilled revenue	127,759	204,359
Materials and supplies, at average cost	88,778	82,353
Fuel and gas inventory, at average cost	82,866	85,547
Unrealized gain on derivative instruments	6,643	6,869
Income taxes	0	4,796
Prepaid expense and other	38,162	13,413
Deferred income taxes	41,081	68,015
Total current assets	630,683	926,267
Other long-term and regulatory assets:
Regulatory asset for deferred income taxes	138,564	119,279
Power cost adjustment mechanism	0	3,773
Other regulatory assets	806,486	813,171
Unrealized gain on derivative instruments	6,644	14,814
Other	78,459	90,330
Total other long-term and regulatory assets	1,030,153	1,041,367
Total assets	10,436,408	10,627,971
Capitalization:
Common stock	859	859
Additional paid-in capital	3,246,205	3,246,205
Earnings reinvested in the business	228,041	344,280
Accumulated other comprehensive income (loss), net of tax	(176,479)	(187,198)
Total common shareholder's equity	3,298,626	3,404,146
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	7,061,884	7,167,404
Current liabilities:
Accounts payable	238,282	321,952
Short-term debt	129,000	181,000
Short-term note owed to parent	29,598	29,598
Current Maturities of long-term debt	10,000	13,000
Purchased gas adjustment liability	4,651	32,587
Accrued expenses:
Taxes	75,146	95,623
Salaries and wages	36,850	38,438
Interest	56,790	55,806
Unrealized loss on derivative instruments	96,245	170,948
Other	66,648	69,882
Total current liabilities	743,210	1,008,834
Long-term and regulatory liabilities:
Deferred income taxes	1,378,195	1,274,602
Unrealized loss on derivative instruments	46,830	68,323
Power Cost Adjustment Mechanism	16,489	0
Regulatory liabilities	656,098	596,324
Other deferred credits	533,702	512,484
Total long-term and regulatory liabilities	2,631,314	2,451,733
Commitments and contingencies
Total capitalization and liabilities	$ 10,436,408	$ 10,627,971