CONSOLIDATED BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Utility plant (at original cost, including construction work in progress):
Electric plant	$ 6,924,356	$ 6,750,400
Gas plant	2,459,063	2,385,784
Common plant	497,169	487,931
Less: Accumulated depreciation and amortization	(1,228,030)	(1,067,424)
Net utility plant	8,652,558	8,556,691
Other property and investments:
Goodwill	1,656,513	1,656,513
Other property and investments	104,676	112,367
Total other property and investments	1,761,189	1,768,880
Current assets:
Cash and cash equivalents	27,604	135,542
Restricted cash	4,723	3,700
Accounts receivable, net of allowance for doubtful accounts	265,022	321,480
Unbilled revenue	124,617	204,359
Materials and supplies, at average cost	84,537	82,353
Fuel and gas inventory, at average cost	77,248	88,953
Unrealized gain on derivative instruments	10,026	6,869
Income taxes	1	4,796
Prepaid expense and other	20,846	13,571
Power contract acquisition adjustment gain	49,746	50,785
Deferred income taxes	30,823	53,437
Total current assets	695,193	965,845
Other long-term and regulatory assets:
Regulatory asset for deferred income taxes	135,269	119,844
Power cost adjustment mechanism	0	3,773
Regulatory assets related to power contracts	35,177	37,655
Other regulatory assets	793,884	815,785
Unrealized gain on derivative instruments	6,865	14,814
Power contract acquisition adjustment gain	419,488	456,225
Other	92,497	95,763
Total other long-term and regulatory assets	1,483,180	1,543,859
Total assets	12,592,120	12,835,275
Capitalization:
Common stock	0	0
Additional paid-in capital	3,308,957	3,308,957
Earnings reinvested in the business	291,871	208,100
Accumulated other comprehensive income (loss), net of tax	(31,981)	(32,829)
Total common shareholder's equity	3,568,847	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	1,779,000	1,834,000
Debt discount	(235,339)	(264,072)
Total long-term debt	5,306,933	5,333,200
Total capitalization	8,875,780	8,817,428
Current liabilities:
Accounts payable	229,045	321,755
Short-term debt	0	181,000
Current Maturities of long-term debt	13,000	13,000
Purchased gas adjustment liability	14,509	32,587
Accrued expenses:
Taxes	84,034	95,623
Salaries and wages	32,113	38,438
Interest	77,913	82,262
Unrealized loss on derivative instruments	115,038	177,519
Power contract acquisition adjustment loss	3,865	3,902
Other	79,483	72,799
Total current liabilities	649,000	1,018,885
Long-term and regulatory liabilities:
Deferred income taxes	1,324,539	1,261,636
Unrealized loss on derivative instruments	59,392	83,276
Power Cost Adjustment Mechanism	5,726	0
Regulatory liabilities	650,665	600,697
Regulatory liabilities related to power contracts	469,234	507,009
Power contract acquisition adjustment loss	31,311	33,753
Other deferred credits	526,473	512,591
Total long-term and regulatory liabilities	3,067,340	2,998,962
Commitments and contingencies
Total capitalization and liabilities	12,592,120	12,835,275
Subsidiaries [Member]
Utility plant (at original cost, including construction work in progress):
Electric plant	9,164,460	9,048,356
Gas plant	3,068,902	2,998,188
Common plant	558,231	555,549
Less: Accumulated depreciation and amortization	(4,139,035)	(4,045,402)
Net utility plant	8,652,558	8,556,691
Other property and investments:
Other property and investments	95,845	103,646
Total other property and investments	95,845	103,646
Current assets:
Cash and cash equivalents	27,463	135,530
Restricted cash	4,723	3,700
Accounts receivable, net of allowance for doubtful accounts	271,067	321,685
Unbilled revenue	124,617	204,359
Materials and supplies, at average cost	84,537	82,353
Fuel and gas inventory, at average cost	74,443	85,547
Unrealized gain on derivative instruments	10,026	6,869
Income taxes	1	4,796
Prepaid expense and other	20,753	13,413
Deferred income taxes	45,419	68,015
Total current assets	663,049	926,267
Other long-term and regulatory assets:
Regulatory asset for deferred income taxes	134,741	119,279
Power cost adjustment mechanism	0	3,773
Other regulatory assets	792,839	813,171
Unrealized gain on derivative instruments	6,865	14,814
Other	83,553	90,330
Total other long-term and regulatory assets	1,017,998	1,041,367
Total assets	10,429,450	10,627,971
Capitalization:
Common stock	859	859
Additional paid-in capital	3,246,205	3,246,205
Earnings reinvested in the business	362,815	344,280
Accumulated other comprehensive income (loss), net of tax	(179,964)	(187,198)
Total common shareholder's equity	3,429,915	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	7,193,173	7,167,404
Current liabilities:
Accounts payable	235,090	321,953
Short-term debt	0	181,000
Short-term note owed to parent	29,598	29,598
Current Maturities of long-term debt	13,000	13,000
Purchased gas adjustment liability	14,509	32,587
Accrued expenses:
Taxes	84,034	95,623
Salaries and wages	32,113	38,438
Interest	53,867	55,806
Unrealized loss on derivative instruments	108,520	170,948
Other	72,392	69,881
Total current liabilities	643,123	1,008,834
Long-term and regulatory liabilities:
Deferred income taxes	1,362,304	1,274,602
Unrealized loss on derivative instruments	52,205	68,323
Power Cost Adjustment Mechanism	5,726	0
Regulatory liabilities	647,335	596,324
Other deferred credits	525,584	512,484
Total long-term and regulatory liabilities	2,593,154	2,451,733
Commitments and contingencies
Total capitalization and liabilities	$ 10,429,450	$ 10,627,971