CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Utility plant (at original cost, including construction work in progress):
Electric plant	$ 7,289,865	$ 7,135,206
Gas plant	2,757,784	2,680,067
Common plant	471,626	472,926
Less: Accumulated depreciation and amortization	(1,739,918)	(1,611,220)
Net utility plant	8,779,357	8,676,979
Other property and investments:
Goodwill	1,656,513	1,656,513
Other property and investments	88,419	91,139
Total other property and investments	1,744,932	1,747,652
Current assets:
Cash and cash equivalents	14,535	37,527
Restricted cash	7,964	32,863
Accounts receivable, net of allowance for doubtful accounts	240,756	306,923
Unbilled revenue	135,624	168,039
Purchased Gas Adjustment Receivable	0	21,073
Materials and supplies, at average cost	83,494	83,189
Fuel and gas inventory, at average cost	58,090	69,433
Unrealized gain on derivative instruments	17,951	21,178
Income taxes	1	301
Prepaid expense and other	24,183	20,905
Power contract acquisition adjustment gain	42,033	43,843
Deferred income taxes	124,472	161,445
Total current assets	749,103	966,719
Other long-term and regulatory assets:
Regulatory asset for deferred income taxes	75,268	95,432
Power Cost Adjustment Mechanism Asset, Noncurrent	4,688	4,623
Regulatory assets related to power contracts	27,592	29,816
Other regulatory assets	892,075	866,835
Unrealized gain on derivative instruments	3,137	3,170
Power contract acquisition adjustment gain	318,184	347,547
Other	98,817	96,275
Total other long-term and regulatory assets	1,419,761	1,443,698
Total assets	12,693,153	12,835,048
Capitalization:
Common stock	0	0
Additional paid-in capital	3,308,957	3,308,957
Earnings reinvested in the business	220,206	271,414
Accumulated Other Comprehensive Income (Loss), Net of Tax	(34,854)	(37,043)
Total common shareholder's equity	3,494,309	3,543,328
Long-term debt:
First mortgage bonds and senior notes	3,364,412	3,189,412
Pollution control bonds	161,860	161,860
Junior subordinated notes	250,000	250,000
Long-term debt	1,800,000	1,699,000
Debt discount	(214,705)	(218,664)
Total long-term debt	5,361,567	5,081,608
Total capitalization	8,855,876	8,624,936
Current liabilities:
Accounts payable	226,109	307,578
Short-term debt	0	85,000
Long-term Debt, Current Maturities	12,000	162,000
Purchased gas adjustment liability	14,890	0
Accrued expenses:
Taxes	99,231	107,782
Salaries and wages	32,762	40,970
Interest	73,525	78,914
Unrealized loss on derivative instruments	130,153	142,195
Power contract acquisition adjustment loss	3,551	3,593
Other	60,959	62,464
Total current liabilities	653,180	990,496
Long-term and regulatory liabilities:
Deferred income taxes	1,524,944	1,522,357
Unrealized loss on derivative instruments	45,889	62,913
Regulatory liabilities	613,287	633,471
Regulatory liabilities related to power contracts	360,217	391,389
Power contract acquisition adjustment loss	24,041	26,223
Other deferred credits	615,719	583,263
Total long-term and regulatory liabilities	$ 3,184,097	$ 3,219,616
Commitments and contingencies
Total capitalization and liabilities	$ 12,693,153	$ 12,835,048
Subsidiaries [Member]
Utility plant (at original cost, including construction work in progress):
Electric plant	9,471,915	9,330,999
Gas plant	3,353,746	3,282,818
Common plant	511,542	512,842
Less: Accumulated depreciation and amortization	(4,557,846)	(4,449,680)
Net utility plant	8,779,357	8,676,979
Other property and investments:
Other property and investments	84,202	86,913
Total other property and investments	84,202	86,913
Current assets:
Cash and cash equivalents	13,320	37,466
Restricted cash	7,964	32,863
Accounts receivable, net of allowance for doubtful accounts	241,179	307,046
Unbilled revenue	135,624	168,039
Purchased Gas Adjustment Receivable	0	21,073
Materials and supplies, at average cost	83,494	83,189
Fuel and gas inventory, at average cost	55,922	66,656
Unrealized gain on derivative instruments	17,951	21,178
Income taxes	1	301
Prepaid expense and other	24,181	20,907
Deferred income taxes	170,716	208,447
Total current assets	750,352	967,165
Other long-term and regulatory assets:
Regulatory asset for deferred income taxes	74,759	94,913
Power Cost Adjustment Mechanism Asset, Noncurrent	4,688	4,623
Other regulatory assets	892,048	866,793
Unrealized gain on derivative instruments	3,137	3,170
Other	89,296	89,306
Total other long-term and regulatory assets	1,063,928	1,058,805
Total assets	10,677,839	10,789,862
Capitalization:
Common stock	859	859
Additional paid-in capital	3,275,105	3,246,205
Earnings reinvested in the business	230,034	202,622
Accumulated Other Comprehensive Income (Loss), Net of Tax	(162,638)	(170,957)
Total common shareholder's equity	3,343,360	3,278,729
Long-term debt:
First mortgage bonds and senior notes	3,364,412	3,189,412
Pollution control bonds	161,860	161,860
Junior subordinated notes	250,000	250,000
Debt discount	(1,919)	(13)
Total long-term debt	3,774,353	3,601,259
Total capitalization	7,117,713	6,879,988
Current liabilities:
Accounts payable	226,109	307,572
Short-term debt	0	85,000
Short-term note owed to parent	0	28,933
Long-term Debt, Current Maturities	12,000	162,000
Purchased gas adjustment liability	14,890	0
Accrued expenses:
Taxes	99,231	107,782
Salaries and wages	32,762	40,970
Interest	47,424	55,346
Unrealized loss on derivative instruments	124,139	135,973
Other	60,959	62,464
Total current liabilities	617,514	986,040
Long-term and regulatory liabilities:
Deferred income taxes	1,672,405	1,649,857
Unrealized loss on derivative instruments	43,609	60,063
Regulatory liabilities	611,091	630,651
Other deferred credits	615,507	583,263
Total long-term and regulatory liabilities	$ 2,942,612	$ 2,923,834
Commitments and contingencies
Total capitalization and liabilities	$ 10,677,839	$ 10,789,862