CONSOLIDATED BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Current assets
Cash and cash equivalents	$ 5,150	$ 3,763
Accounts receivable, net	277,461	317,039
Accounts receivable from affiliates	93,544	12,063
Investments in utility money pool arrangement	0	52,000
Accrued unbilled revenues	285,624	304,544
Inventories	223,794	253,997
Regulatory assets	143,689	196,311
Deferred income taxes	0	33,349
Derivative instruments	4,889	4,930
Prepayments and other	22,970	19,504
Total current assets	1,057,121	1,197,500
Property, plant and equipment, net	10,030,991	9,475,571
Other assets
Regulatory assets	934,728	809,011
Derivative instruments	10,868	15,357
Other	50,461	36,066
Total other assets	996,057	860,434
Total assets	12,084,169	11,533,505
Current liabilities
Current portion of long-term debt	256,297	605,633
Short-term debt	154,000	0
Accounts payable	359,969	362,580
Accounts payable to affiliates	30,001	48,371
Regulatory liabilities	33,723	68,809
Taxes accrued	153,614	116,376
Accrued interest	48,014	53,749
Dividends payable to parent	66,803	66,926
Derivative instruments	8,753	85,518
Other	72,669	75,671
Total current liabilities	1,183,843	1,483,633
Deferred credits and other liabilities
Deferred income taxes	1,782,828	1,775,448
Deferred investment tax credits	42,097	44,725
Regulatory liabilities	417,404	444,442
Asset retirement obligations	43,751	42,207
Derivative instruments	30,605	38,325
Customer advances	229,498	226,097
Pension and employee benefit obligations	324,625	222,707
Other	69,307	69,561
Total deferred credits and other liabilities	2,940,115	2,863,512
Commitments and contingencies
Capitalization, Long-term Debt and Equity [Abstract]
Long-term debt	3,374,476	2,880,642
Common stock - authorized 100 shares of $0.01 par value; outstanding 100 shares	0	0
Additional paid in capital	3,415,669	3,316,386
Retained earnings	1,192,937	1,001,709
Accumulated other comprehensive loss	(22,871)	(12,377)
Total common stockholder's equity	4,585,735	4,305,718
Total liabilities and equity	$ 12,084,169	$ 11,533,505