CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Operating activities
Net income	$ 841,172	$ 755,834	$ 680,887
Remove (income) loss from discontinued operations	202	(3,878)	4,637
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	908,853	872,186	835,597
Conservation and demand side management program amortization	9,816	21,700	29,418
Nuclear fuel amortization	100,902	105,369	80,104
Deferred income taxes	466,567	414,460	407,517
Amortization of investment tax credits	(6,194)	(6,353)	(6,426)
Allowance for equity funds used during construction	(51,223)	(56,152)	(75,686)
Equity earnings of unconsolidated subsidiaries	(30,527)	(29,948)	(24,664)
Dividends from unconsolidated subsidiaries	34,034	32,538	29,059
Provision for bad debts	44,521	44,068	49,023
Share-based compensation expense	45,006	35,807	29,672
Net derivative losses (gains)	9,966	(35,552)	39,029
Changes in operating assets and liabilities:
Accounts receivable	(79,701)	(29,749)	122,503
Accrued unbilled revenues	19,951	(14,642)	49,430
Inventories	(57,432)	9,239	100,504
Other current assets	62,458	10,461	(84,783)
Accounts payable	13,748	(188,855)	(50,638)
Net regulatory assets and liabilities	149,282	36,096	(38,403)
Other current liabilities	112,353	13,192	49,388
Pension and other employee benefit obligations	(150,717)	(62,625)	(245,987)
Change in other noncurrent assets	24,069	5,936	(1,991)
Change in other noncurrent liabilities	(61,584)	(35,190)	(65,284)
Net cash provided by operating activities	2,405,522	1,893,942	1,912,906
Investing activities
Utility capital/construction expenditures	(2,205,567)	(2,216,193)	(1,777,608)
Allowance for equity funds used during construction	51,223	56,152	75,686
Merricourt refund	101,261	0	0
Merricourt deposit	(90,833)	(1,134)	(9,294)
Purchase of investments in external decommissioning fund	(2,098,642)	(3,781,438)	(1,644,278)
Proceeds from the sale of investments in external decommissioning fund	2,098,642	3,786,373	1,664,957
Proceeds from the sale of assets	0	87,823	0
Acquisition of generation assets	0	(732,495)	0
Investment in WYCO Development LLC	(2,446)	(8,046)	(42,490)
Change in restricted cash	(95,287)	89	264
Other, net	(6,152)	2,145	(1,917)
Net cash used in investing activities	(2,247,801)	(2,806,724)	(1,734,680)
Financing activities
(Repayments of) proceeds from short-term borrowings, net	(247,400)	7,400	3,750
Proceeds from issuance of long-term debt	688,598	1,433,406	689,915
Repayments of long-term debt, including reacquisition premiums	(105,623)	(560,383)	(621,296)
Proceeds from issuance of common stock	38,691	457,258	20,133
Redemption of preferred stock	(104,980)	0	0
Dividends paid	(474,760)	(432,110)	(414,922)
Net cash (used in) provided by financing activities	(205,474)	905,571	(322,420)
Net change in cash and cash equivalents	(47,753)	(7,211)	(144,194)
Cash and cash equivalents at beginning of period	108,437	115,648	259,842
Cash and cash equivalents at end of period	60,684	108,437	115,648
Supplemental disclosure of cash flow information:
Cash paid for interest (net of amounts capitalized)	(531,148)	(530,072)	(514,675)
Cash received (paid) for income taxes, net	55,764	(16,635)	21,154
Supplemental disclosure of non-cash investing and financing transactions:
Property, plant and equipment additions in accounts payable	137,558	174,903	68,417
Storage assets under capital lease	3,688	6,314	71,553
Issuance of common stock for reinvested dividends and 401(k) plans	$ 71,715	$ 63,905	$ 54,638