Condensed Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Current assets:
Cash and cash equivalents	$ 515,038	$ 831,251
Accounts receivable (less accumulated provision for doubtful accounts June 30, 2012 and December 31, 2011: $12,429 and $16,371)	105,800	153,321
Unbilled revenues	7,626	30,257
Inventory	77,907	123,960
Derivative instruments, at fair value	467,587	512,161
Prepaid expenses and other	39,595	39,184
Total current assets	1,213,553	1,690,134
Equity in nonconsolidated investments	132,221	136,972
Property, plant and equipment	9,423,950	8,768,713
Less: accumulated depreciation and depletion	2,167,508	1,962,404
Net property, plant and equipment	7,256,442	6,806,309
Regulatory assets	101,267	94,095
Other assets	44,688	45,209
Total assets	8,748,171	8,772,719
Current liabilities:
Current portion of long-term debt	229,944	219,315
Accounts payable	202,483	256,757
Derivative instruments, at fair value	109,214	123,306
Other current liabilities	154,969	205,532
Total current liabilities	696,610	804,910
Long-term debt	2,506,739	2,527,627
Deferred income taxes and investment tax credits	1,672,584	1,618,944
Other credits	226,230	227,408
Total liabilities	5,102,163	5,178,889
Common stockholders' equity:
Common stock, no par value, authorized 320,000 shares; shares issued June 30, 2012 and December 31, 2011: 175,684 and 175,684	1,754,884	1,734,994
Treasury stock, shares at cost: June 30, 2012 and December 31, 2011: 26,089 and 26,207	(471,075)	(473,215)
Retained earnings	2,181,504	2,143,910
Accumulated other comprehensive income	180,695	188,141
Total common stockholders' equity	3,646,008	3,593,830
Total liabilities and stockholders' equity	$ 8,748,171	$ 8,772,719