Condensed Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 16,700	$ 182,055
Accounts receivable (less accumulated provision for doubtful accounts of $13,189 at June 30, 2013 and $12,586 at December 31, 2012)	237,372	205,479
Unbilled revenues	4,653	27,699
Inventory	55,034	76,787
Derivative instruments, at fair value	230,139	304,237
Prepaid expenses and other	33,510	56,588
Total current assets	577,408	852,845
Equity in nonconsolidated investments	129,695	130,368
Property, plant and equipment	10,929,867	10,139,903
Less: accumulated depreciation and depletion	2,729,035	2,424,605
Net property, plant and equipment	8,200,832	7,715,298
Regulatory assets	110,123	111,915
Other assets	36,217	39,436
Total assets	9,054,275	8,849,862
Current liabilities:
Current portion of long-term debt	6,212	23,204
Short-term loans	55,000
Accounts payable	271,027	289,032
Derivative instruments, at fair value	36,471	75,562
Other current liabilities	126,694	182,667
Total current liabilities	495,404	570,465
Long-term debt	2,499,074	2,502,969
Deferred income taxes and investment tax credits	1,719,918	1,666,029
Other credits	237,654	221,597
Total liabilities	4,952,050	4,961,060
Stockholders' equity:
Common stock, no par value, authorized 320,000 shares, shares issued: 175,684 at June 30, 2013 and December 31, 2012	1,805,500	1,770,545
Treasury stock, shares at cost: 25,090 at June 30, 2013 and 25,575 at December 31, 2012	(452,992)	(461,774)
Retained earnings	2,373,575	2,195,502
Accumulated other comprehensive income	84,993	99,547
Total common stockholders' equity	3,811,076	3,603,820
Noncontrolling interests in consolidated subsidiaries	291,149	284,982
Total equity	4,102,225	3,888,802
Total liabilities and equity	$ 9,054,275	$ 8,849,862