Condensed Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 423,897	$ 182,055
Accounts receivable (less accumulated provision for doubtful accounts of $10,104 at September 30, 2013 and $12,586 at December 31, 2012)	186,061	205,479
Unbilled revenues	8,305	27,699
Inventory	67,911	76,787
Derivative instruments, at fair value	203,002	304,237
Prepaid expenses and other	44,115	56,588
Total current assets	933,291	852,845
Equity in nonconsolidated investments	127,185	130,368
Property, plant and equipment	11,374,145	10,139,903
Less: accumulated depreciation and depletion	2,897,981	2,424,605
Net property, plant and equipment	8,476,164	7,715,298
Regulatory assets	113,475	111,915
Other assets	32,137	39,436
Total assets	9,682,252	8,849,862
Current liabilities:
Current portion of long-term debt	6,162	23,204
Accounts payable	306,200	289,032
Derivative instruments, at fair value	18,353	75,562
Other current liabilities	209,967	182,667
Total current liabilities	540,682	570,465
Long-term debt	2,495,717	2,502,969
Deferred income taxes and investment tax credits	1,670,562	1,666,029
Other credits	239,968	221,597
Total liabilities	4,946,929	4,961,060
Stockholders' equity:
Common stock, no par value, authorized 320,000 shares, shares issued: 175,684 at September 30, 2013 and 25,575 at December 31, 2012	1,827,413	1,770,545
Treasury stock, shares at cost: 24,968 at September 30, 2013 and 25,575 at December 31, 2012	(450,786)	(461,774)
Retained earnings	2,457,304	2,195,502
Accumulated other comprehensive income	77,175	99,547
Total common stockholders' equity	3,911,106	3,603,820
Noncontrolling interests in consolidated subsidiaries	824,217	284,982
Total equity	4,735,323	3,888,802
Total liabilities and equity	$ 9,682,252	$ 8,849,862