Condensed Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 160,325	$ 182,055
Accounts receivable (less accumulated provision for doubtful accounts of $14,696 at March 31, 2013 and $12,586 at December 31, 2012)	219,705	205,479
Unbilled revenues	20,335	27,699
Inventory	41,405	76,787
Derivative instruments, at fair value	183,727	304,237
Prepaid expenses and other	31,587	56,588
Total current assets	657,084	852,845
Equity in nonconsolidated investments	127,851	130,368
Property, plant and equipment	10,436,556	10,139,903
Less: accumulated depreciation and depletion	2,567,762	2,424,605
Net property, plant and equipment	7,868,794	7,715,298
Regulatory assets	110,557	111,915
Other assets	38,198	39,436
Total assets	8,802,484	8,849,862
Current liabilities:
Current portion of long-term debt	6,212	23,204
Accounts payable	271,007	289,032
Derivative instruments, at fair value	92,107	75,562
Other current liabilities	130,032	182,667
Total current liabilities	499,358	570,465
Long-term debt	2,499,437	2,502,969
Deferred income taxes and investment tax credits	1,651,737	1,666,029
Other credits	224,916	221,597
Total liabilities	4,875,448	4,961,060
Stockholders' equity:
Common stock, no par value, authorized 320,000 shares, shares issued: 175,684 at March 31, 2013 and December 31, 2012	1,780,966	1,770,545
Treasury stock, shares at cost: 25,249 at March 31, 2013 and 25,575 at December 31, 2012	(455,872)	(461,774)
Retained earnings	2,291,241	2,195,502
Accumulated other comprehensive income	21,582	99,547
Total common stockholders' equity	3,637,917	3,603,820
Noncontrolling interests in consolidated subsidiaries	289,119	284,982
Total equity	3,927,036	3,888,802
Total liabilities and equity	$ 8,802,484	$ 8,849,862