CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDERS' EQUITY AND COMPREHENSIVE INCOME (USD $) In Thousands, except Share data, unless otherwise specified	Common Stock [Member]	Additional Paid In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]	Total
Balance at Dec. 31, 2008	$ 1,134,480	$ 4,695,019	$ 1,187,911	$ (53,669)	$ 6,963,741
Balance (in shares) at Dec. 31, 2008	453,792,000
Net income			680,887		680,887
Pension and retiree medical benefit adjustments, net of tax				(3,129)	(3,129)
Net derivative instrument changes, net of tax				6,678	6,678
Unrealized gain (loss) - marketable securities, net of tax				411	411
Comprehensive income for the period					684,847
Dividends declared:
Cumulative preferred stock			(4,241)		(4,241)
Common stock			(445,356)		(445,356)
Premium on redemption of preferred stock					0
Issuances of common stock	9,293	48,679			57,972
Issuances of common stock (in shares)	3,717,000
Share-based compensation		26,282			26,282
Balance at Dec. 31, 2009	1,143,773	4,769,980	1,419,201	(49,709)	7,283,245
Balance (in shares) at Dec. 31, 2009	457,509,000
Net income			755,834		755,834
Pension and retiree medical benefit adjustments, net of tax				(1,855)	(1,855)
Net derivative instrument changes, net of tax				(1,659)	(1,659)
Unrealized gain (loss) - marketable securities, net of tax				130	130
Comprehensive income for the period					752,450
Dividends declared:
Cumulative preferred stock			(4,241)		(4,241)
Common stock			(469,091)		(469,091)
Premium on redemption of preferred stock					0
Issuances of common stock	62,061	426,717			488,778
Issuances of common stock (in shares)	24,825,000
Share-based compensation		32,378			32,378
Balance at Dec. 31, 2010	1,205,834	5,229,075	1,701,703	(53,093)	8,083,519
Balance (in shares) at Dec. 31, 2010	482,334,000				482,333,750
Net income			841,172		841,172
Pension and retiree medical benefit adjustments, net of tax				(3,205)	(3,205)
Net derivative instrument changes, net of tax				(37,644)	(37,644)
Unrealized gain (loss) - marketable securities, net of tax				(93)	(93)
Comprehensive income for the period					800,230
Dividends declared:
Cumulative preferred stock			(3,534)		(3,534)
Common stock			(503,525)		(503,525)
Premium on redemption of preferred stock			(3,260)		(3,260)
Issuances of common stock	10,400	54,514			64,914
Issuances of common stock (in shares)	4,160,000
Share-based compensation		43,854			43,854
Balance at Dec. 31, 2011	$ 1,216,234	$ 5,327,443	$ 2,032,556	$ (94,035)	$ 8,482,198
Balance (in shares) at Dec. 31, 2011	486,494,000				486,493,933