Business Segments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Financial data by business segment
Operating revenues	$ 1,109,475	$ 1,124,841	$ 2,608,682	$ 2,573,487
Net income (loss) attributable to common shareholders	244,823	255,359	358,911	326,909
Assets	13,460,286		13,460,286		13,111,018
Regulated electricity segment
Financial data by business segment
Operating revenues	1,109,000	1,124,000	2,607,000	2,571,000
Net income (loss) attributable to common shareholders	246,000	246,000	364,000	318,000
Assets	13,426,000		13,426,000		13,068,000
All other
Financial data by business segment
Operating revenues		1,000	2,000	2,000
Net income (loss) attributable to common shareholders	(1,000)	9,000	(5,000)	9,000
Assets	$ 34,000		$ 34,000		$ 43,000