Business Segments (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Financial data by business segment
Operating revenues	$ 1,124,841	$ 1,116,710	$ 2,573,487	$ 2,531,767
Net income attributable to common shareholders	255,359	233,920	326,909	342,703
Assets	13,150,363		13,150,363		12,392,998
Regulated electricity segment
Financial data by business segment
Operating revenues	1,124,000	1,116,000	2,571,000	2,527,000
Net income attributable to common shareholders	246,000	225,000	318,000	320,000
Assets	13,112,000		13,112,000		12,285,000
All other
Financial data by business segment
Operating revenues	1,000	1,000	2,000	5,000
Net income attributable to common shareholders	9,000	9,000	9,000	23,000
Assets	$ 38,000		$ 38,000		$ 108,000