Business Segments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Financial data by business segment
Operating revenues	$ 878,576	$ 799,799	$ 1,499,207	$ 1,448,646
Net income (loss) attributable to common shareholders	122,345	86,685	114,088	71,550
Assets	13,214,407		13,214,407		13,111,018
Regulated electricity segment
Financial data by business segment
Operating revenues	878,000	799,000	1,498,000	1,447,000
Net income (loss) attributable to common shareholders	123,000	86,000	117,000	71,000
Assets	13,178,000		13,178,000		13,068,000
All other
Financial data by business segment
Operating revenues	1,000	1,000	1,000	2,000
Net income (loss) attributable to common shareholders	(1,000)	1,000	(3,000)	1,000
Assets	$ 36,000		$ 36,000		$ 43,000