Segment Information (Summarized Segment Information) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting Information [Line Items]
Net operating revenues	$ 639,779	$ 760,869	$ 668,873	$ 596,715	$ 625,773	$ 744,296	$ 630,761	$ 554,205	$ 2,666,236	$ 2,555,035	$ 2,290,446
Customers in number of states	16								16
Depreciation and amortization									351,821	330,264	309,874
Impairment charge									0	0	428,036
Total operating expenses, net									1,863,100	1,826,913	2,106,611
Income (loss) from continuing operations before income taxes									503,680	429,424	(106,206)
Total assets	14,776,391				14,086,246				14,776,391	14,086,246	13,459,368
Assets of discontinued operations (included in total assets above)	929,858				937,705				929,858	937,705	932,775
Capital expenditures									924,858	765,636	785,265
Capital expenditures of discontinued operations (included in above)									21,138	26,868	43,189
Number of operating segments									2
Number of utility subsidiaries									17
Regulated Businesses [Member]
Segment Reporting Information [Line Items]
Net operating revenues									2,368,891	2,285,656	2,076,563
Depreciation and amortization									321,540	301,087	286,390
Impairment charge											0
Total operating expenses, net									1,609,276	1,587,963	1,497,453
Income (loss) from continuing operations before income taxes									535,445	478,629	371,919
Total assets	12,843,820				12,290,798				12,843,820	12,290,798	11,673,685
Assets of discontinued operations (included in total assets above)	904,391				911,905				904,391	911,905	905,938
Capital expenditures									920,210	758,150	779,428
Capital expenditures of discontinued operations (included in above)									21,052	26,725	43,078
Market-Based Operations [Member]
Segment Reporting Information [Line Items]
Net operating revenues									327,815	294,723	238,180
Depreciation and amortization									6,822	7,014	5,306
Impairment charge											0
Total operating expenses, net									290,854	269,060	217,643
Income (loss) from continuing operations before income taxes									39,250	30,443	25,168
Total assets	280,862				240,718				280,862	240,718	246,201
Assets of discontinued operations (included in total assets above)	0				6,590				0	6,590	9,333
Capital expenditures									4,648	7,486	5,837
Capital expenditures of discontinued operations (included in above)									86	143	111
Other [Member]
Segment Reporting Information [Line Items]
Net operating revenues									(30,470)	(25,344)	(24,297)
Depreciation and amortization									23,459	22,163	18,178
Impairment charge											428,036
Total operating expenses, net									(37,030)	(30,110)	391,515
Income (loss) from continuing operations before income taxes									(71,015)	(79,648)	(503,293)
Total assets	1,651,709				1,554,730				1,651,709	1,554,730	1,539,482
Assets of discontinued operations (included in total assets above)	25,467				19,210				25,467	19,210	17,504
Capital expenditures									0	0	0
Capital expenditures of discontinued operations (included in above)									$ 0	$ 0	$ 0