Segments and Related 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]
Equity investments in unconsolidated subsidiaries	$ 92,700				$ 97,600				$ 92,700	$ 97,600
Operating revenues from external customers									10,654,770	10,310,947	9,644,303
Intersegment revenues									0	0	0
Total revenues	2,568,410	2,831,598	2,438,222	2,816,540	2,566,934	2,628,787	2,307,764	2,807,462	10,654,770	10,310,947	9,644,303
Depreciation and amortization									890,619	858,882	818,052
Interest charges and financing costs									562,917	548,621	521,855
Income tax expense (benefit)									468,316	436,635	371,314
Income (loss) from continuing operations	140,941	338,295	158,671	203,467	136,503	312,488	135,625	167,340	841,374	751,956	685,524
Regulated Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers									8,766,593	8,451,845	7,704,723
Intersegment revenues									1,269	1,015	816
Total revenues									8,767,862	8,452,860	7,705,539
Depreciation and amortization									773,392	748,815	711,090
Interest charges and financing costs									402,668	380,074	371,525
Income tax expense (benefit)									473,848	434,756	357,128
Income (loss) from continuing operations									788,967	665,155	611,851
Regulated Natural Gas [Member]
Segment Reporting Information [Line Items]
Equity investments in unconsolidated subsidiaries	92,700				97,600				92,700	97,600
Operating revenues from external customers									1,811,926	1,782,582	1,865,703
Intersegment revenues									2,358	5,653	2,931
Total revenues									1,814,284	1,788,235	1,868,634
Depreciation and amortization									106,870	99,220	95,633
Interest charges and financing costs									52,115	49,314	44,572
Income tax expense (benefit)									57,408	59,790	81,956
Income (loss) from continuing operations									101,842	114,554	108,948
All Other [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers									76,251	76,520	73,877
Intersegment revenues									0	0	0
Total revenues									76,251	76,520	73,877
Depreciation and amortization									10,357	10,847	11,329
Interest charges and financing costs									108,134	119,233	105,758
Income tax expense (benefit)									(62,940)	(57,911)	(67,770)
Income (loss) from continuing operations									(49,435)	(27,753)	(35,275)
Reconciling Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers									0	0	0
Intersegment revenues									(3,627)	(6,668)	(3,747)
Total revenues									(3,627)	(6,668)	(3,747)
Depreciation and amortization									0	0	0
Interest charges and financing costs									0	0	0
Income tax expense (benefit)									0	0	0
Income (loss) from continuing operations									$ 0	$ 0	$ 0