Segment Information (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Segment Reporting Information [Line Items]
Equity investments in unconsolidated subsidiaries	$ 86,300,000		$ 87,100,000
Operating revenues	3,202,604,000	2,782,849,000
Net income (loss)	261,221,000	236,570,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	2,302,063,000	2,092,497,000
Net income (loss)	185,433,000	174,106,000
Regulated Natural Gas
Segment Reporting Information [Line Items]
Equity investments in unconsolidated subsidiaries	86,300,000		87,100,000
Operating revenues	882,940,000	670,096,000
Net income (loss)	77,336,000	64,910,000
All Other
Segment Reporting Information [Line Items]
Operating revenues	21,206,000	21,057,000
Net income (loss)	(1,548,000)	(2,446,000)
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	3,202,604,000	2,782,849,000
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	2,301,710,000	2,092,196,000
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	879,688,000	669,596,000
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	21,206,000	21,057,000
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(3,605,000)	(801,000)
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	353,000	301,000
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	3,252,000	500,000
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0	$ 0