Segment Information (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Segment Reporting Information [Line Items]
Equity investments in unconsolidated subsidiaries	$ 84,500,000		$ 84,500,000		$ 87,100,000
Operating revenues	2,869,807,000	2,822,338,000	8,757,507,000	8,184,100,000
Net income (loss)	368,582,000	364,752,000	824,967,000	798,179,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	2,616,823,000	2,600,271,000	7,216,961,000	6,912,953,000
Net income (loss)	360,656,000	365,156,000	731,766,000	740,347,000
Regulated Natural Gas
Segment Reporting Information [Line Items]
Equity investments in unconsolidated subsidiaries	84,500,000		84,500,000		87,100,000
Operating revenues	237,246,000	206,464,000	1,490,431,000	1,218,438,000
Net income (loss)	3,996,000	(174,000)	96,629,000	80,698,000
All Other
Segment Reporting Information [Line Items]
Operating revenues	16,807,000	17,055,000	56,344,000	55,827,000
Net income (loss)	3,930,000	(230,000)	(3,428,000)	(22,866,000)
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	2,869,807,000	2,822,338,000	8,757,507,000	8,184,100,000
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	2,616,351,000	2,599,925,000	7,215,699,000	6,911,998,000
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	236,649,000	205,358,000	1,485,464,000	1,216,275,000
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	16,807,000	17,055,000	56,344,000	55,827,000
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(1,069,000)	(1,452,000)	(6,229,000)	(3,118,000)
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	472,000	346,000	1,262,000	955,000
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	597,000	1,106,000	4,967,000	2,163,000
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0	$ 0	$ 0	$ 0