Segment Information (Details) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Segment Reporting Information [Line Items]
Operating revenues	$ 1,049,111,000		$ 1,044,300,000		$ 3,246,358,000		$ 3,128,186,000
Net income	154,159,000		166,266,000		362,354,000		380,170,000
Affiliate electric revenue	2,000,000		2,000,000		8,000,000		6,000,000
Affiliate other revenue	1,000,000		1,000,000		3,000,000		3,000,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	881,197,000		898,878,000		2,377,189,000		2,368,256,000
Net income	139,751,000		158,498,000		289,460,000		322,569,000
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	159,837,000		137,352,000		839,469,000		730,644,000
Net income	7,235,000		3,555,000		56,230,000		46,141,000
All Other
Segment Reporting Information [Line Items]
Operating revenues	8,201,000		8,140,000		30,091,000		29,576,000
Net income	7,173,000		4,213,000		16,664,000		11,460,000
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	1,049,111,000	[1],[2]	1,044,300,000	[1],[2]	3,246,358,000	[3],[4]	3,128,186,000	[3],[4]
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	881,102,000	[1]	898,811,000	[1]	2,376,935,000	[4]	2,368,041,000	[4]
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	159,808,000		137,349,000		839,332,000		730,569,000
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	8,201,000	[2]	8,140,000	[2]	30,091,000	[3]	29,576,000	[3]
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(124,000)		(70,000)		(391,000)		(290,000)
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	95,000		67,000		254,000		215,000
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	29,000		3,000		137,000		75,000
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0		$ 0		$ 0		$ 0

[1] Operating revenues include $2 million of affiliate electric revenue for the three months ended Sept. 30, 2014 and 2013.

[2] Operating revenues include $1 million of other affiliate revenue for the three months ended Sept. 30, 2014 and 2013.

[3] Operating revenues include $3 million of other affiliate revenue for the nine months ended Sept. 30, 2014 and 2013.

[4] Operating revenues include $8 million and $6 million of affiliate electric revenue for the nine months ended Sept. 30, 2014 and 2013, respectively.