Segment Information (Details) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Segment Reporting Information [Line Items]
Operating revenues	$ 993,704,000		$ 966,429,000		$ 2,197,247,000		$ 2,083,886,000
Net income	89,792,000		97,299,000		208,195,000		213,904,000
Affiliate electric revenue	3,000,000		2,000,000		5,000,000		4,000,000
Affiliate other revenue	1,000,000		1,000,000		2,000,000		2,000,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	761,631,000		747,943,000		1,495,992,000		1,469,379,000
Net income	75,741,000		85,603,000		149,709,000		164,071,000
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	223,236,000		209,321,000		679,632,000		593,292,000
Net income	10,846,000		8,786,000		48,995,000		42,585,000
All Other
Segment Reporting Information [Line Items]
Operating revenues	8,948,000		9,251,000		21,890,000		21,436,000
Net income	3,205,000		2,910,000		9,491,000		7,248,000
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	993,704,000	[1],[2]	966,429,000	[1],[2]	2,197,247,000	[3],[4]	2,083,886,000	[3],[4]
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	761,569,000	[2]	747,882,000	[2]	1,495,833,000	[3]	1,469,230,000	[3]
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	223,187,000		209,296,000		679,524,000		593,220,000
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	8,948,000	[1]	9,251,000	[1]	21,890,000	[4]	21,436,000	[4]
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(111,000)		(86,000)		(267,000)		(221,000)
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	62,000		61,000		159,000		149,000
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	49,000		25,000		108,000		72,000
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0		$ 0		$ 0		$ 0

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

[2] Operating revenues include $3 million and $2 million of affiliate electric revenue for the three months ended June 30, 2014 and 2013, respectively.

[3] Operating revenues include $5 million and $4 million of affiliate electric revenue for the six months ended June 30, 2014 and 2013, respectively.

[4] Operating revenues include $2 million of other affiliate revenue for the six months ended June 30, 2014 and 2013.