Segment Information (Details) (USD $)	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Segment Reporting Information [Line Items]
Operating revenues	$ 1,203,543,000		$ 1,117,457,000
Net income	118,403,000		116,605,000
Related Party Transaction, Electric Domestic Regulated Revenue	2,000,000		2,000,000
Related Party Transaction, Other Revenue, Net	1,000,000		1,000,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	734,361,000		721,436,000
Net income	73,968,000		78,468,000
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	456,396,000		383,971,000
Net income	38,149,000		33,799,000
All Other
Segment Reporting Information [Line Items]
Operating revenues	12,942,000		12,185,000
Net income	6,286,000		4,338,000
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	1,203,543,000	[1],[2]	1,117,457,000	[1],[2]
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	734,264,000	[2]	721,348,000	[2]
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	456,337,000		383,924,000
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	12,942,000	[1]	12,185,000	[1]
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(156,000)		(135,000)
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	97,000		88,000
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	59,000		47,000
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0		$ 0

[1] Operating revenues include $1 million and $1 million of other intercompany revenue for the three months ended March 31, 2014 and 2013, respectively.

[2] Operating revenues include $2 million and $2 million of intercompany electric revenue for the three months ended March 31, 2014 and 2013, respectively.