Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Segment Reporting Information [Line Items]
Operating revenues	$ 1,117,457	$ 1,076,052
Net income	116,605	93,285
Regulated Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	721,348	682,279
Net income	78,468	61,333
Regulated Natural Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	383,924	383,004
Net income	33,799	28,310
All Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	12,185	10,769
Net income	4,338	3,642
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	1,117,457	1,076,052
Operating Segments [Member] | Regulated Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	721,436	682,371
Operating Segments [Member] | Regulated Natural Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	383,971	383,059
Operating Segments [Member] | All Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	12,185	10,769
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	(135)	(147)
Net income	0	0
Intersegment Eliminations [Member] | Regulated Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	88	92
Intersegment Eliminations [Member] | Regulated Natural Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	47	55
Intersegment Eliminations [Member] | All Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	$ 0	$ 0