Property, Plant And Equipment (Major Classifications Of Property, Plant And Equipment) (Details) (USD $)	Dec. 31, 2012		Dec. 31, 2011
Public Utility, Property, Plant and Equipment [Line Items]
Construction work-in-progress (CWIP) and other	$ 143,098,000		$ 79,322,000
Total	4,197,742,000		3,966,706,000
Other intangibles, property and investments-net	46,542,000		45,687,000
Total	4,250,570,000		4,014,511,000
Ecova [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Other intangibles, property and investments-net	30,138,000	[1]	25,763,000	[1]
Accumulated depreciation	23,400,000		20,300,000
Other [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Other intangibles, property and investments-net	22,690,000	[1]	22,042,000	[1]
Accumulated depreciation	13,700,000		13,100,000
Electric [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Production	1,112,670,000		1,094,223,000
Transmission	546,019,000		522,930,000
Distribution	1,217,827,000		1,157,012,000
Construction work-in-progress (CWIP) and other	244,761,000		205,437,000
Total	3,121,277,000		2,979,602,000
Natural Gas [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Distribution	704,839,000		683,948,000
Construction work-in-progress (CWIP) and other	57,745,000		41,077,000
Natural gas underground storage	40,890,000		40,430,000
Total	803,474,000		765,455,000
Common Plant [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Total	$ 272,991,000		$ 221,649,000

[1] Included in other property and investments-net on the Consolidated Balance Sheets. Accumulated depreciation was $23.4 million as of December 31, 2012 and $20.3 million as of December 31, 2011 for Ecova and $13.7 million as of December 31, 2012 and $13.1 million as of December 31, 2011 for the other businesses.