Property, Plant and Equipment (details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Public Utilities, Property, Plant and Equipment [Abstract]
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service	2.80%	2.80%	2.80%
Weighted Average Depreciation Rates Non Utility	2.20%	2.20%	2.50%
Asset Removal Costs	$ 281,200	$ 267,400
Property, Plant and Equipment, Net, by Type [Abstract]
Utility plant in service	2,786,008	2,661,102
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	812,396	767,226
Total property, plant, and equipment, net	1,973,612	1,893,876
Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Utility plant in service	2,435,886	2,323,467
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	46,831	36,051
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	789,201	749,603
Total property, plant, and equipment, net	1,693,516	1,609,915
Non Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Utility plant in service	296,781	293,205
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	6,510	8,379
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	23,195	17,623
Total property, plant, and equipment, net	$ 280,096	$ 283,961