Property, Plant and Equipment (details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
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%	2.20%
Regulatory liabilities	$ 303,485	$ 288,113
Property, Plant and Equipment, Net, by Type [Abstract]
Utility plant in service	2,918,739	2,786,008
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	855,865	812,396
Total property, plant, and equipment, net	2,062,874	1,973,612
Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Utility plant in service	2,585,901	2,435,886
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	28,855	46,831
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	827,380	789,201
Total property, plant, and equipment, net	1,787,376	1,693,516
Non Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Utility plant in service	297,330	296,781
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	6,653	6,510
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	28,485	23,195
Total property, plant, and equipment, net	275,498	280,096
Accrued Asset Removal Costs [Member]
Public Utilities, Property, Plant and Equipment [Abstract]
Regulatory liabilities	$ 296,294	$ 281,213