Property, Plant and Equipment (details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant, and Equipment, Gross	$ 2,939,760	$ 2,918,739	$ 2,808,673
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	868,257	855,865	824,561
Total property, plant, and equipment, net	2,071,503	2,062,874	1,984,112
Property, Plant, Equipment non-cash	7,769	10,456	7,621
Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant, and Equipment, Gross	2,605,018	2,585,901	2,452,419
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	30,699	28,855	53,474
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	838,285	827,380	799,864
Total property, plant, and equipment, net	1,797,432	1,787,376	1,706,029
Non Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant, and Equipment, Gross	297,352	297,330	296,228
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	6,691	6,653	6,552
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	29,972	28,485	24,697
Total property, plant, and equipment, net	$ 274,071	$ 275,498	$ 278,083