Property, Plant and Equipment (details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant, and Equipment, Gross	$ 2,965,226	$ 2,918,739	$ 2,833,083
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	879,296	855,865	833,851
Total property, plant, and equipment, net	2,085,930	2,062,874	1,999,232
Property, Plant, Equipment non-cash	9,826	10,456	7,374
Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant, and Equipment, Gross	2,624,774	2,585,901	2,468,853
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	36,798	28,855	61,283
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	847,828	827,380	807,652
Total property, plant, and equipment, net	1,813,744	1,787,376	1,722,484
Non Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant, and Equipment, Gross	297,269	297,330	296,167
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	6,385	6,653	6,780
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	31,468	28,485	26,199
Total property, plant, and equipment, net	$ 272,186	$ 275,498	$ 276,748