Selected Balance Sheet Data Balance Sheet Related Disclosures, Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 5,871,790	$ 5,615,125
Less accumulated depreciation	(1,922,951)	(1,871,984)
Property, plant and equipment, net	3,948,839	3,743,141
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	5,580,225	5,376,606
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 291,565	$ 238,519