Selected Balance Sheet Data, Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 18,822,136	$ 18,675,239
Less accumulated depreciation	(6,354,488)	(6,251,498)
Property, plant and equipment, net	12,828,607	12,807,338
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	16,298,761	16,256,887
Natural gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	1,249,933	1,248,408
Common and other property
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	622,749	624,409
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	650,693	545,535
Nuclear fuel
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,450,363	2,447,251
Less accumulated depreciation	$ (2,089,404)	$ (2,063,654)