Selected Balance Sheet Data Balance Sheet Related Disclosures, Property, Plant and Equipment (Details) - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		$ 16,382,625	$ 15,865,971
Less accumulated depreciation		(4,416,323)	(4,239,015)
Property, plant and equipment, net		11,966,302	11,626,956
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		11,689,760	10,927,867
Natural gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		3,341,157	3,210,242
Common and other property
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		823,239	827,708
Plant to be retired
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	[1]	42,336	71,534
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		$ 486,133	$ 828,620

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