Selected Balance Sheet Data Balance Sheet Related Disclosures, Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 7,507,289	$ 7,117,246
Less accumulated depreciation	(2,073,102)	(2,021,637)
Property, plant and equipment, net	5,434,187	5,095,609
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	7,059,344	6,765,371
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 447,945	$ 351,875