Selected Balance Sheet Data Balance Sheet Related Disclosures, Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 5,345,308	$ 5,102,721
Less accumulated depreciation	(1,824,578)	(1,818,691)
Property, plant and equipment, net	3,520,730	3,284,030
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	4,991,826	4,714,398
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 353,482	$ 388,323