Selected Balance Sheet Data Balance Sheet Related Disclosures, Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 5,235,726	$ 5,102,721
Less accumulated depreciation	(1,840,452)	(1,818,691)
Property, plant and equipment, net	3,395,274	3,284,030
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	4,789,975	4,714,398
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 445,751	$ 388,323