Selected Balance Sheet Data Balance Sheet Related Disclosures, Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 4,980,409	$ 4,616,344
Less accumulated depreciation	(1,806,222)	(1,754,588)
Property, plant and equipment, net	3,174,187	2,861,756
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	4,660,418	4,379,208
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 319,991	$ 237,136