Selected Balance Sheet Data, Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 2,265,280	$ 2,193,812
Less accumulated depreciation	(921,567)	(895,576)
Property, plant and equipment, net	1,343,713	1,298,236
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	1,848,303	1,795,239
Natural gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	227,790	224,625
Common and other property
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	112,043	111,319
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 77,144	$ 62,629