Selected Balance Sheet Data, Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 2,833,237	$ 2,674,485
Less accumulated depreciation	(1,043,903)	(1,000,204)
Property, plant and equipment, net	1,789,334	1,674,281
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	2,332,155	2,061,669
Natural gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	264,390	255,465
Common and other property
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	124,584	125,938
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 112,108	$ 231,413