Selected Balance Sheet Data, Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 3,069,525	$ 2,885,022
Less accumulated depreciation	(1,121,888)	(1,056,943)
Property, plant and equipment, net	1,947,637	1,828,079
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	2,499,401	2,411,562
Natural gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	294,986	275,376
Common and other property
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	156,316	132,329
CWIP
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 118,822	$ 65,755