Selected Balance Sheet Data Balance Sheet Related Disclosures, Property, Plant and Equipment (Details) - USD ($) $ in Thousands		Sep. 30, 2016	Dec. 31, 2015
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		$ 17,191,343	$ 16,586,427
Less accumulated depreciation		(4,562,971)	(4,414,216)
Property, plant and equipment, net		12,628,372	12,172,211
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		12,136,597	11,856,126
Natural gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		3,590,746	3,420,249
Common and other property
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		867,340	862,840
Plant to be retired
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	[1]	36,852	38,249
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		$ 559,808	$ 408,963

[1] (a) In 2017, PSCo expects to both early retire Valmont Unit 5 and convert Cherokee Unit 4 from a coal-fueled generating facility to natural gas. PSCo also expects Craig Unit 1 to be early retired in approximately 2025. Amounts are presented net of accumulated depreciation.