Selected Balance Sheet Data Balance Sheet Related Disclosures, Property, Plant and Equipment (Details) - USD ($) $ in Thousands		Dec. 31, 2015	Dec. 31, 2014
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		$ 16,586,427	$ 15,865,971
Less accumulated depreciation		(4,414,216)	(4,239,015)
Property, plant and equipment, net		12,172,211	11,626,956
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		11,856,126	10,927,867
Natural gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		3,420,249	3,210,242
Common and other property
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		862,840	827,708
Plant to be retired
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	[1]	38,249	71,534
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		$ 408,963	$ 828,620

[1] PSCo’s Cherokee Unit 3 was retired in August 2015. 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, as approved by the Colorado Public Utilities Commission (CPUC). Amounts are presented net of accumulated depreciation.