Regulatory Assets and Liabilities, Regulatory Liabilities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 79,499		$ 33,723
Regulatory Liability, Noncurrent	424,690		417,404
Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	0		0
Regulatory Liability, Noncurrent	359,387		365,331
Regulatory liability, remaining amortization period	Plant lives
Deferred Electric, Gas, and Steam Production Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	33,247		22,022
Regulatory Liability, Noncurrent	0		0
Regulatory liability, remaining amortization period	Less than one year
Investment Tax Credit Deferrals
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	0		0
Regulatory Liability, Noncurrent	24,038		25,790
Regulatory liability, remaining amortization period	Various
Deferred Income Tax Adjustment
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	0		0
Regulatory Liability, Noncurrent	18,770		20,218
Regulatory liability, remaining amortization period	Various
Conservation Programs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	12,188	[1]	2,522	[1]
Regulatory Liability, Noncurrent	0	[1]	0	[1]
Regulatory liability, remaining amortization period	Less than one year
Renewable Resources and Environmental Initiatives
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	0		0
Regulatory Liability, Noncurrent	1,412		1,412
Regulatory liability, remaining amortization period	Various
Low Income Discount Program
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	4,084		4,205
Regulatory Liability, Noncurrent	0		0
Regulatory liability, remaining amortization period	Less than one year
Gain From Asset Sales
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	1,687		2,414
Regulatory Liability, Noncurrent	368		2,039
Regulatory liability remaining amortization period, minimum	1 year
Regulatory liability remaining amortization period, maximum	3 years
PSCo Earnings Test
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	22,892		1,732
Regulatory Liability, Noncurrent	19,203		1,732
Regulatory liability remaining amortization period, minimum	1 year
Regulatory liability remaining amortization period, maximum	2 years
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	5,401		828
Regulatory Liability, Noncurrent	$ 1,512		$ 882
Regulatory liability, remaining amortization period	Various

[1] Includes costs for conservation programs, as well as incentives allowed in certain jurisdictions.