Regulatory Assets and Liabilities, Regulatory Liabilities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 134,459		$ 79,499
Regulatory Liability, Noncurrent	464,421		424,690
Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	0		0
Regulatory Liability, Noncurrent	366,359		359,387
Regulatory Noncurrent Liability, Amortization Period	Plant lives
Deferred Electric, Gas, and Steam Production Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	24,035		33,247
Regulatory Liability, Noncurrent	0		0
Regulatory Noncurrent Liability, Amortization Period	Less than one year
Investment Tax Credit Deferrals
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	0		0
Regulatory Liability, Noncurrent	22,225		24,038
Regulatory Noncurrent Liability, Amortization Period	Various
Deferred Income Tax Adjustment
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	0		0
Regulatory Liability, Noncurrent	18,672		18,770
Regulatory Noncurrent Liability, Amortization Period	Various
Conservation Programs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	32,226	[1]	12,188	[1]
Regulatory Liability, Noncurrent	0	[1]	0	[1]
Regulatory Noncurrent Liability, Amortization Period	Less than one year
Renewable Resources and Environmental Initiatives
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	3,308		0
Regulatory Liability, Noncurrent	10,376		1,412
Regulatory Noncurrent Liability, Amortization Period	Various
Low Income Discount Program
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	1,680		4,084
Regulatory Liability, Noncurrent	0		0
Regulatory Noncurrent Liability, Amortization Period	Less than one year
Gain From Asset Sales
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	316		1,687
Regulatory Liability, Noncurrent	4		368
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	57,127		22,892
Regulatory Liability, Noncurrent	42,819		19,203
Regulatory liability remaining amortization period, minimum	1 year
Regulatory liability remaining amortization period, maximum	2 years
Gas Pipeline Inspection Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	13,970		0
Regulatory Liability, Noncurrent	642		0
Regulatory liability remaining amortization period, minimum	1 year
Regulatory liability remaining amortization period, maximum	4 years
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	1,797		5,401
Regulatory Liability, Noncurrent	$ 3,324		$ 1,512
Regulatory Noncurrent Liability, Amortization Period	Various

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