Regulatory Assets and Liabilities, Regulatory Liabilities (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[1]	$ 152,823	$ 134,459
Regulatory Liability, Noncurrent		471,421	464,421
Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		0	0
Regulatory Liability, Noncurrent		$ 364,291	366,359
Regulatory Noncurrent Liability, Amortization Period		Plant lives
Deferred Electric, Gas, and Steam Production Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 66,696	24,035
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		$ 20,515	22,225
Regulatory Noncurrent Liability, Amortization Period		Various
Deferred Income Tax Adjustment
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 0	0
Regulatory Liability, Noncurrent		$ 16,891	18,672
Regulatory Noncurrent Liability, Amortization Period		Various
Conservation Programs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[2]	$ 33,460	32,226
Regulatory Liability, Noncurrent	[2]	$ 0	0
Regulatory Noncurrent Liability, Amortization Period		Less than one year
Renewable Resources and Environmental Initiatives
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 3,311	3,308
Regulatory Liability, Noncurrent		$ 40,988	10,376
Regulatory Noncurrent Liability, Amortization Period		Various
Low Income Discount Program
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 1,393	1,680
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		$ 0	316
Regulatory Liability, Noncurrent		$ 0	4
Regulatory Noncurrent Liability, Amortization Period		One to three years
PSCo Earnings Test
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 42,868	57,127
Regulatory Liability, Noncurrent		$ 9,472	42,819
Regulatory Noncurrent Liability, Amortization Period		One to two years
Gas Pipeline Inspection Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 1,140	13,970
Regulatory Liability, Noncurrent		$ 4,273	642
Regulatory Noncurrent Liability, Amortization Period		One to two years
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 3,955	1,797
Regulatory Liability, Noncurrent		$ 14,991	3,324
Regulatory Noncurrent Liability, Amortization Period		Various
Other Current Liabilities [Member]
Regulatory Liabilities [Line Items]
Entity's Recorded Provision for Revenue Subject To Refund		$ 9,100	$ 4,400

[1] Revenue subject to refund of $9.1 million and $4.4 million for 2015 and 2014, respectively, is included in other current liabilities.

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