Regulatory Assets and Liabilities, Regulatory Liabilities (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[1]	$ 101,110	$ 152,823
Regulatory Liability, Noncurrent		$ 512,933	471,421
Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory Noncurrent Liability, Amortization Period		Plant lives
Regulatory Liability, Current		$ 0	0
Regulatory Liability, Noncurrent		$ 367,440	364,291
Renewable Resources and Environmental Initiatives
Regulatory Liabilities [Line Items]
Regulatory Noncurrent Liability, Amortization Period		Various
Regulatory Liability, Current		$ 3,600	3,311
Regulatory Liability, Noncurrent		$ 67,728	40,988
Investment Tax Credit Deferrals
Regulatory Liabilities [Line Items]
Regulatory Noncurrent Liability, Amortization Period		Various
Regulatory Liability, Current		$ 0	0
Regulatory Liability, Noncurrent		$ 18,797	20,515
Deferred Income Tax Adjustment
Regulatory Liabilities [Line Items]
Regulatory Noncurrent Liability, Amortization Period		Various
Regulatory Liability, Current		$ 0	0
Regulatory Liability, Noncurrent		$ 16,260	16,891
Gain From Asset Sales
Regulatory Liabilities [Line Items]
Regulatory Noncurrent Liability, Amortization Period		Pending rate case
Regulatory Liability, Current		$ 0	0
Regulatory Liability, Noncurrent		$ 1,469	0
PSCo Earnings Test
Regulatory Liabilities [Line Items]
Regulatory Noncurrent Liability, Amortization Period		One to two years
Regulatory Liability, Current		$ 8,300	42,868
Regulatory Liability, Noncurrent		$ 914	9,472
Deferred Electric, Gas, and Steam Production Costs
Regulatory Liabilities [Line Items]
Regulatory Noncurrent Liability, Amortization Period		Less than one year
Regulatory Liability, Current		$ 35,123	66,696
Regulatory Liability, Noncurrent		$ 0	0
Conservation Programs
Regulatory Liabilities [Line Items]
Regulatory Noncurrent Liability, Amortization Period		Less than one year
Regulatory Liability, Current	[2]	$ 24,077	33,460
Regulatory Liability, Noncurrent	[2]	$ 0	0
Gas Pipeline Inspection Costs
Regulatory Liabilities [Line Items]
Regulatory Noncurrent Liability, Amortization Period		Less than one year
Regulatory Liability, Current		$ 5,108	1,140
Regulatory Liability, Noncurrent		$ 0	4,273
Low Income Discount Program
Regulatory Liabilities [Line Items]
Regulatory Noncurrent Liability, Amortization Period		Less than one year
Regulatory Liability, Current		$ 881	1,393
Regulatory Liability, Noncurrent		$ 0	0
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory Noncurrent Liability, Amortization Period		Various
Regulatory Liability, Current		$ 24,021	3,955
Regulatory Liability, Noncurrent		40,325	14,991
Other Current Liabilities
Regulatory Liabilities [Line Items]
Entity's Recorded Provision for Revenue Subject To Refund		$ 2,400	$ 9,100

[1] (b) Revenue subject to refund of $2.4 million and $9.1 million for 2016 and 2015, respectively, is included in other current liabilities.

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