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]	$ 306,830	$ 410,729
Regulatory Liability, Noncurrent		1,332,889	1,163,429
Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		0	0
Regulatory Liability, Noncurrent		$ 1,131,023	953,660
Regulatory liability, remaining amortization period		Plant lives
Deferred Electric, Gas, and Steam Production Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 146,235	88,527
Regulatory Liability, Noncurrent		$ 0	0
Regulatory liability remaining amortization period, maximum		1 year
Conservation Programs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[2]	$ 34,444	103,351
Regulatory Liability, Noncurrent	[2]	$ 0	0
Regulatory liability remaining amortization period, maximum		1 year
DOE Settlement
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 16,139	49,492
Regulatory Liability, Noncurrent		$ 0	0
Regulatory liability remaining amortization period, minimum		1 year
Regulatory liability remaining amortization period, maximum		2 years
Investment Tax Credit Deferrals
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 0	0
Regulatory Liability, Noncurrent		$ 48,985	52,666
Regulatory liability, remaining amortization period		Various
PSCo Earnings Test
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 42,868	57,127
Regulatory Liability, Noncurrent		$ 9,472	42,819
Regulatory liability remaining amortization period, minimum		1 year
Regulatory liability remaining amortization period, maximum		2 years
Deferred Income Tax Adjustment
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 0	0
Regulatory Liability, Noncurrent		$ 46,737	48,622
Regulatory liability, remaining amortization period		Various
Gas Pipeline Inspection Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 1,140	13,970
Regulatory Liability, Noncurrent		$ 4,273	642
Regulatory liability, remaining amortization period		Various
Contract Valuation Adjustments
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[3]	$ 21,661	55,751
Regulatory Liability, Noncurrent	[3]	$ 0	2,521
Regulatory liability, remaining amortization period		Term of related contract
Gain From Asset Sales
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 2,640	2,893
Regulatory Liability, Noncurrent		$ 2,584	4,472
Regulatory liability, remaining amortization period		Various
Renewable Resources and Environmental Initiatives
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 6,271	10,427
Regulatory Liability, Noncurrent		$ 41,869	10,376
Regulatory liability, remaining amortization period		Various
Low Income Discount Program
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 2,475	3,355
Regulatory Liability, Noncurrent		$ 0	0
Regulatory liability remaining amortization period, maximum		1 year
Excess depreciation reserve [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 60	10,999
Regulatory Liability, Noncurrent		$ 0	0
Regulatory liability remaining amortization period, maximum		1 year
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 32,897	14,837
Regulatory Liability, Noncurrent		$ 47,946	47,651
Regulatory liability, remaining amortization period		Various
Other Current Liabilities [Member]
Regulatory Liabilities [Line Items]
Entity's Recorded Provision for Revenue Subject To Refund		$ 75,000	$ 128,300

[1] Revenue subject to refund of $75.0 million and $128.3 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.

[3] Includes the fair value of certain long-term PPAs used to meet energy capacity requirements and valuation adjustments on natural gas commodity purchases.