Rate Matters (Details) - USD ($) $ in Millions		1 Months Ended		9 Months Ended
		Oct. 31, 2017	Jun. 30, 2017	Sep. 30, 2017
CPUC Proceeding - 2017 Multi-Year Gas Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Number of Years Which Rates are Requested to Increase			3 years
Public Utilities, Requested Return on Equity, Percentage			10.00%
Public Utilities, Requested Equity Capital Structure, Percentage			55.25%
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 139.0
Public Utilities, Rate Impact			232.9
CPUC Proceeding - 2017 Multi-Year Gas Rate Case, Gas Rates 2018
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			63.2
Public Utilities, Rate Impact			63.2
Public Utilities, Expected Year-End Rate Base	[1]		1,500.0
CPUC Proceeding - 2017 Multi-Year Gas Rate Case, Gas Rates 2019
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			32.9
Public Utilities, Rate Impact			126.8
Public Utilities, Expected Year-End Rate Base	[1]		2,300.0
CPUC Proceeding - 2017 Multi-Year Gas Rate Case, Gas Rates 2020
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			42.9
Public Utilities, Rate Impact			42.9
Public Utilities, Expected Year-End Rate Base	[1]		2,400.0
CPUC Proceeding - Annual Electric Earnings Test 2015 through 2017
Public Utilities, General Disclosures [Line Items]
Public Utilities, Return On Equity Threshold For Earnings Sharing				9.83%
PSCo | CPUC Proceeding - 2017 Multi-Year Gas Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			139.0
Subsequent Event | CPUC Staff | CPUC Proceeding - 2017 Multi-Year Gas Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Recommended Length of Average Rate Base		13 months
Public Utilities, Recommended Equity Capital Structure, Percentage		48.73%
Public Utilities, Recommended ROE, Percentage		9.00%
Public Utilities, Impact of Change in Test Year		$ 4.4
Public Utilities, Increase (Decrease) Filed Historic Test Year Revenue Request, Amount		67.6
Public Utilities, Increase (Decrease) of Recommended ROE		(13.5)
Public Utilities, Increase (Decrease) of Recommended Capital Structure and Cost of Debt		(10.2)
Public Utilities, Increase (Decrease) of Recommended Change in Amortization Period		(5.4)
Public Utilities, Increase (Decrease) of Recommended Prepaid pension and Retiree Medical Assets		(5.2)
Public Utilities, Increase (Decrease) of Recommended Change to Average Rate Base		(4.8)
Public Utilities, Increase (Decrease) Other, net		(5.0)
Public Utilities, Total Recommended Adjustments		(44.1)
Public Utilities, Total Recommended Rate Increase (Decrease)		$ 23.5
Subsequent Event | Office of Consumer Council (OCC) | CPUC Proceeding - 2017 Multi-Year Gas Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Recommended Length of Average Rate Base		13 months
Public Utilities, Recommended Equity Capital Structure, Percentage		51.20%
Public Utilities, Recommended ROE, Percentage		9.00%
Public Utilities, Impact of Change in Test Year		$ 4.4
Public Utilities, Increase (Decrease) Filed Historic Test Year Revenue Request, Amount		67.6
Public Utilities, Increase (Decrease) of Recommended ROE		(13.5)
Public Utilities, Increase (Decrease) of Recommended Capital Structure and Cost of Debt		(7.5)
Public Utilities, Increase (Decrease) of Recommended Change in Amortization Period		0.0
Public Utilities, Increase (Decrease) of Recommended Prepaid pension and Retiree Medical Assets		0.0
Public Utilities, Increase (Decrease) of Recommended Change to Average Rate Base		(4.8)
Public Utilities, Increase (Decrease) Other, net		(5.5)
Public Utilities, Total Recommended Adjustments		(31.3)
Public Utilities, Total Recommended Rate Increase (Decrease)		36.3
Subsequent Event | PSCo | CPUC Proceeding - Colorado 2017 Multi-Year Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approximate Rate Increase (Decrease), Amount		$ 245.0
Public Utilities, Number of Years Which Rates are Requested to Increase		4 years
Public Utilities, Requested Return on Equity, Percentage		10.00%
Public Utilities, Requested Equity Capital Structure, Percentage		55.25%
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 244.9
Public Utilities, Rate Impact	[2]	378.0
Subsequent Event | PSCo | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2018
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		74.6
Public Utilities, Rate Impact	[2]	207.7
Public Utilities, Expected Year-End Rate Base	[2]	6,800.0
Subsequent Event | PSCo | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2019
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		74.9
Public Utilities, Rate Impact	[2]	74.9
Public Utilities, Expected Year-End Rate Base	[2]	7,100.0
Subsequent Event | PSCo | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2020
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		59.7
Public Utilities, Rate Impact	[2]	59.7
Public Utilities, Expected Year-End Rate Base	[2]	7,300.0
Subsequent Event | PSCo | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2021
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		35.7
Public Utilities, Rate Impact	[2]	35.7
Public Utilities, Expected Year-End Rate Base	[2]	7,400.0
Clean Air Clean Jobs Act (CACJA) Recovery Rider | Subsequent Event | CPUC Proceeding - Colorado 2017 Multi-Year Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates	[3]	90.4
Clean Air Clean Jobs Act (CACJA) Recovery Rider | Subsequent Event | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2018
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates	[3]	90.4
Clean Air Clean Jobs Act (CACJA) Recovery Rider | Subsequent Event | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2019
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates	[3]	0.0
Clean Air Clean Jobs Act (CACJA) Recovery Rider | Subsequent Event | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2020
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates	[3]	0.0
Clean Air Clean Jobs Act (CACJA) Recovery Rider | Subsequent Event | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2021
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates	[3]	0.0
Transmission Cost Adjustment (TCA) Rider | Subsequent Event | CPUC Proceeding - Colorado 2017 Multi-Year Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates	[3]	42.7
Transmission Cost Adjustment (TCA) Rider | Subsequent Event | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2018
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates	[3]	42.7
Transmission Cost Adjustment (TCA) Rider | Subsequent Event | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2019
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates	[3]	0.0
Transmission Cost Adjustment (TCA) Rider | Subsequent Event | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2020
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates	[3]	0.0
Transmission Cost Adjustment (TCA) Rider | Subsequent Event | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2021
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates	[3]	$ 0.0
Pipeline System Integrity Adjustment (PSIA) Rider | CPUC Proceeding - 2017 Multi-Year Gas Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates	[4]		93.9
Pipeline System Integrity Adjustment (PSIA) Rider | CPUC Proceeding - 2017 Multi-Year Gas Rate Case, Gas Rates 2018
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates	[4]		0.0
Pipeline System Integrity Adjustment (PSIA) Rider | CPUC Proceeding - 2017 Multi-Year Gas Rate Case, Gas Rates 2019
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates	[4]		93.9
Pipeline System Integrity Adjustment (PSIA) Rider | CPUC Proceeding - 2017 Multi-Year Gas Rate Case, Gas Rates 2020
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates	[4]		$ 0.0

[1] (b) The additional rate base in 2019 predominantly reflects the roll-in of capital associated with the PSIA rider.

[2] (b) This base rate request does not include the impacts associated with the renewable energy standard adjustment and retail electric commodity adjustment for the Rush Creek wind investments or any impacts of the proposed Colorado Energy Plan.

[3] (a) The roll-in of each of the TCA and CACJA rider revenues into base rates will not have an impact on total customer bills or total revenue as these costs are already being recovered through a rider. Transmission investments for 2019 through 2021 will be recovered through the TCA rider.

[4] (a) The roll-in of PSIA rider revenue into base rates will not have an impact on customer bills or total revenue as these costs are already being recovered through the rider. PSCo plans to request new PSIA rates for 2018 in November 2017. The recovery of incremental PSIA related investments in 2019 and 2020 are included in the base rate request.