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Rate Matters (Details) - USD ($)
$ in Millions
 1 Months Ended 3 Months Ended
Apr. 30, 2018
Feb. 28, 2018
Oct. 31, 2017
Jun. 30, 2017
Mar. 31, 2018
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  
Public Utilities, Rate Impact       233  
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  
Public Utilities, Rate Impact       63  
Public Utilities, Expected Year-End Rate Base [1]       1,500  
CPUC Proceeding - 2017 Multi-Year Gas Rate Case, Gas Rates 2019          
Public Utilities, General Disclosures [Line Items]          
Public Utilities, Requested Rate Increase (Decrease), Amount       33  
Public Utilities, Rate Impact       127  
Public Utilities, Expected Year-End Rate Base [1]       2,300  
CPUC Proceeding - 2017 Multi-Year Gas Rate Case, Gas Rates 2020          
Public Utilities, General Disclosures [Line Items]          
Public Utilities, Requested Rate Increase (Decrease), Amount       43  
Public Utilities, Rate Impact       43  
Public Utilities, Expected Year-End Rate Base [1]       2,400  
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%    
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%    
PSCo | CPUC Proceeding - 2017 Multi-Year Gas Rate Case          
Public Utilities, General Disclosures [Line Items]          
Public Utilities, Reduction of Provisional Rates   $ 20      
Public Utilities, Requested Rate Increase (Decrease), Amount       139  
Public Utilities, Provisional Rates Subject to Refund         $ 63
PSCo | CPUC Proceeding - Colorado 2017 Multi-Year Electric Rate Case          
Public Utilities, General Disclosures [Line Items]          
Public Utilities, Approximate Rate Increase (Decrease), Amount     $ 245    
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     $ 245    
Public Utilities, Rate Impact     378    
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    
Public Utilities, Rate Impact     207    
Public Utilities, Expected Year-End Rate Base     6,800    
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     75    
Public Utilities, Rate Impact     75    
Public Utilities, Expected Year-End Rate Base     7,100    
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     60    
Public Utilities, Rate Impact     60    
Public Utilities, Expected Year-End Rate Base     7,300    
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     36    
Public Utilities, Rate Impact     36    
Public Utilities, Expected Year-End Rate Base     7,400    
PSCo | CPUC Staff | CPUC Proceeding - 2017 Multi-Year Gas Rate Case, Gas Rates 2018          
Public Utilities, General Disclosures [Line Items]          
Public Utilities, Total Recommended Rate Increase (Decrease)     30    
PSCo | Office of Consumer Council (OCC) | CPUC Proceeding - 2017 Multi-Year Gas Rate Case, Gas Rates 2018          
Public Utilities, General Disclosures [Line Items]          
Public Utilities, Total Recommended Rate Increase (Decrease)     39    
Subsequent Event | PSCo | CPUC Proceeding - Tax Cuts and Jobs Act of 2017 [Member]          
Public Utilities, General Disclosures [Line Items]          
Tax Cuts and Jobs Act of 2017, 2018 Electric Rate Reduction Under Settlement $ 101        
Subsequent Event | PSCo | Colorado Public Utilities Commission [Member] | CPUC Proceeding - Tax Cuts and Jobs Act of 2017 [Member]          
Public Utilities, General Disclosures [Line Items]          
Tax Cuts and Jobs Act of 2017, 2018 Electric Refund of TCJA Benefits Under Settlement 42        
Tax Cuts and Jobs Act of 2017, Amount of Accelerated Prepaid Pension Asset Amortization Under Settlement $ 59        
Clean Air Clean Jobs Act (CACJA) Recovery Rider | CPUC Proceeding - Colorado 2017 Multi-Year Electric Rate Case          
Public Utilities, General Disclosures [Line Items]          
Public Utilities, Rider Conversion to Base Rates     90    
Clean Air Clean Jobs Act (CACJA) Recovery Rider | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2018          
Public Utilities, General Disclosures [Line Items]          
Public Utilities, Rider Conversion to Base Rates     90    
Clean Air Clean Jobs Act (CACJA) Recovery Rider | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2019          
Public Utilities, General Disclosures [Line Items]          
Public Utilities, Rider Conversion to Base Rates     0    
Clean Air Clean Jobs Act (CACJA) Recovery Rider | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2020          
Public Utilities, General Disclosures [Line Items]          
Public Utilities, Rider Conversion to Base Rates     0    
Clean Air Clean Jobs Act (CACJA) Recovery Rider | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2021          
Public Utilities, General Disclosures [Line Items]          
Public Utilities, Rider Conversion to Base Rates     0    
Transmission Cost Adjustment (TCA) Rider | CPUC Proceeding - Colorado 2017 Multi-Year Electric Rate Case          
Public Utilities, General Disclosures [Line Items]          
Public Utilities, Rider Conversion to Base Rates     43    
Transmission Cost Adjustment (TCA) Rider | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2018          
Public Utilities, General Disclosures [Line Items]          
Public Utilities, Rider Conversion to Base Rates     43    
Transmission Cost Adjustment (TCA) Rider | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2019          
Public Utilities, General Disclosures [Line Items]          
Public Utilities, Rider Conversion to Base Rates     0    
Transmission Cost Adjustment (TCA) Rider | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2020          
Public Utilities, General Disclosures [Line Items]          
Public Utilities, Rider Conversion to Base Rates     0    
Transmission Cost Adjustment (TCA) Rider | CPUC Proceeding - 2017 Multi-Year Electric Rate Case, Electric Rates 2021          
Public Utilities, General Disclosures [Line Items]          
Public Utilities, Rider Conversion to Base Rates     $ 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 [2]       94  
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 [2]       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 [2]       94  
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 [2]       $ 0  
[1] The additional rate base in 2019 predominantly reflects the roll-in of capital associated with the PSIA rider.
[2] The roll-in of PSIA rider revenue into base rates will not have an impact on customer bills or revenue as these costs are already being recovered through the rider. The recovery of incremental PSIA related investments in 2019 and 2020 are included in the base rate request.