Borrowings and Other Financing Instruments, Long-Term Borrowings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Debt Instrument, Unamortized Discount (Premium), Net	$ (14)	$ (13)
Unamortized Debt Issuance Expense	(33)	(29)
Long-term Debt, Current Maturities	(406)	(306)
Long-term Debt	4,592	4,303
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	400
Long-term Debt, Maturities, Repayments of Principal in Year Two	400
Long-term Debt, Maturities, Repayments of Principal in Year Three	0
Long-term Debt, Maturities, Repayments of Principal in Year Four	300
Long-term Debt, Maturities, Repayments of Principal in Year Five	250
Deferred Finance Costs, Noncurrent, Net	33	29
First Mortgage Bonds
Debt Instrument [Line Items]
Long-term Debt, Gross	4,900	4,500
First Mortgage Bonds | Series Due June 15, 2028 [Member]
Debt Instrument [Line Items]
Face amount	$ 350
Interest rate, stated percentage (in hundredths)	3.70%
Debt Instrument, Maturity Date	Jun. 15, 2028
First Mortgage Bonds | Series Due June 15, 2048 [Member]
Debt Instrument [Line Items]
Face amount	$ 350
Interest rate, stated percentage (in hundredths)	4.10%
Debt Instrument, Maturity Date	Jun. 15, 2048
First Mortgage Bonds | Series Due June 15, 2047 [Member]
Debt Instrument [Line Items]
Face amount		$ 400
Interest rate, stated percentage (in hundredths)		3.80%
Debt Instrument, Maturity Date		Jun. 15, 2047
Capital Lease Obligations
Debt Instrument [Line Items]
Capital Lease Obligations	$ 145	$ 151
PSCo [Member] | First Mortgage Bonds
Debt Instrument [Line Items]
Debt Instrument, Maturity Date Range, Start	Jun. 01, 2019
Debt Instrument, Maturity Date Range, End	Jun. 15, 2048
PSCo [Member] | First Mortgage Bonds | Minimum
Debt Instrument [Line Items]
Interest rate, stated percentage (in hundredths)	2.25%	2.25%
PSCo [Member] | First Mortgage Bonds | Maximum
Debt Instrument [Line Items]
Interest rate, stated percentage (in hundredths)	6.50%	6.50%
PSCo [Member] | Capital Lease Obligations
Debt Instrument [Line Items]
Debt Instrument, Maturity Date Range, Start	Jan. 01, 2025
Debt Instrument, Maturity Date Range, End	Jan. 01, 2060
PSCo [Member] | Capital Lease Obligations | Minimum
Debt Instrument [Line Items]
Interest rate, stated percentage (in hundredths)	11.20%	11.20%
PSCo [Member] | Capital Lease Obligations | Maximum
Debt Instrument [Line Items]
Interest rate, stated percentage (in hundredths)	14.30%	14.30%