Borrowings and Other Financing Instruments (Tables)

12 Months Ended

Dec. 31, 2018

Dec. 31, 2017

Debt Disclosure [Abstract]

Money Pool [Table Text Block]

Money pool borrowings for PSCo were as follows:

	Three Months Ended Dec. 31, 2018			Year Ended Dec. 31
(Amounts in Millions, Except Interest Rates)				2018			2017			2016
Borrowing limit	$	250		$	250		$	250		$	250
Amount outstanding at period end	—			—			—			—
Average amount outstanding	26			25			—			21
Maximum amount outstanding	96			156			20			141
Weighted average interest rate, computed on a daily basis	2.27		%	1.93		%	0.92		%	0.73		%
Weighted average interest rate at end of period	N/A			N/A			N/A			N/A

Schedule of Short-term Debt [Table Text Block]

Commercial paper borrowings for PSCo were as follows:

	Three Months Ended Dec. 31, 2018			Year Ended Dec. 31
(Amounts in Millions, Except Interest Rates)				2018			2017			2016
Borrowing limit	$	700		$	700		$	700		$	700
Amount outstanding at period end	307			307			—			129
Average amount outstanding	87			55			54			24
Maximum amount outstanding	309			309			268			154
Weighted average interest rate, computed on a daily basis	2.64		%	2.28		%	1.08		%	0.70		%
Weighted average interest rate at end of period	2.95			2.95			N/A			0.95

Schedule Of Debt To Total Capitalization Ratio [Table Text Block]

Debt-to-Total Capitalization Ratio(a)				Amount Facility May Be Increased (millions)		Additional Periods For Which a One-Year Extension May Be Requested (b)
2018		2017
46	%	44	%	$	100	2

Schedule of Line of Credit Facilities [Table Text Block]

Dec. 31, 2018 (millions):

Credit Facility (a)		Drawn(b)		Available
$	700	$	317	$	383

Schedule of Capitalization [Table Text Block]

Long-term debt obligations for PSCo as of Dec. 31:

(Millions of Dollars)	Maturity Range	Interest Rate Range 2018	Interest Rate Range 2017	2018			2017
Capital lease obligations	2025-2060	11.20% - 14.30%	11.20% - 14.30%	$	145		$	151
Mortgage bonds	2019-2048	2.25% - 6.50%	2.25% - 6.50%	4,900			4,500
Unamortized discount				(14		)	(13		)
Unamortized debt issuance cost				(33		)	(29		)
Current maturities				(406		)	(306		)
Total				$	4,592		$	4,303

Schedule of Maturities of Long-term Debt [Table Text Block]

Maturities of long-term debt:

(Millions of Dollars)
2019	$	400
2020	400
2021	—
2022	300
2023	250

Schedule of Long-Term Debt Issuances [Table Text Block]

2018 financings:

Amount	Financing Instrument	Interest Rate		Maturity Date
$350 million	First mortgage bonds	3.70	%	June 15, 2028
350 million	First mortgage bonds	4.10		June 15, 2048

2017 financings:

Amount	Financing Instrument	Interest Rate		Maturity Date
$400 million	First mortgage bonds	3.80	%	June 15, 2047