Borrowings and Other Financing Instruments (Tables)

12 Months Ended

Dec. 31, 2019

Debt Disclosure [Abstract]

Money Pool [Table Text Block]

Money pool borrowings for SPS were as follows:

(Millions of Dollars, Except Interest Rates)	Three Months Ended Dec. 31, 2019			Year Ended Dec. 31
				2019			2018			2017
Borrowing limit	$	100		$	100		$	100		$	100
Amount outstanding at period end	—			—			—			—
Average amount outstanding	1			8			29			13
Maximum amount outstanding	12			100			100			100
Weighted average interest rate, computed on a daily basis	1.63		%	2.42		%	1.96		%	1.12		%
Weighted average interest rate at end of period	N/A			N/A			N/A			N/A

Short-Term Borrowings

Commercial paper outstanding for SPS was as follows:

(Millions of Dollars, Except Interest Rates)	Three Months Ended Dec. 31, 2019		Year Ended Dec. 31
			2019			2018			2017
Borrowing limit	$	500	$	500		$	400		$	400
Amount outstanding at period end	—		—			42			—
Average amount outstanding	—		72			30			69
Maximum amount outstanding	—		316			144			176
Weighted average interest rate, computed on a daily basis	N/A		2.68		%	2.27		%	1.13		%
Weighted average interest rate at end of period	N/A		N/A			2.80			NA

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

Features of SPS’ credit facility:

Debt-to-Total Capitalization Ratio(a)		Amount Facility May Be Increased (millions)	Additional Periods for Which a One-Year Extension May Be Requested (b)
2019	2018
46%	46%	$50	2

^(a)  The SPS credit facility has a financial covenant requiring that the debt-to-total capitalization ratio be less than or equal to 65%.

^(b) 

All extension requests are subject to majority bank group approval.

Credit Facilities

SPS had the following committed credit facilities available as of Dec. 31, 2019.

Credit Facility (a)	Drawn (b)	Available
$500	$2	$498

^(a) This credit facility matures in June 2024.

^(b)

Includes letters of credit and outstanding commercial paper.

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

Long-term debt obligations for SPS as of Dec. 31 (millions of dollars):

Financing Instrument	Interest Rate		Maturity Date	2019			2018
First mortgage bonds	3.30	%	June 15, 2024	$	150		$	150
First mortgage bonds	3.30		June 15, 2024	200			200
Unsecured senior notes	6.00		Oct. 1, 2033	100			100
Unsecured senior notes	6.00		Oct. 1, 2036	250			250
First mortgage bonds	4.50		Aug. 15, 2041	200			200
First mortgage bonds	4.50		Aug. 15, 2041	100			100
First mortgage bonds	4.50		Aug. 15, 2041	100			100
First mortgage bonds	3.40		Aug. 15, 2046	300			300
First mortgage bonds	3.70		Aug. 15, 2047	450			450
First mortgage bonds (b)	4.40		Nov. 15, 2048	300			300
First mortgage bonds (a)	3.75		June 15, 2049	300			—
Unamortized discount				(7		)	(4		)
Unamortized debt issuance cost				(23		)	(20		)
Total long-term debt				$	2,420		$	2,126

^(a)  2019 financing

^(b) 

2018 financing

Dividend Payment Restrictions [Text Block]

Requirements and actuals as of Dec. 31, 2019:

Equity to Total Capitalization Ratio - Required Range				Equity to Total Capitalization Ratio - Actual (a)
Low		High		2019
45.0	%	55.0	%	54.4	%

^(a)  Excludes short-term debt.

Unrestricted Retained Earnings			Total Capitalization			Limit on Total Capitalization (a)
$	535.0	million	$	5.3	billion	N/A

^(a) SPS may not pay a dividend that would cause it to lose its investment grade bond rating.

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

Maturities of long-term debt:

(Millions of Dollars)
2020	$	—
2021	—
2022	—
2023	—
2024	350

Schedule of Stock by Class [Table Text Block]

SPS has the following preferred stock:

Preferred Stock Authorized (Shares)	Par Value of Preferred Stock	Preferred Stock Outstanding (Shares)  2019 and 2018
10,000,000	1.00	—