Borrowings and Other Financing Instruments (Tables)

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Money Pool [Table Text Block]

Money pool borrowings for SPS were as follows:

	Three Months Ended Dec. 31, 2018			Year Ended Dec. 31
(Amounts in Millions, Except Interest Rates)				2018			2017			2016
Borrowing limit	$	100		$	100		$	100		$	100
Amount outstanding at period end	—			—			—			—
Average amount outstanding	14			29			13			28
Maximum amount outstanding	74			100			100			100
Weighted average interest rate, computed on a daily basis	2.13		%	1.96		%	1.12		%	0.67		%
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:

	Three Months Ended Dec. 31, 2018			Year Ended Dec. 31
(Amounts in Millions, Except Interest Rates)				2018			2017			2016
Borrowing limit	$	400		$	400		$	400		$	400
Amount outstanding at period end	42			42			—			50
Average amount outstanding	20			30			69			43
Maximum amount outstanding	100			144			176			140
Weighted average interest rate, computed on a daily basis	2.45		%	2.27		%	1.13		%	0.67		%
Weighted average interest rate at end of period	2.80			2.80			NA			0.95

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)
2018	2017
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, 2018.

Credit Facility (a)	Drawn (b)	Available
$400	$44	$356

^(a) This credit facility matures in June 2021.

^(b) Includes letters of credit and outstanding commercial paper.

Schedule of Capitalization [Table Text Block]

Long term debt obligations for SPS as of Dec. 31:

(Millions of Dollars)	Maturity Range	Interest Rate Range 2018	Interest Rate Range 2017	2018			2017
Mortgage bonds	2024 - 2048	3.30% - 4.50%	3.30% - 4.50%	$	1,800		$	1,500
Unsecured senior notes	2033 - 2036	6.00%	6.00% - 8.75%	350			350
Unamortized discount				(4		)	(2		)
Unamortized debt issuance cost				(20		)	(18		)
Current maturities				—			—
Total long term debt				$	2,126		$	1,830

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

2018 financings:

Amount	Financing Instrument	Interest Rate		Maturity Date
$300 million	First mortgage bonds	4.40	%	Nov 15, 2048

2017 financings:

Amount	Financing Instrument	Interest Rate		Maturity Date
$450 million	First mortgage bonds	3.70	%	Aug 15, 2047

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

SPS has the following preferred stock:

	Preferred Stock Authorized (Shares)	Par Value of Preferred Stock	Preferred Stock Outstanding (Shares)2018 and 2017
SPS	10,000,000	1.00	0

Dividend Payment Restrictions [Text Block]

Requirements and actuals as of Dec. 31, 2018:

Equity to Total Capitalization Ratio - Required Range				Equity to Total Capitalization Ratio - Actual (a)
Low		High		2018
45.0	%	55.0	%	54.4	%

^(a)  SPS excludes short-term debt.

	Unrestricted Retained Earnings			Total Capitalization			Limit on Total Capitalization
	2018			2018			2018
SPS (a)	$	605.7	million	$	4.7	billion	N/A

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