STATEMENTS OF CAPITALIZATION (Parenthetical) - $ / shares	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Common Stockholder's Equity
Common stock, shares authorized (in shares)	200	200
Common stock, par value (in dollars per share)	$ 1.00	$ 1.00
Common stock, shares outstanding (in shares)	100	100
First Mortgage Bonds, Series due: \| Series Due June 15, 2024
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Maturity Date	Jun. 15, 2024	Jun. 15, 2024
Debt instrument, Interest Rate, Stated Percentage	3.30%	3.30%
First Mortgage Bonds, Series due: \| Series Due Aug. 15, 2041
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Maturity Date	Aug. 15, 2041	Aug. 15, 2041
Debt instrument, Interest Rate, Stated Percentage	4.50%	4.50%
First Mortgage Bonds, Series due: \| Series Due Aug. 15, 2046
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Maturity Date	Aug. 15, 2046	Aug. 15, 2046
Debt instrument, Interest Rate, Stated Percentage	3.40%	3.40%
Unsecured Senior Notes \| Series Due August 15, 2047
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Maturity Date	Aug. 15, 2047
Debt instrument, Interest Rate, Stated Percentage	3.70%
Unsecured Senior Notes \| Senior G Due Dec. 1, 2018
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Maturity Date	Dec. 01, 2018	Dec. 01, 2018
Debt instrument, Interest Rate, Stated Percentage	8.75%	8.75%
Unsecured Senior Notes \| Senior C and D Due Oct. 1, 2033
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Maturity Date	Oct. 01, 2033	Oct. 01, 2033
Debt instrument, Interest Rate, Stated Percentage	6.00%	6.00%
Unsecured Senior Notes \| Senior F Due Oct. 1, 2036
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Maturity Date	Oct. 01, 2036	Oct. 01, 2036
Debt instrument, Interest Rate, Stated Percentage	6.00%	6.00%

-- None. No documentation exists for this element. --

No definition available.

Name:	sps_CommonStockholdersEquityAbstract
Namespace Prefix:	sps_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Face amount or stated value per share of common stock.

+ References

+ Details

Name:	us-gaap_CommonStockParOrStatedValuePerShare
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

The maximum number of common shares permitted to be issued by an entity's charter and bylaws.

+ References

+ Details

Name:	us-gaap_CommonStockSharesAuthorized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21463-112644

+ Details

Name:	us-gaap_CommonStockSharesOutstanding
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

+ Details

Name:	us-gaap_DebtInstrumentMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ScheduleOfCapitalizationLongtermDebtLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_BondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=sps_SeriesDueJune152024Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=sps_SeriesDueAug152041Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=sps_SeriesDueAugust152046Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_UnsecuredDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=sps_SeriesDueAugust152047Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=sps_SeniorGDueDec12018Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=sps_SeniorCAndDDueOct12033Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=sps_SeniorFDueOct12036Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: