Borrowings and Other Financing Instruments, Long-Term Borrowings (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Deferred Finance Costs, Noncurrent, Net	$ 18	$ 15
Equity to total capitalization ratio (excluding short-term debt), low end of range	45.00%
Equity to total capitalization ratio (excluding short-term debt), high end of range	55.00%
Equity to total capitalization ratio (excluding short-term debt)	53.80%
Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions	$ 542
First Mortgage Bonds, Series due: \| Series Due Aug. 15, 2046
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	3.40%	3.40%
Debt Instrument, Maturity Date	Aug. 15, 2046	Aug. 15, 2046
SPS \| First Mortgage Bonds, Series due: \| Series Due August 15, 2047
Debt Instrument [Line Items]
Face Amount	$ 450
Interest Rate, Stated Percentage (in hundredths)	3.70%
Debt Instrument, Maturity Date	Aug. 15, 2047
SPS \| First Mortgage Bonds, Series due: \| Series Due Aug. 15, 2046
Debt Instrument [Line Items]
Face Amount		$ 300
Interest Rate, Stated Percentage (in hundredths)		3.40%
Debt Instrument, Maturity Date		Aug. 15, 2046

The actual ratio of equity to total capitalization, excluding short-term debt.

No definition available.

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- Definition

The high end of the range of the ratio of equity to total capitalization, excluding short-term debt, required to be maintained by the entity in order to pay dividends.

+ References

No definition available.

+ Details

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- Definition

The low end of the range of the ratio of equity to total capitalization, excluding short-term debt, required to be maintained by the entity in order to pay dividends.

+ References

No definition available.

+ Details

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Balance Type:	na
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- Definition

Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions

+ References

No definition available.

+ Details

Name:	sps_UnrestrictedRetainedEarningsPerStateRegulatoryCommissionsDividendRestrictions
Namespace Prefix:	sps_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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

+ 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 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Balance Type:	na
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- 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

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ 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 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Amount, after accumulated amortization, of debt issuance costs classified as noncurrent. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Balance Type:	debit
Period Type:	instant

- Details

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- Details

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- Details

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- Details

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