Debt Instrument Discount Rate

No definition available.

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Data Type:	num:percentItemType
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Period Type:	instant

- Definition

Line Of Credit Facility, Amount Classified As Long-Term Debt

+ References

No definition available.

+ Details

Name:	swx_LineOfCreditFacilityAmountClassifiedAsLongTermDebt
Namespace Prefix:	swx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line Of Credit Facility, Working Capital Purposes

+ References

No definition available.

+ Details

Name:	swx_LineOfCreditFacilityWorkingCapitalPurposes
Namespace Prefix:	swx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of assets pledged to secure a debt instrument.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

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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Namespace Prefix:	us-gaap_
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/disclosureRef
-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

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

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

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

Name:	us-gaap_LineOfCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Date the credit facility terminates, in CCYY-MM-DD format.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityExpirationDate1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value portion of notes payable.

+ References

+ Details

Name:	us-gaap_NotesPayableFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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Namespace Prefix:
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Balance Type:
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- Details

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- Details

Name:	us-gaap_LongtermDebtTypeAxis=swx_DebenturesNotesThreePointSevenPercentageDueTwoThousandAndTwentyEightMember
Namespace Prefix:
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Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=swx_SouthwestGasCreditFacilityMember
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Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ExtinguishmentOfDebtAxis=us-gaap_LongTermDebtMember
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Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
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- Details

Name:	us-gaap_VariableRateAxis=swx_AlternativeBaseRateMember
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- Details

Name:	us-gaap_LongtermDebtTypeAxis=swx_CommercialPaperProgramMember
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Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_LineOfCreditMember
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- Details

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- Details

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Long-Term Debt - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended	6 Months Ended
	Mar. 31, 2018	Jun. 30, 2018	Jun. 30, 2018
Secured Revolving Credit and Term Loan Facility [Member] \| Centuri Construction Group Inc [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		$ 450,000,000	$ 450,000,000
Credit facility expiration date			Nov. 30, 2022
Borrowings outstanding under facility		278,000,000	$ 278,000,000
Debt secured by assets		659,000,000	659,000,000
Notes 3.7% due 2028 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	$ 300,000,000
Debt instrument interest rate	3.70%
Debt instrument discount rate	0.185%
Debt instrument due date	Apr. 30, 2028
Southwest Gas Credit Facility [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		400,000,000	400,000,000
Line of credit designated as long term debt		150,000,000	150,000,000
Line of credit designated for working capital purposes		250,000,000	$ 250,000,000
Credit facility expiration date			Mar. 01, 2022
Southwest Gas Credit Facility [Member] \| Long-term Debt [Member]
Debt Instrument [Line Items]
Borrowings outstanding under facility		48,000,000	$ 48,000,000
Southwest Gas Credit Facility [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Applicable margin			1.00%
Southwest Gas Credit Facility [Member] \| Alternative Base Rate [Member]
Debt Instrument [Line Items]
Applicable margin			0.00%
Commercial Paper Program [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		0	$ 0
Borrowings outstanding under facility		50,000,000	50,000,000
Line of Credit [Member] \| Secured Revolving Credit and Term Loan Facility [Member] \| Centuri Construction Group Inc [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		250,000,000	250,000,000
Centuri Term Loan Facility [Member] \| Secured Revolving Credit and Term Loan Facility [Member] \| Centuri Construction Group Inc [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		200,000,000	200,000,000
Equipment Loans [Member] \| Secured Revolving Credit and Term Loan Facility [Member] \| Centuri Construction Group Inc [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		$ 40,000,000	$ 40,000,000
Credit facility expiration date		May 31, 2023