Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	$ (26,064)	$ (50,101)
Long-term debt, less current maturities	1,564,132	1,549,983
Less: current maturities	(26,064)	(50,101)
Long-term debt, less current maturities	1,564,132	1,549,983
Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities		(25,000)
Long-term debt, less current maturities	1,385,315	1,375,080
Less: current maturities		(25,000)
Long-term debt, less current maturities	1,385,315	1,375,080
Variable-rate bonds [Member] \| 2009 Series A, due 2039 [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Unsecured debt, fair value	50,000	50,000
Carrying Amount [Member]
Long-term Debt, Current and Noncurrent Abstract
Long-term debt	1,590,196	1,600,084
Less: current maturities	(26,064)	(50,101)
Long-term debt, less current maturities	1,564,132	1,549,983
Less: current maturities	(26,064)	(50,101)
Long-term debt, less current maturities	1,564,132	1,549,983
Carrying Amount [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities		(25,000)
Long-term debt, less current maturities	1,385,315	1,375,080
Unamortized discount and debt issuance costs	(2,397)	(2,489)
Less: current maturities		(25,000)
Long-term debt, less current maturities	1,385,315	1,375,080
Carrying Amount [Member] \| Centuri Construction Group Inc [Member]
Debt Instrument [Line Items]
Unamortized debt issuance costs	(485)	(516)
Centuri term loan/secured revolving credit facility	104,873	106,184
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(26,064)	(25,101)
Long-term debt, less current maturities	178,817	174,903
Less: current maturities	(26,064)	(25,101)
Long-term debt, less current maturities	178,817	174,903
Carrying Amount [Member] \| Centuri Secured Revolving Credit Facility [Member] \| Centuri Construction Group Inc [Member]
Debt Instrument [Line Items]
Centuri term loan/secured revolving credit facility	51,054	41,185
Carrying Amount [Member] \| IDRBs [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Unsecured debt	197,603	197,511
Carrying Amount [Member] \| Centuri Term Loan Facility [Member] \| Centuri Construction Group Inc [Member]
Debt Instrument [Line Items]
Centuri term loan/secured revolving credit facility	105,358	106,700
Carrying Amount [Member] \| Centuri Other Debt Obligations [Member] \| Centuri Construction Group Inc [Member]
Long-term Debt, Current and Noncurrent Abstract
Centuri other debt obligations	48,954	52,635
Carrying Amount [Member] \| Debentures [Member]
Long-term Debt, Current and Noncurrent Abstract
Notes payable	1,172,712	1,197,569
Carrying Amount [Member] \| Debentures [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Unamortized discount and debt issuance costs	(9,788)	(9,931)
Carrying Amount [Member] \| Debentures [Member] \| Notes, 4.45%, due 2020 [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Notes payable	125,000	125,000
Carrying Amount [Member] \| Debentures [Member] \| Notes, 6.1%, due 2041 [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Notes payable	125,000	125,000
Carrying Amount [Member] \| Debentures [Member] \| Notes, 3.875%, due 2022 [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Notes payable	250,000	250,000
Carrying Amount [Member] \| Debentures [Member] \| Notes, 4.875%, due 2043 [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Notes payable	250,000	250,000
Carrying Amount [Member] \| Debentures [Member] \| Notes, 3.8%, due 2046 [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Notes payable	300,000	300,000
Carrying Amount [Member] \| Debentures [Member] \| 8% Series, due 2026 [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Notes payable	75,000	75,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 7.59% series, due 2017 [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Notes payable		25,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 7.78% series, due 2022 [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Notes payable	25,000	25,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 7.92% series, due 2027 [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Notes payable	25,000	25,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 6.76% series, due 2027 [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Notes payable	7,500	7,500
Carrying Amount [Member] \| Debentures [Member] \| Revolving Credit Facility and Commercial Paper [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Line of credit facility	15,000	5,000
Carrying Amount [Member] \| Variable-rate bonds [Member] \| Tax-exempt Series A, due 2028 [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Unsecured debt	50,000	50,000
Carrying Amount [Member] \| Variable-rate bonds [Member] \| 2003 Series A, due 2038 [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Unsecured debt	50,000	50,000
Carrying Amount [Member] \| Variable-rate bonds [Member] \| 2008 Series A, due 2038 [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Unsecured debt	50,000	50,000
Carrying Amount [Member] \| Variable-rate bonds [Member] \| 2009 Series A, due 2039 [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Unsecured debt	50,000	50,000
Market Value [Member] \| Centuri Construction Group Inc [Member]
Debt Instrument [Line Items]
Centuri term loan/secured revolving credit facility	105,944	106,819
Long-term Debt, Current and Noncurrent Abstract
Centuri other debt obligations	49,078	52,840
Market Value [Member] \| Centuri Secured Revolving Credit Facility [Member] \| Centuri Construction Group Inc [Member]
Debt Instrument [Line Items]
Centuri term loan/secured revolving credit facility	51,144	41,292
Market Value [Member] \| Debentures [Member] \| Notes, 4.45%, due 2020 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	130,193	129,703
Market Value [Member] \| Debentures [Member] \| Notes, 6.1%, due 2041 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	150,255	149,734
Market Value [Member] \| Debentures [Member] \| Notes, 3.875%, due 2022 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	257,328	254,900
Market Value [Member] \| Debentures [Member] \| Notes, 4.875%, due 2043 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	264,968	266,793
Market Value [Member] \| Debentures [Member] \| Notes, 3.8%, due 2046 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	282,501	283,029
Market Value [Member] \| Debentures [Member] \| 8% Series, due 2026 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	94,822	94,691
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 7.59% series, due 2017 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure		25,040
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 7.78% series, due 2022 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	29,269	29,290
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 7.92% series, due 2027 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	31,947	31,905
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 6.76% series, due 2027 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	8,792	8,769
Market Value [Member] \| Debentures [Member] \| Revolving Credit Facility and Commercial Paper [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	15,000	5,000
Market Value [Member] \| Variable-rate bonds [Member] \| Tax-exempt Series A, due 2028 [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Unsecured debt, fair value	50,000	50,000
Market Value [Member] \| Variable-rate bonds [Member] \| 2003 Series A, due 2038 [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Unsecured debt, fair value	50,000	50,000
Market Value [Member] \| Variable-rate bonds [Member] \| 2008 Series A, due 2038 [Member] \| Southwest Gas Corporation [Member]
Long-term Debt, Current and Noncurrent Abstract
Unsecured debt, fair value	$ 50,000	$ 50,000

Debt instrument unamortized discount and debt issuance costs.

No definition available.

Name:	swx_DebtInstrumentUnamortizedDiscountAndDebtIssuanceCosts
Namespace Prefix:	swx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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://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))
-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 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-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

+ Details

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

- Definition

Fair value portion of contractual arrangement with a lender under which borrowings can be made up to a specific amount at any point in time.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Line-of-Credit Arrangement
-URI http://asc.fasb.org/extlink&oid=6517033

+ Details

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

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-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://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))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

+ Details

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

- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

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

- Definition

Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer.

+ 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))
-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 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-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

+ Details

Name:	us-gaap_NotesPayable
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

- Definition

Amount of long-term debt classified as other.

+ 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))
-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 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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

- Definition

Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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 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(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

- Definition

The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ References

+ Details

Name:	us-gaap_UnamortizedDebtIssuanceExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Including the current and noncurrent portions, carrying value as of the balance sheet date of uncollateralized debt obligations (with maturities initially due after one year or beyond the operating cycle if longer).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-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

+ Details

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

- Details

Name:	dei_LegalEntityAxis=swx_SouthwestGasCorporationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=swx_CenturiConstructionGroupMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=swx_CenturiSecuredRevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: