SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Inter-company Transactions (Details) - Affiliated Entity - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Regulated Operation \| Laclede Gas \| Spire Marketing
Related Party Transaction [Line Items]
Purchases of natural gas from Spire Marketing	$ 21.0	$ 11.6	$ 41.5	$ 24.8
Regulated Operation \| Laclede Gas \| Laclede Insurance Risk Services
Related Party Transaction [Line Items]
Insurance services received from Laclede Insurance Risk Services	1.0	0.3	2.1	0.5
Regulated Operation \| Spire Marketing \| Laclede Gas
Related Party Transaction [Line Items]
Sales of natural gas to Spire Marketing	2.7	1.0	6.3	1.7
Unregulated Operation \| Laclede Gas \| Laclede Pipeline Company
Related Party Transaction [Line Items]
Transportation services received from Laclede Pipeline Company	$ 0.2	$ 0.2	$ 0.5	$ 0.5

Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

+ Details

Name:	us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_RelatedPartyTransactionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Purchases during the period (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

+ References

No definition available.

+ Details

Name:	us-gaap_RelatedPartyTransactionPurchasesFromRelatedParty
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of selling, general and administrative expenses resulting from transactions, excluding transactions that are eliminated in consolidated or combined financial statements, with related party.

+ References

No definition available.

+ Details

Name:	us-gaap_RelatedPartyTransactionSellingGeneralAndAdministrativeExpensesFromTransactionsWithRelatedParty
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(e))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Related Parties
-URI http://asc.fasb.org/extlink&oid=16382449

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07.1(c))
-URI http://asc.fasb.org/extlink&oid=6488393&loc=d3e606610-122999

+ Details

Name:	us-gaap_RevenueFromRelatedParties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_AffiliatedEntityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis=us-gaap_RegulatedOperationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis=lg_LacledeGasCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis=lg_SpireMarketingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis=us-gaap_UnregulatedOperationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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