PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Amounts Recognized in Consolidated Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent liabilities	$ (237.4)	$ (303.7)
Pension Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	(0.5)	(0.6)
Noncurrent liabilities	(216.7)	(253.6)
Total	(217.2)	(254.2)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	163.0	179.4
Prior service (credit) cost	(13.4)	8.2
Subtotal	149.6	187.6
Adjustments for amounts included in regulatory assets	(147.1)	(184.8)
Total	2.5	2.8
Postretirement Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	(0.4)	(0.4)
Noncurrent liabilities	(21.0)	(50.1)
Total	27.0	(12.8)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	1.5	38.0
Prior service (credit) cost	(6.6)	(5.2)
Subtotal	(5.1)	32.8
Adjustments for amounts included in regulatory assets	5.1	(32.8)
Total	0.0	0.0
Spire Missouri
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent liabilities	(173.0)	(211.8)
Spire Missouri \| Pension Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	(0.5)	(0.6)
Noncurrent liabilities	(153.2)	(163.7)
Total	(153.7)	(164.3)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	126.2	135.5
Prior service (credit) cost	7.3	8.2
Subtotal	133.5	143.7
Adjustments for amounts included in regulatory assets	(131.0)	(140.9)
Total	2.5	2.8
Spire Missouri \| Postretirement Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	(0.4)	(0.4)
Noncurrent liabilities	(19.8)	(48.1)
Total	(17.6)	(48.2)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	12.3	43.4
Prior service (credit) cost	(3.7)	(3.4)
Subtotal	8.6	40.0
Adjustments for amounts included in regulatory assets	(8.6)	(40.0)
Total	0.0	0.0
Spire Alabama
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent liabilities	(50.2)	(74.3)
Spire Alabama \| Pension Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	0.0	0.0
Noncurrent liabilities	(50.3)	(74.3)
Total	(50.3)	(74.3)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	40.9	43.9
Prior service (credit) cost	(20.7)	0.0
Subtotal	20.2	43.9
Adjustments for amounts included in regulatory assets	(20.2)	(43.9)
Total	0.0	0.0
Spire Alabama \| Postretirement Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	0.0	0.0
Noncurrent liabilities	0.0	0.0
Total	45.8	37.4
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	(9.7)	(5.4)
Prior service (credit) cost	(2.9)	(1.8)
Subtotal	(12.6)	(7.2)
Adjustments for amounts included in regulatory assets	12.6	7.2
Total	0.0	0.0
Current Asset [Member] \| Postretirement Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	1.4	0.3
Current Asset [Member] \| Spire Missouri \| Postretirement Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	1.4	0.3
Current Asset [Member] \| Spire Alabama \| Postretirement Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	0.0	0.0
Noncurrent Asset [Member] \| Postretirement Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	47.0	37.4
Noncurrent Asset [Member] \| Spire Missouri \| Postretirement Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	1.2	0.0
Noncurrent Asset [Member] \| Spire Alabama \| Postretirement Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	$ 45.8	$ 37.4

Adjustments for defined benefit plan amount included in regulatory assets.

No definition available.

Name:	lg_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeAdjustmentsForAmountsIncludedInRegulatoryAssets
Namespace Prefix:	lg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The pretax total, net of regulatory adjustments, of net gain (loss), prior service cost (credit), and transition assets (obligations), included in accumulated other comprehensive income associated with a defined benefit pension or other postretirement plan(s) because they have yet to be recognized as components of net periodic benefit cost net of adjustments for amounts included in regulatory assets.

+ References

No definition available.

+ Details

Name:	lg_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTaxNetRegulatoryAdjustments
Namespace Prefix:	lg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, before tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (i-k)
-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669646-108580

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

+ References

+ Details

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

- Definition

Amount, before tax, of accumulated other comprehensive (income) loss for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (j)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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

- Definition

Amount of asset (liability), recognized in statement of financial position, for defined benefit pension and other postretirement plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64852052&loc=d3e1703-114919

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of asset, recognized in statement of financial position, for overfunded defined benefit pension and other postretirement plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64852052&loc=d3e1731-114919

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(17))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64852052&loc=d3e1703-114919

Reference 5: 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)(10))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 6: 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(10))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Amount of liability, recognized in statement of financial position, for defined benefit pension and other postretirement plans, classified as current.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64852052&loc=d3e1731-114919

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(20))
-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 715
-SubTopic 20
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64852052&loc=d3e1703-114919

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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

- Definition

Amount of liability, recognized in statement of financial position, for defined benefit pension and other postretirement plans, classified as noncurrent.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64852052&loc=d3e1731-114919

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(24))
-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 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64852052&loc=d3e1703-114919

+ Details

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

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_BalanceSheetLocationAxis=lg_CurrentAssetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=lg_NoncurrentAssetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: