REGULATORY MATTERS - Narrative (Details) - USD ($)			3 Months Ended		9 Months Ended		12 Months Ended
	Mar. 07, 2018	Apr. 11, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2017
Public Utilities, General Disclosures [Line Items]
Remaining recovery period for certain regulatory assets for which no return on investment during recovery period is provided (in years)					20 years
Remaining recovery period for regulatory assets for which no return on investment during recovery period is provided (in years)							15 years
Effects of the Tax Cuts and Jobs Act	$ 33,000,000		$ 0	$ 0	$ (54,000,000)	$ 0
Regulatory asset			$ 791,800,000	$ 985,100,000	$ 791,800,000	$ 985,100,000	$ 966,600,000
Pension and postretirement benefit costs | Spire Missouri East
Public Utilities, General Disclosures [Line Items]
Remaining recovery period for regulatory assets for which no return on investment during recovery period is provided (in years)					8 years
Amortization of regulatory assets not earning a return					$ 173,000,000
Pension and postretirement benefit costs | Spire Missouri West
Public Utilities, General Disclosures [Line Items]
Amortization of regulatory assets not earning a return					26,200,000
ISRS surcharge | MoPSC
Public Utilities, General Disclosures [Line Items]
Approved rate increase (decrease) amount	0
Approved rate increase (decrease), incentive compensation costs excluded from rate base	6,900,000
Approved rate increase (decrease), incentive compensation costs from buildings sold in 2014 excluded from rate base	1,800,000
Approved rate increase (decrease), rate case expenses excluded from rate base	900,000
Approved rate increase (decrease), regulatory charges					$ 38,400,000
Approved rate increase (decrease), impact to net income (loss)	$ 23,600,000
Approved rate increase (decrease), impact to earnings per share (in usd per share)	$ 0.49
ISRS surcharge | MoPSC | Spire Missouri East
Public Utilities, General Disclosures [Line Items]
Approved rate increase (decrease) amount	$ 18,000,000	$ 32,600,000
Approved rate increase (decrease), annualized revenue	(14,600,000)
ISRS surcharge | MoPSC | Spire Missouri West
Public Utilities, General Disclosures [Line Items]
Approved rate increase (decrease) amount	15,200,000	16,400,000
Approved rate increase (decrease), annualized revenue	(1,200,000)
ISRS surcharge | MoPSC | Pension costs
Public Utilities, General Disclosures [Line Items]
Increase (decrease) in regulatory assets	(28,800,000)
Approved rate increase (decrease), regulatory charges	30,600,000
ISRS surcharge | MoPSC | Pension costs | Spire Missouri East
Public Utilities, General Disclosures [Line Items]
Regulatory asset	29,000,000	15,500,000
ISRS surcharge | MoPSC | Pension costs | Spire Missouri West
Public Utilities, General Disclosures [Line Items]
Regulatory asset	5,500,000	9,900,000
ISRS surcharge | MoPSC | Postretirement health care costs | Spire Missouri East
Public Utilities, General Disclosures [Line Items]
Regulatory asset	$ 8,600,000	$ 9,500,000