REGULATORY MATTERS - Off-system Sales (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Off-System Sales [Line Items]
Customer Share	75.00%	85.00%	100.00%
Company Share		15.00%
Spire Missouri East
Off-System Sales [Line Items]
Customer Share	25.00%
Spire Missouri West
Off-System Sales [Line Items]
Customer Share	25.00%
First $2.0 of pre-tax income | Spire Missouri East
Off-System Sales [Line Items]
Pre-tax Income	$ 2.0
Customer Share	85.00%
Company Share	15.00%
Next $2.0 of pre-tax income | Spire Missouri East
Off-System Sales [Line Items]
Pre-tax Income	$ 2.0
Customer Share	80.00%
Company Share	20.00%
Next $2.0 of pre-tax income | Spire Missouri East
Off-System Sales [Line Items]
Pre-tax Income	$ 2.0
Customer Share	75.00%
Company Share	25.00%
Amounts of pre-tax income exceeding $6.0 | Spire Missouri East
Off-System Sales [Line Items]
Pre-tax Income	$ 6.0
Customer Share	70.00%
Company Share	30.00%
First $1.2 of pre-tax income | Spire Missouri West
Off-System Sales [Line Items]
Pre-tax Income	$ 1.2
Customer Share	85.00%
Company Share	15.00%
Next $1.2 of pre-tax income | Spire Missouri West
Off-System Sales [Line Items]
Pre-tax Income	$ 1.2
Customer Share	80.00%
Company Share	20.00%
Next $1.2 of pre-tax income | Spire Missouri West
Off-System Sales [Line Items]
Pre-tax Income	$ 1.2
Customer Share	75.00%
Company Share	25.00%
Amounts of pre-tax income exceeding $3.6 | Spire Missouri West
Off-System Sales [Line Items]
Pre-tax Income	$ 3.6
Customer Share	70.00%
Company Share	30.00%