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