REGULATORY MATTERS - Schedule of Regulatory Assets (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017
Regulatory Assets [Line Items]
Regulatory assets, current	$ 72.8	$ 175.5
Regulatory assets, non current	669.8	791.1
Total Regulatory Assets	742.6	966.6
Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory assets, current	30.2	42.2
Regulatory assets, non current	364.9	404.7
Unamortized purchased gas adjustments
Regulatory Assets [Line Items]
Regulatory assets, current	8.2	102.6
Future income taxes due from customers
Regulatory Assets [Line Items]
Regulatory assets, non current	96.3	170.5
Accrued cost of removal
Regulatory Assets [Line Items]
Regulatory assets, non current	133.4	123.3
Unamortized purchased gas adjustments
Regulatory Assets [Line Items]
Regulatory assets, non current	0.0	9.9
Energy efficiency
Regulatory Assets [Line Items]
Regulatory assets, non current	32.8	29.0
Other
Regulatory Assets [Line Items]
Regulatory assets, current	34.4	30.7
Regulatory assets, non current	42.4	53.7
Spire Missouri
Regulatory Assets [Line Items]
Regulatory assets, current	30.7	95.6
Regulatory assets, non current	441.1	557.8
Total Regulatory Assets	471.8	653.4
Spire Missouri | Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory assets, current	21.9	34.9
Regulatory assets, non current	292.5	322.7
Spire Missouri | Unamortized purchased gas adjustments
Regulatory Assets [Line Items]
Regulatory assets, current	1.0	57.4
Spire Missouri | Future income taxes due from customers
Regulatory Assets [Line Items]
Regulatory assets, non current	94.4	170.5
Spire Missouri | Accrued cost of removal
Regulatory Assets [Line Items]
Regulatory assets, non current	0.0	0.0
Spire Missouri | Unamortized purchased gas adjustments
Regulatory Assets [Line Items]
Regulatory assets, non current	0.0	9.9
Spire Missouri | Energy efficiency
Regulatory Assets [Line Items]
Regulatory assets, non current	32.8	29.0
Spire Missouri | Other
Regulatory Assets [Line Items]
Regulatory assets, current	7.8	3.3
Regulatory assets, non current	21.4	25.7
Spire Alabama
Regulatory Assets [Line Items]
Regulatory assets, current	26.2	64.6
Regulatory assets, non current	201.5	197.0
Total Regulatory Assets	227.7	261.6
Spire Alabama | Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory assets, current	7.3	7.2
Regulatory assets, non current	64.8	72.6
Spire Alabama | Unamortized purchased gas adjustments
Regulatory Assets [Line Items]
Regulatory assets, current	6.4	45.2
Spire Alabama | Future income taxes due from customers
Regulatory Assets [Line Items]
Regulatory assets, non current	0.0	0.0
Spire Alabama | Accrued cost of removal
Regulatory Assets [Line Items]
Regulatory assets, non current	133.4	123.3
Spire Alabama | Unamortized purchased gas adjustments
Regulatory Assets [Line Items]
Regulatory assets, non current	0.0	0.0
Spire Alabama | Energy efficiency
Regulatory Assets [Line Items]
Regulatory assets, non current	0.0	0.0
Spire Alabama | Other
Regulatory Assets [Line Items]
Regulatory assets, current	12.5	12.2
Regulatory assets, non current	$ 3.3	$ 1.1