REGULATORY MATTERS - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Regulatory Assets [Line Items]
Regulatory assets, current	$ 175.5	$ 93.9
Regulatory assets, non current	791.1	838.0
Total Regulatory Assets	966.6	931.9
Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory assets, current	42.2	27.0
Regulatory assets, non current	404.7	487.9
Unamortized purchased gas adjustments
Regulatory Assets [Line Items]
Regulatory assets, current	102.6	49.7
Future income taxes due from customers
Regulatory Assets [Line Items]
Regulatory assets, non current	170.5	151.3
Accrued cost of removal
Regulatory Assets [Line Items]
Regulatory assets, non current	123.3	130.6
Unamortized purchased gas adjustments
Regulatory Assets [Line Items]
Regulatory assets, non current	9.9	12.6
Energy efficiency
Regulatory Assets [Line Items]
Regulatory assets, non current	29.0	25.5
Other
Regulatory Assets [Line Items]
Regulatory assets, current	30.7	17.2
Regulatory assets, non current	53.7	30.1
Spire Missouri
Regulatory Assets [Line Items]
Regulatory assets, current	95.6	67.0
Regulatory assets, non current	557.8	589.8
Total Regulatory Assets	653.4	656.8
Spire Missouri | Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory assets, current	34.9	20.2
Regulatory assets, non current	322.7	375.7
Spire Missouri | Unamortized purchased gas adjustments
Regulatory Assets [Line Items]
Regulatory assets, current	57.4	43.1
Spire Missouri | Future income taxes due from customers
Regulatory Assets [Line Items]
Regulatory assets, non current	170.5	151.3
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	9.9	12.6
Spire Missouri | Energy efficiency
Regulatory Assets [Line Items]
Regulatory assets, non current	29.0	25.5
Spire Missouri | Other
Regulatory Assets [Line Items]
Regulatory assets, current	3.3	3.7
Regulatory assets, non current	25.7	24.7
Spire Alabama
Regulatory Assets [Line Items]
Regulatory assets, current	64.6	20.5
Regulatory assets, non current	197.0	230.7
Total Regulatory Assets	261.6	251.2
Spire Alabama | Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory assets, current	7.2	6.8
Regulatory assets, non current	72.6	98.9
Spire Alabama | Unamortized purchased gas adjustments
Regulatory Assets [Line Items]
Regulatory assets, current	45.2	5.6
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	123.3	130.6
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	12.2	8.1
Regulatory assets, non current	$ 1.1	$ 1.2