REGULATORY MATTERS - Schedule of Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 306.4	$ 380.3	$ 409.0
Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	160.5	198.5	231.0
Future income taxes due from customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	130.5	170.5	166.6
Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	15.4	11.3	11.4
Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	304.3	380.3	409.0
Spire Missouri | Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	160.5	198.5	231.0
Spire Missouri | Future income taxes due from customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	128.4	170.5	166.6
Spire Missouri | Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 15.4	$ 11.3	$ 11.4