REGULATORY MATTERS - Schedule of Regulatory Assets Not Earnings a Return (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 259.8	$ 380.3
Future income taxes due from customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	148.4	170.5
Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	96.3	198.5
Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	15.1	11.3
Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	257.9	380.3
Spire Missouri | Future income taxes due from customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	148.4	170.5
Spire Missouri | Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	94.4	198.5
Spire Missouri | Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 15.1	$ 11.3