REGULATORY MATTERS - Schedule of Regulatory Assets Not Earnings a Return (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 380.3	$ 404.8
Future income taxes due from customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	170.5	151.3
Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	198.5	240.6
Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	11.3	12.9
Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	380.3	404.8
Spire Missouri | Future income taxes due from customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	170.5	151.3
Spire Missouri | Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	198.5	240.6
Spire Missouri | Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 11.3	$ 12.9