REGULATORY MATTERS - Schedule of Regulatory Assets Not Earnings a Return (Details) - USD ($) $ in Millions	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 318.1	$ 380.3	$ 399.1
Future income taxes due from customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	113.1	170.5	155.5
Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	193.8	198.5	231.4
Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	11.2	11.3	12.2
Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	318.1	380.3	399.1
Spire Missouri | Future income taxes due from customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	113.1	170.5	155.5
Spire Missouri | Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	193.8	198.5	231.4
Spire Missouri | Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 11.2	$ 11.3	$ 12.2