Pension Plans and Other Postretirement Benefits - Schedule of Targeted and Actual Plan Assets by Category (Details)	Sep. 30, 2020	Sep. 30, 2019
Spire Missouri | Pension Plans
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	100.00%	100.00%
Actual allocation of plan assets	100.00%	100.00%
Spire Missouri | Pension Plans | Return Seeking Assets
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	70.00%	56.50%
Actual allocation of plan assets	70.00%	57.00%
Spire Missouri | Pension Plans | Liability Hedging Assets
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	30.00%	43.50%
Actual allocation of plan assets	27.00%	39.80%
Spire Missouri | Pension Plans | Other
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	0.00%	0.00%
Actual allocation of plan assets	3.00%	3.20%
Spire Missouri | Other Postretirement Plans
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	100.00%
Actual allocation of plan assets	100.00%	100.00%
Spire Missouri | Other Postretirement Plans | Other
Targeted and actual plan assets by category [Abstract]
Actual allocation of plan assets	4.80%	2.80%
Spire Missouri | Other Postretirement Plans | Equity markets
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	60.00%
Actual allocation of plan assets	57.40%	58.40%
Spire Missouri | Other Postretirement Plans | Debt securities
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	40.00%
Actual allocation of plan assets	37.80%	38.80%
Spire Alabama | Pension Plans
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	100.00%	100.00%
Actual allocation of plan assets	100.00%	100.00%
Spire Alabama | Pension Plans | Return Seeking Assets
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	70.00%	60.00%
Actual allocation of plan assets	71.00%	58.50%
Spire Alabama | Pension Plans | Liability Hedging Assets
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	30.00%	29.00%
Actual allocation of plan assets	27.00%	29.80%
Spire Alabama | Pension Plans | Other
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	0.00%	11.00%
Actual allocation of plan assets	2.00%	11.70%
Spire Alabama | Other Postretirement Plans
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	100.00%
Actual allocation of plan assets	100.00%	100.00%
Spire Alabama | Other Postretirement Plans | Equity markets
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	60.00%
Actual allocation of plan assets	61.80%	60.30%
Spire Alabama | Other Postretirement Plans | Debt securities
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	40.00%
Actual allocation of plan assets	38.20%	39.70%