Pension Plans and Other Postretirement Benefits - Schedule of Targeted and Actual Plan Assets by Category (Details)	Sep. 30, 2021	Sep. 30, 2020
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%	70.00%
Actual allocation of plan assets	74.50%	70.00%
Spire Missouri | Pension Plans | Liability Hedging Assets
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	30.00%	30.00%
Actual allocation of plan assets	23.10%	27.00%
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	2.40%	3.00%
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	1.90%	4.80%
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	59.20%	57.40%
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	38.90%	37.80%
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%	70.00%
Actual allocation of plan assets	72.80%	71.00%
Spire Alabama | Pension Plans | Liability Hedging Assets
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	30.00%	30.00%
Actual allocation of plan assets	25.50%	27.00%
Spire Alabama | 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	1.70%	2.00%
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	60.50%	61.80%
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	39.50%	38.20%