Pension Plans and Other Postretirement Benefits - Schedule of Targeted and Actual Plan Assets by Category (Details)	Sep. 30, 2019	Sep. 30, 2018
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 | Equity markets
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	56.50%	56.40%
Actual allocation of plan assets	57.00%	55.30%
Spire Missouri | Pension Plans | Debt securities
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	43.50%	43.60%
Actual allocation of plan assets	39.80%	42.40%
Spire Missouri | Pension Plans | Cash equivalents
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	0.00%	0.00%
Actual allocation of plan assets	3.20%	2.30%
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 | Equity markets
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	60.00%
Actual allocation of plan assets	58.40%	58.80%
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	38.80%	39.10%
Spire Missouri | Other Postretirement Plans | Other
Targeted and actual plan assets by category [Abstract]
Actual allocation of plan assets	2.80%	2.10%
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 | Equity markets
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	60.00%	60.00%
Actual allocation of plan assets	58.50%	59.40%
Spire Alabama | Pension Plans | Debt securities
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	29.00%	29.00%
Actual allocation of plan assets	29.80%	28.60%
Spire Alabama | Pension Plans | Other
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	11.00%	11.00%
Actual allocation of plan assets	11.70%	12.00%
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	60.30%	60.00%
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	39.70%	40.00%