REGULATORY MATTERS - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Regulatory Liabilities [Line Items]
Total Current Regulatory Liabilities	$ 22.6	$ 30.6
Total Noncurrent Regulatory Liabilities	157.2	130.7
Total Regulatory Liabilities	179.8	161.3
RSE adjustment
Regulatory Liabilities [Line Items]
Total Current Regulatory Liabilities	1.4	7.5
Pension and postretirement costs
Regulatory Liabilities [Line Items]
Total Current Regulatory Liabilities	8.2	9.3
Total Noncurrent Regulatory Liabilities	4.1	9.4
Unbilled service margin
Regulatory Liabilities [Line Items]
Total Current Regulatory Liabilities	0.0	5.9
Unamortized purchased gas adjustments
Regulatory Liabilities [Line Items]
Total Current Regulatory Liabilities	1.0	1.7
Other
Regulatory Liabilities [Line Items]
Total Current Regulatory Liabilities	12.0	6.2
Total Noncurrent Regulatory Liabilities	37.1	17.6
Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Total Noncurrent Regulatory Liabilities	32.2	28.9
Accrued cost of removal
Regulatory Liabilities [Line Items]
Total Noncurrent Regulatory Liabilities	83.8	74.8
Spire Missouri
Regulatory Liabilities [Line Items]
Total Current Regulatory Liabilities	2.7	1.3
Total Noncurrent Regulatory Liabilities	81.2	67.3
Total Regulatory Liabilities	83.9	68.6
Spire Missouri | RSE adjustment
Regulatory Liabilities [Line Items]
Total Current Regulatory Liabilities	0.0	0.0
Spire Missouri | Pension and postretirement costs
Regulatory Liabilities [Line Items]
Total Current Regulatory Liabilities	0.0	0.0
Total Noncurrent Regulatory Liabilities	0.0	0.0
Spire Missouri | Unbilled service margin
Regulatory Liabilities [Line Items]
Total Current Regulatory Liabilities	0.0	0.0
Spire Missouri | Unamortized purchased gas adjustments
Regulatory Liabilities [Line Items]
Total Current Regulatory Liabilities	0.0	0.0
Spire Missouri | Other
Regulatory Liabilities [Line Items]
Total Current Regulatory Liabilities	2.7	1.3
Total Noncurrent Regulatory Liabilities	26.7	12.2
Spire Missouri | Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Total Noncurrent Regulatory Liabilities	0.0	0.0
Spire Missouri | Accrued cost of removal
Regulatory Liabilities [Line Items]
Total Noncurrent Regulatory Liabilities	54.5	55.1
Spire Alabama
Regulatory Liabilities [Line Items]
Total Current Regulatory Liabilities	12.0	22.7
Total Noncurrent Regulatory Liabilities	39.6	41.7
Total Regulatory Liabilities	51.6	64.4
Spire Alabama | RSE adjustment
Regulatory Liabilities [Line Items]
Total Current Regulatory Liabilities	1.4	5.0
Spire Alabama | Pension and postretirement costs
Regulatory Liabilities [Line Items]
Total Current Regulatory Liabilities	8.2	9.3
Total Noncurrent Regulatory Liabilities	4.1	9.4
Spire Alabama | Unbilled service margin
Regulatory Liabilities [Line Items]
Total Current Regulatory Liabilities	0.0	5.9
Spire Alabama | Unamortized purchased gas adjustments
Regulatory Liabilities [Line Items]
Total Current Regulatory Liabilities	0.0	0.0
Spire Alabama | Other
Regulatory Liabilities [Line Items]
Total Current Regulatory Liabilities	2.4	2.5
Total Noncurrent Regulatory Liabilities	3.3	3.4
Spire Alabama | Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Total Noncurrent Regulatory Liabilities	32.2	28.9
Spire Alabama | Accrued cost of removal
Regulatory Liabilities [Line Items]
Total Noncurrent Regulatory Liabilities	$ 0.0	$ 0.0