PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Amounts Recognized in Consolidated Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent liabilities	$ (303.7)	$ (253.4)
Pension Plans
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Current liabilities	(0.6)	(0.5)
Noncurrent liabilities	(253.6)	(202.9)
Total	(254.2)	(203.4)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	179.4	143.9
Prior service costs	8.2	3.5
Subtotal	187.6	147.4
Adjustments for amounts included in regulatory assets	(184.8)	(144.9)
Total	2.8	2.5
Postretirement Plans
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Current assets	0.3	0.0
Noncurrent assets	37.4	35.5
Current liabilities	(0.4)	(0.3)
Noncurrent liabilities	(50.1)	(51.1)
Total	(12.8)	(15.9)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	38.0	40.8
Prior service costs	(5.2)	(3.1)
Subtotal	32.8	37.7
Adjustments for amounts included in regulatory assets	(32.8)	(37.7)
Total	0.0	0.0
Laclede Gas
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent liabilities	(211.8)	(207.8)
Laclede Gas | Pension Plans
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Current liabilities	(0.6)	(0.5)
Noncurrent liabilities	(163.7)	(157.2)
Total	(164.3)	(157.7)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	135.5	121.9
Prior service costs	8.2	3.5
Subtotal	143.7	125.4
Adjustments for amounts included in regulatory assets	(140.9)	(122.9)
Total	2.8	2.5
Laclede Gas | Postretirement Plans
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Current assets	0.3	0.0
Noncurrent assets	0.0	3.1
Current liabilities	(0.4)	(0.3)
Noncurrent liabilities	(48.1)	(51.1)
Total	(48.2)	(48.3)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	43.4	45.8
Prior service costs	(3.4)	(3.1)
Subtotal	40.0	42.7
Adjustments for amounts included in regulatory assets	(40.0)	(42.7)
Total	0.0	0.0
Alagasco
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent liabilities	(74.3)	(45.6)
Alagasco | Pension Plans
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Current liabilities	0.0	0.0
Noncurrent liabilities	(74.3)	(45.7)
Total	(74.3)	(45.7)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	43.9	22.0
Prior service costs	0.0	0.0
Subtotal	43.9	22.0
Adjustments for amounts included in regulatory assets	(43.9)	(22.0)
Total	0.0	0.0
Alagasco | Postretirement Plans
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Current assets	0.0	0.0
Noncurrent assets	37.4	32.4
Current liabilities	0.0	0.0
Noncurrent liabilities	0.0	0.0
Total	37.4	32.4
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	(5.4)	(5.0)
Prior service costs	(1.8)	0.0
Subtotal	(7.2)	(5.0)
Adjustments for amounts included in regulatory assets	7.2	5.0
Total	$ 0.0	$ 0.0