PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Amounts Recognized in Consolidated Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2015	Sep. 30, 2014
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent liabilities	$ (253.4)	$ (244.9)
Pension Plans
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Current liabilities	(0.5)	(0.5)
Noncurrent liabilities	(202.9)	(185.4)
Total	(203.4)	(185.9)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	143.9	7.7
Prior service costs	3.5	0.5
Subtotal	147.4	8.2
Adjustments for amounts included in Regulatory Assets	(144.9)	(7.9)
Total	2.5	0.3
Postretirement Plans
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent assets	35.5	25.0
Current liabilities	(0.3)	(0.3)
Noncurrent liabilities	(51.1)	(60.7)
Total	(15.9)	(36.0)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	40.8	54.4
Prior service costs	(3.1)	2.5
Subtotal	37.7	56.9
Adjustments for amounts included in Regulatory Assets	(37.7)	(56.9)
Total	0.0	0.0
Laclede Gas
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent liabilities	(207.8)	(215.3)
Laclede Gas | Pension Plans
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Current liabilities	(0.5)	(0.5)
Noncurrent liabilities	(157.2)	(155.7)
Total	(157.7)	(156.2)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	121.9	7.7
Prior service costs	3.5	0.5
Subtotal	125.4	8.2
Adjustments for amounts included in Regulatory Assets	(122.9)	(7.9)
Total	2.5	0.3
Laclede Gas | Postretirement Plans
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent assets	3.1	0.3
Current liabilities	(0.3)	(0.3)
Noncurrent liabilities	(51.1)	(60.7)
Total	(48.3)	(60.7)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	45.8	53.3
Prior service costs	(3.1)	2.5
Subtotal	42.7	55.8
Adjustments for amounts included in Regulatory Assets	(42.7)	(55.8)
Total	0.0	0.0
Alagasco
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent liabilities	(45.6)	(29.6)
Alagasco | Pension Plans
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Current liabilities	0.0	0.0
Noncurrent liabilities	(45.7)	(29.6)
Total	(45.7)	(29.6)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	22.0	1.5
Prior service costs	0.0	0.0
Subtotal	22.0	1.5
Adjustments for amounts included in Regulatory Assets	(22.0)	(1.5)
Total	0.0	0.0
Alagasco | Postretirement Plans
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent assets	32.4	24.7
Current liabilities	0.0	0.0
Noncurrent liabilities	0.0	0.0
Total	32.4	24.7
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	(5.0)	1.1
Prior service costs	0.0	0.0
Subtotal	(5.0)	1.1
Adjustments for amounts included in Regulatory Assets	5.0	(1.1)
Total	$ 0.0	$ 0.0