PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Benefit Obligation Rollforward (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year		$ 652.3	$ 692.4
Service cost		15.3	17.3	$ 10.2
Interest cost		28.0	29.5	24.5
Actuarial loss (gain)		85.8	(12.8)
Plan amendments		5.1	0.0
EnergySouth acquisition		60.4	0.0
Settlement loss		1.1	16.5
Special termination benefits		1.6	0.0
Settlement benefits paid		(16.6)	(71.1)	(22.1)
Regular benefits paid		(38.3)	(19.5)
Benefit obligation, end of year	$ 692.4	794.7	652.3	692.4
Accumulated benefit obligation, end of year		724.5	591.4
Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year		239.2	258.5
Service cost		10.9	12.8	11.3
Interest cost		10.2	11.2	8.9
Actuarial loss (gain)		7.1	(23.7)
Plan amendments		(1.8)	(4.9)
EnergySouth acquisition		5.9	0.0
Special termination benefits		2.6	0.0
Retiree drug subsidy program		0.2	0.4
Regular benefits paid		(15.1)	(15.1)
Benefit obligation, end of year	258.5	259.2	239.2	258.5
Laclede Gas | Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year		497.6	543.6
Service cost		10.0	11.5	9.7
Interest cost		21.7	23.3	24.0
Actuarial loss (gain)		59.2	(20.7)
Plan amendments		5.1	0.0
EnergySouth acquisition		0.0	0.0
Settlement loss		0.0	14.5
Special termination benefits		1.6
Settlement benefits paid		0.0	(58.2)
Regular benefits paid		(35.2)	(16.4)
Benefit obligation, end of year	543.6	560.0	497.6	543.6
Accumulated benefit obligation, end of year		517.7	456.9
Laclede Gas | Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year		191.9	197.9
Service cost		10.6	12.3	11.2
Interest cost		8.1	8.6	8.7
Actuarial loss (gain)		6.7	(10.9)
Plan amendments		0.0	(4.9)
EnergySouth acquisition		0.0	0.0
Special termination benefits		2.6	0.0
Retiree drug subsidy program		0.0	0.0
Regular benefits paid		(12.0)	(11.1)
Benefit obligation, end of year	197.9	207.9	191.9	197.9
Alagasco | Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year		154.7	148.8
Service cost	5.1	5.3	5.8
Interest cost	4.1	6.3	6.2
Actuarial loss (gain)		26.6	7.9
Plan amendments		0.0	0.0
EnergySouth acquisition		0.0	0.0
Settlement loss		1.1	2.0
Special termination benefits			0.0
Settlement benefits paid		(16.6)	(12.9)
Regular benefits paid		(3.1)	(3.1)
Benefit obligation, end of year	148.8	174.3	154.7	148.8
Accumulated benefit obligation, end of year		149.8	134.5
Alagasco | Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year		47.3	60.6
Service cost	0.4	0.3	0.5
Interest cost	1.9	2.1	2.6
Actuarial loss (gain)		0.4	(12.8)
Plan amendments		(1.8)	0.0
EnergySouth acquisition		0.0	0.0
Special termination benefits		0.0	0.0
Retiree drug subsidy program		0.2	0.4
Regular benefits paid		(3.1)	(4.0)
Benefit obligation, end of year	$ 60.6	$ 45.4	$ 47.3	$ 60.6