Information by Operating Segment - Schedule of Operating Segment Information (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2018
Operating Segment Information [Abstract]
Total Operating Revenues	$ 602.0	$ 561.8
Gas Utility
Natural and propane gas	251.7	240.8
Other operation and maintenance expenses	102.5	99.0
Depreciation and amortization	43.7	40.3
Taxes, other than income taxes	39.2	36.7
Total Gas Utility Operating Expenses	437.1	416.8
Gas Marketing and other	59.8	41.0
Total Operating Expenses	496.9	457.8
Operating Income	105.1	104.0
Net Economic Earnings (Loss)	65.9	57.9
Total Assets	7,232.2	6,701.1	$ 6,843.6
Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues	(4.4)	(2.6)
Gas Utility
Natural and propane gas	(40.1)	(22.6)
Other operation and maintenance expenses	(2.4)	(1.9)
Depreciation and amortization	0.0	0.0
Taxes, other than income taxes	0.0	0.0
Total Gas Utility Operating Expenses	(42.5)	(24.5)
Gas Marketing and other	38.1	21.9
Total Operating Expenses	(4.4)	(2.6)
Operating Income	0.0	0.0
Net Economic Earnings (Loss)	0.0	0.0
Total Assets	(1,563.8)	(1,571.8)	(1,566.4)
Gas Utility
Operating Segment Information [Abstract]
Total Operating Revenues	573.8	541.9
Gas Utility | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues	1.4	0.1
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues	575.2	542.0
Gas Utility
Natural and propane gas	291.8	263.4
Other operation and maintenance expenses	104.9	100.9
Depreciation and amortization	43.7	40.3
Taxes, other than income taxes	39.2	36.7
Total Gas Utility Operating Expenses	479.6	441.3
Gas Marketing and other	0.0	0.0
Total Operating Expenses	479.6	441.3
Operating Income	95.6	100.7
Net Economic Earnings (Loss)	66.4	59.5
Total Assets	5,843.7	5,611.7	5,606.7
Gas Marketing
Operating Segment Information [Abstract]
Total Operating Revenues	25.8	19.6
Gas Marketing | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues	0.0	0.0
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues	25.8	19.6
Gas Utility
Natural and propane gas	0.0	0.0
Other operation and maintenance expenses	0.0	0.0
Depreciation and amortization	0.0	0.0
Taxes, other than income taxes	0.0	0.0
Total Gas Utility Operating Expenses	0.0	0.0
Gas Marketing and other	13.3	14.6
Total Operating Expenses	13.3	14.6
Operating Income	12.5	5.0
Net Economic Earnings (Loss)	8.3	3.6
Total Assets	416.7	233.5	295.3
Corporate, Non-Segment
Operating Segment Information [Abstract]
Total Operating Revenues	2.4	0.3
Corporate, Non-Segment | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues	3.0	2.5
Corporate, Non-Segment | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues	5.4	2.8
Gas Utility
Natural and propane gas	0.0	0.0
Other operation and maintenance expenses	0.0	0.0
Depreciation and amortization	0.0	0.0
Taxes, other than income taxes	0.0	0.0
Total Gas Utility Operating Expenses	0.0	0.0
Gas Marketing and other	8.4	4.5
Total Operating Expenses	8.4	4.5
Operating Income	(3.0)	(1.7)
Net Economic Earnings (Loss)	(8.8)	(5.2)
Total Assets	$ 2,535.6	$ 2,427.7	$ 2,508.0