Information by Operating Segment - Schedule of Operating Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018
Operating Segment Information [Abstract]
Total Operating Revenues	$ 803.5	$ 813.4	$ 1,405.5	$ 1,375.2
Gas Utility
Natural and propane gas	337.4	383.7	589.1	624.5
Other operation and maintenance expenses	109.5	135.3	212.0	234.3
Depreciation and amortization	44.4	41.1	88.1	81.4
Taxes, other than income taxes	57.4	58.0	96.6	94.7
Total Gas Utility Operating Expenses	548.7	618.1	985.8	1,034.9
Gas Marketing and other	45.3	45.2	105.1	86.2
Total Operating Expenses	594.0	663.3	1,090.9	1,121.1
Operating Income	209.5	150.1	314.6	254.1
Net Economic Earnings (Loss)	147.9	137.2	213.8	195.1
Total Assets	7,273.6	6,586.8	7,273.6	6,586.8	$ 6,843.6
Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues	(3.1)	(3.1)	(7.5)	(5.7)
Gas Utility
Natural and propane gas	(29.3)	(19.5)	(69.4)	(42.1)
Other operation and maintenance expenses	(2.5)	(2.2)	(4.9)	(4.1)
Depreciation and amortization	0.0	0.0
Taxes, other than income taxes	0.0	0.0
Total Gas Utility Operating Expenses	(31.8)	(21.7)	(74.3)	(46.2)
Gas Marketing and other	28.7	18.6	66.8	40.5
Total Operating Expenses	(3.1)	(3.1)	(7.5)	(5.7)
Operating Income	0.0	0.0
Net Economic Earnings (Loss)	0.0	0.0
Total Assets	(1,340.0)	(1,318.0)	(1,340.0)	(1,318.0)	(1,566.4)
Gas Utility
Operating Segment Information [Abstract]
Total Operating Revenues	776.7	790.6	1,350.5	1,332.5
Gas Utility | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues	0.1	0.2	1.5	0.3
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues	776.8	790.8	1,352.0	1,332.8
Gas Utility
Natural and propane gas	366.7	403.2	658.5	666.6
Other operation and maintenance expenses	112.0	137.5	216.9	238.4
Depreciation and amortization	44.4	41.1	88.1	81.4
Taxes, other than income taxes	57.4	58.0	96.6	94.7
Total Gas Utility Operating Expenses	580.5	639.8	1,060.1	1,081.1
Gas Marketing and other	0.0	0.0
Total Operating Expenses	580.5	639.8	1,060.1	1,081.1
Operating Income	196.3	151.0	291.9	251.7
Net Economic Earnings (Loss)	146.7	131.7	213.1	191.2
Total Assets	5,850.1	5,484.3	5,850.1	5,484.3	5,606.7
Gas Marketing
Operating Segment Information [Abstract]
Total Operating Revenues	25.4	21.3	51.2	40.9
Gas Marketing | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues	0.1	0.0	0.1
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues	25.5	21.3	51.3	40.9
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	8.7	20.2	22.0	34.8
Total Operating Expenses	8.7	20.2	22.0	34.8
Operating Income	16.8	1.1	29.3	6.1
Net Economic Earnings (Loss)	6.2	10.2	14.5	13.8
Total Assets	366.7	208.6	366.7	208.6	295.3
Corporate, Non-Segment
Operating Segment Information [Abstract]
Total Operating Revenues	1.4	1.5	3.8	1.8
Corporate, Non-Segment | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues	2.9	2.9	5.9	5.4
Corporate, Non-Segment | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues	4.3	4.4	9.7	7.2
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	7.9	6.4	16.3	10.9
Total Operating Expenses	7.9	6.4	16.3	10.9
Operating Income	(3.6)	(2.0)	(6.6)	(3.7)
Net Economic Earnings (Loss)	(5.0)	(4.7)	(13.8)	(9.9)
Total Assets	$ 2,396.8	$ 2,211.9	$ 2,396.8	$ 2,211.9	$ 2,508.0