Business Separation, Restructuring, and Cost Reduction Actions (Narrative) (Details) $ in Millions	3 Months Ended								6 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($) position
Restructuring Cost and Reserve [Line Items]
Restructuring charge net of business separation costs										$ (2.0)	$ (4.7)	$ 0.7
Disposal Group, Including Discontinued Operation, Assets												$ 17.9
Number of positions eliminated | position												90
Restructuring charge	$ 3.9	$ 6.7	$ 8.2	$ 1.8	$ 10.2	$ 6.0	$ 6.1	$ 3.2		20.6	25.5	$ 0.9
Onsite Ammonia Plant
Restructuring Cost and Reserve [Line Items]
Gain from sale of plants									$ 1.4
Severance
Restructuring Cost and Reserve [Line Items]
Restructuring charge net of business separation costs										(2.0)	(4.7)	(2.5)
Asset actions
Restructuring Cost and Reserve [Line Items]
Restructuring charge net of business separation costs										$ 0.0	$ 0.0	3.2
Separation
Restructuring Cost and Reserve [Line Items]
Costs incurred from separation												$ 1.6