Plant and Equipment, Net (Narrative) (Details) $ in Millions	1 Months Ended	3 Months Ended								6 Months Ended	12 Months Ended
	Jan. 31, 2016 USD ($) plant	Sep. 30, 2018 USD ($)	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 ($)
Property, Plant and Equipment [Abstract]
Depreciation expense											$ 43.6	$ 38.4	$ 39.4
Long Lived Assets Held-for-sale [Line Items]
Income tax provision		$ 30.1	$ 19.6	$ 14.2	$ 55.0	$ 11.9	$ 14.4	$ 11.5	$ 15.2		$ 118.9	$ 53.0	$ 58.8
Onsite Ammonia Plant
Long Lived Assets Held-for-sale [Line Items]
Number of plants sold | plant	2
Proceeds from sale of plants	$ 17.1
Gain from sale of plants										$ 1.4
Income tax provision										$ 1.5