Plant and Equipment, Net (Narrative) (Details) $ in Millions	1 Months Ended	3 Months Ended								12 Months Ended
	Jan. 31, 2016 USD ($) plant	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)
Property, Plant and Equipment [Abstract]
Depreciation expense										$ 38.4	$ 39.4	$ 48.1
Long Lived Assets Held-for-sale [Line Items]
Income tax provision		$ 11.6	$ 14.4	$ 11.5	$ 15.3	$ 15.6	$ 17.6	$ 12.2	$ 13.4	$ 52.8	$ 58.8	$ 31.7
Onsite Ammonia Plant
Long Lived Assets Held-for-sale [Line Items]
Number of plants sold | plant	2
Proceeds from sale of plants	$ 17.1							17.1
Gain from sale of plants								1.4
Income tax provision								$ 1.5