Segment and Geographic Data - Summarized Financial Information (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2017
Summarized financial information
Sales and operating revenues	$ 1,041.0	$ 917.7	$ 1,970.6	$ 1,712.5
Gross profit	115.7	102.2	222.6	186.6
Selling, general & administrative expenses	85.9	80.0	170.7	154.5
Transaction related costs	0.0	0.3	0.1	1.1
Change in fair value of contingent consideration obligations	12.6	10.0	(6.0)	20.6
Operating income	17.2	11.9	57.8	10.4
Other income	0.4	0.2	0.5	2.6
Interest income	0.1	0.1	0.2	0.2
Interest expense	(12.7)	(12.5)	(25.7)	(24.5)
Net income (loss) before income taxes	5.0	(0.3)	32.8	(11.3)
Segment assets
Assets	2,317.9		2,317.9		$ 2,253.5
Operating Segments
Segment assets
Assets	1,748.5		1,748.5		1,647.3
Unallocated
Segment assets
Assets	569.4		569.4		606.2
Chemicals
Summarized financial information
Sales and operating revenues	487.8	415.0	919.7	767.4
Gross profit	62.0	50.6	120.4	93.3
Chemicals \| Operating Segments
Segment assets
Assets	840.3		840.3		793.6
Plastics
Summarized financial information
Sales and operating revenues	516.9	471.7	979.1	884.2
Gross profit	47.0	45.8	88.9	81.8
Plastics \| Operating Segments
Segment assets
Assets	814.1		814.1		762.7
Other
Summarized financial information
Sales and operating revenues	36.3	31.0	71.8	60.9
Gross profit	6.7	$ 5.8	13.3	$ 11.5
Other \| Operating Segments
Segment assets
Assets	$ 94.1		$ 94.1		$ 91.0

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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