Segment and Geographic Data - Summarized Financial Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2017
Summarized financial information
Sales and operating revenues	$ 1,046.4	$ 942.7	$ 3,017.0	$ 2,655.2
Gross profit	120.2	102.7	342.8	289.3
Selling, general & administrative expenses	88.8	79.4	259.5	233.9
Transaction related costs	0.0	0.2	0.1	1.3
Change in fair value of contingent consideration obligations	(8.7)	(0.8)	(14.7)	19.8
Operating income	40.1	23.9	97.9	34.3
Other income, net	0.4	5.7	0.9	8.3
Interest income	0.2	0.0	0.4	0.2
Interest expense	(13.5)	(13.5)	(39.2)	(38.0)
Net income before income taxes	27.2	16.1	60.0	4.8
Segment assets
Assets	2,278.7		2,278.7		$ 2,253.5
Operating Segments
Segment assets
Assets	1,723.4		1,723.4		1,647.3
Unallocated
Segment assets
Assets	555.3		555.3		606.2
Chemicals
Summarized financial information
Sales and operating revenues	494.9	443.9	1,414.6	1,211.3
Gross profit	64.9	54.3	185.3	147.6
Chemicals | Operating Segments
Segment assets
Assets	845.1		845.1		793.6
Plastics
Summarized financial information
Sales and operating revenues	512.6	466.2	1,491.7	1,350.4
Gross profit	49.6	43.1	138.5	124.9
Plastics | Operating Segments
Segment assets
Assets	782.8		782.8		762.7
Other
Summarized financial information
Sales and operating revenues	38.9	32.6	110.7	93.5
Gross profit	5.7	$ 5.3	19.0	$ 16.8
Other | Operating Segments
Segment assets
Assets	$ 95.5		$ 95.5		$ 91.0