Segment and Geographic Data - Summarized Financial Information (Details) - USD ($) $ in Millions	2 Months Ended	3 Months Ended			8 Months Ended	9 Months Ended
	Jun. 08, 2016	Jun. 30, 2017	Jun. 30, 2016		Jun. 08, 2016	Jun. 30, 2017	Jun. 30, 2016		Sep. 30, 2016
Segment assets
Assets		$ 2,236.9				$ 2,236.9			$ 2,078.9
Period acquiree is included in operating activities		22 days				22 days
Operating Segments
Segment assets
Assets		$ 1,640.5				$ 1,640.5			1,450.5
Unallocated
Segment assets
Assets		596.4				596.4			628.4
Chemicals | Operating Segments
Segment assets
Assets		775.0				775.0			656.8
Plastics | Operating Segments
Segment assets
Assets		780.5				780.5			708.7
Other | Operating Segments
Segment assets
Assets		85.0				85.0			$ 85.0
Successor
Summarized financial information
Sales and operating revenues		942.7	$ 214.3	[1]		2,655.2	$ 214.3	[1]
Gross profit		102.7	18.8	[1]		289.3	18.8	[1]
Selling, general & administrative expenses		79.4	19.1	[1]		233.9	19.2	[1]
Transaction related costs		0.2	15.9	[1]		1.3	18.0	[1]
Change in fair value of contingent consideration obligations		(0.8)	(2.3)	[1]		19.8	(2.3)	[1]
Operating income (loss)		23.9	(13.9)	[1]		34.3	(16.1)	[1]
Other income		5.7	0.0	[1]		8.3	0.0	[1]
Interest income		0.0	0.3	[1]		0.2	0.9	[1]
Interest expense		(13.5)	(3.2)	[1]		(38.0)	(3.2)	[1]
Net income (loss) from continuing operations before income taxes		16.1	(16.8)	[1]		4.8	(18.4)	[1]
Successor | Chemicals
Summarized financial information
Sales and operating revenues		443.9	94.2			1,211.3	94.2
Gross profit		54.3	9.6			147.6	9.6
Successor | Plastics
Summarized financial information
Sales and operating revenues		466.2	109.9			1,350.4	109.9
Gross profit		43.1	6.7			124.9	6.7
Successor | Other
Summarized financial information
Sales and operating revenues		32.6	10.2			93.5	10.2
Gross profit		$ 5.3	$ 2.5			$ 16.8	$ 2.5
Predecessor
Summarized financial information
Sales and operating revenues	$ 650.2				$ 2,340.1
Gross profit	75.4				271.9
Selling, general & administrative expenses	57.5				208.9
Transaction related costs	26.1				33.4
Change in fair value of contingent consideration obligations	0.0				0.0
Operating income (loss)	(8.2)				29.6
Other income	0.3				2.9
Interest income	0.0				0.1
Interest expense	(11.2)				(42.3)
Net income (loss) from continuing operations before income taxes	(19.1)				(9.7)
Predecessor | Chemicals
Summarized financial information
Sales and operating revenues	298.7				1,066.4
Gross profit	38.8				136.2
Predecessor | Plastics
Summarized financial information
Sales and operating revenues	329.8				1,192.2
Gross profit	32.2				117.6
Predecessor | Other
Summarized financial information
Sales and operating revenues	21.7				81.5
Gross profit	$ 4.4				$ 18.1

[1] The three and nine months ended June 30, 2016 include 22 days of the acquired business’ operating activities as a result of the consummation of the Business Combination on June 9, 2016.