Supplementary Financial Statement Information - Property, Plant and Equipment (Details 5) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Proceeds from AFSG Holdings, Inc. distribution		$ 14.0	$ 0.0	$ 0.0
Charge for facility idling		28.1	0.0	0.0
Property, plant and equipment	$ 6,466.0	6,466.0	6,388.4
Less accumulated depreciation	(4,379.5)	(4,379.5)	(4,175.2)
Property, plant and equipment, net	2,086.5	2,086.5	2,213.2
Capitalized interest on capital projects		2.1	2.7	$ 2.7
Asset retirement obligations	6.6	6.6	6.0
Land, land improvements and leaseholds [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	263.0	263.0	260.7
Buildings [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	465.9	465.9	466.7
Machinery and equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	5,628.2	5,628.2	5,571.0
Construction in progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	108.9	108.9	$ 90.0
Ashland Works Hot End [Member]
Property, Plant and Equipment [Line Items]
Charge for facility idling	28.1
Ashland Works Hot End [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Restructuring and Related Cost, Expected Cost	2.0	2.0
Ashland Works Hot End [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Restructuring and Related Cost, Expected Cost	3.0	$ 3.0
Supplemental Unemployment Benefits [Member] | Ashland Works Hot End [Member]
Property, Plant and Equipment [Line Items]
Charge for facility idling	22.2
Equipment Idling And Other Costs [Member] | Ashland Works Hot End [Member]
Property, Plant and Equipment [Line Items]
Charge for facility idling	$ 5.9