Supplementary Financial Statement Information - Facility Idling (Details9) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restructuring Cost and Reserve [Line Items]
Depreciation		$ 192.6	$ 220.2	$ 226.0
Asset impairment charge		0.0	0.0	75.6
Ashland Works [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Charges	$ 77.4	69.3
Ashland Works [Member] | Take-Or-Pay Supply Agreements [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Charges		18.5
Ashland Works [Member] | Supplemental Unemployment Benefits [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Charges		20.1
Restructuring Reserve		19.1
Restructuring Reserve [Roll Forward]
Payments for restructuring		(1.0)
Ashland Works [Member] | Multiemployer Plan Withdrawal [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Charges		10.0
Ashland Works [Member] | Equipment Idling And Other Costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Charges		7.4
Ashland Works [Member] | Accelerated Depreciation Due To Facility Closure [Member]
Restructuring Cost and Reserve [Line Items]
Depreciation		4.0
Ashland Works Hot End [Member]
Restructuring Cost and Reserve [Line Items]
Costs For Idled Operations		12.6	20.0	21.2
Asset impairment charge				75.6
Ashland Works Hot End [Member] | Supplemental Unemployment Benefits [Member]
Restructuring Reserve [Roll Forward]
Balance at beginning of year	$ 0.0	0.0	1.5	6.2
Payments for restructuring			(1.5)	(5.3)
Additions to the reserve			0.0	0.6
Balance at end of year			$ 0.0	$ 1.5
Pension and OPEB Expense (Income) [Member] | Ashland Works [Member] | Other Pension, Postretirement and Supplemental Plans [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Charges		$ 13.3