Supplementary Financial Statement Information Facility Closure (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019
Restructuring Cost and Reserve [Line Items]
Accelerated Depreciation Expense	$ 47.4	$ 55.0	$ 147.4	$ 164.8
Ashland Works [Member]
Restructuring Cost and Reserve [Line Items]
Ashland Works closure			77.4
Ashland Works Hot End [Member]
Restructuring Cost and Reserve [Line Items]
Costs For Temporarily Idled Operations	$ 2.1	$ 4.7	7.0	$ 15.1
Take-Or-Pay Supply Agreements [Member] | Ashland Works [Member]
Restructuring Cost and Reserve [Line Items]
Ashland Works closure			18.5
Supplemental Unemployment Benefits [Member] | Ashland Works [Member]
Restructuring Cost and Reserve [Line Items]
Ashland Works closure			20.2
Withdrawal from Multiemployer Defined Benefit Plan [Member] | Ashland Works [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Charges Change In Multiemployer Plans Withdrawal Obligation			18.0
Equipment Idling And Other Costs [Member] | Ashland Works [Member]
Restructuring Cost and Reserve [Line Items]
Ashland Works closure			7.4
Forecast [Member] | Ashland Works [Member]
Restructuring Cost and Reserve [Line Items]
Cash Payments Related To Facility Closure					$ 30.0	$ 15.0
Charges For Facility Closure						14.0
Forecast [Member] | Facility Closing [Member] | Ashland Works Hot End [Member]
Restructuring Cost and Reserve [Line Items]
Costs For Temporarily Idled Operations						10.0
Forecast [Member] | Accelerated Depreciation Due To Facility Closure [Member] | Ashland Works [Member]
Restructuring Cost and Reserve [Line Items]
Accelerated Depreciation Expense						$ 4.0
Pension and OPEB Expense (Income) [Member] | Other Pension, Postretirement and Supplemental Plans [Member] | Ashland Works [Member]
Restructuring Cost and Reserve [Line Items]
Ashland Works closure			$ 13.3