Supplementary Financial Statement Information Facility Closure (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2020	Dec. 31, 2019
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	$ 2.4		$ 5.0	$ 4.9	$ 10.4
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	5.0	15.2
Withdrawal from Multiemployer Defined Benefit Plan [Member] | Ashland Works [Member]
Restructuring Cost and Reserve [Line Items]
Ashland Works closure		23.0
Restructuring Charges Change In Multiemployer Plans Withdrawal Obligation	$ (5.0)
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
Cash Charge For Facility Closure							14.0
Forecast [Member] | Facility Closing [Member] | Ashland Works [Member]
Restructuring Cost and Reserve [Line Items]
Cash Charge For Facility Closure							10.0
Forecast [Member] | Accelerated Depreciation Due To Facility Closure [Member] | Ashland Works [Member]
Restructuring Cost and Reserve [Line Items]
Cash Charge For Facility Closure							$ 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