Supplementary Financial Statement Information - Facility Idling (Details9) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2019	Dec. 31, 2017	Dec. 31, 2015	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restructuring Cost and Reserve [Line Items]
Asset impairment charge						$ 0.0	$ 75.6	$ 0.0
Ashland Works Hot End [Member]
Restructuring Cost and Reserve [Line Items]
Charge For Facility Idling			$ 28.1
Costs For Temporarily Idled Operations						20.0	21.2	22.1
Asset impairment charge		$ 75.6					75.6
Restructuring Reserve [Roll Forward]
Balance at beginning of year	$ 0.0				$ 0.0	1.5	6.2	22.2
Payments for restructuring						(1.5)	(5.3)	(20.1)
Additions to the reserve						0.0	0.6	4.1
Balance at end of year		$ 1.5	22.2			$ 0.0	$ 1.5	$ 6.2
Ashland Works Hot End [Member] | Supplemental Unemployment Benefits [Member]
Restructuring Cost and Reserve [Line Items]
Charge For Facility Idling			22.2
Ashland Works Hot End [Member] | Equipment Idling And Other Costs [Member]
Restructuring Cost and Reserve [Line Items]
Charge For Facility Idling			$ 5.9
Scenario, Forecast [Member] | Ashland Works Hot End [Member]
Restructuring Cost and Reserve [Line Items]
Charge For Facility Closure	80.0
Cash Payments Related To Facility Closure				$ 30.0	15.0
Cash Charge For Facility Closure					14.0
Scenario, Forecast [Member] | Ashland Works Hot End [Member] | Take-Or-Pay Supply Agreements [Member]
Restructuring Cost and Reserve [Line Items]
Charge For Facility Closure	20.0
Scenario, Forecast [Member] | Ashland Works Hot End [Member] | Supplemental Unemployment Benefits [Member]
Restructuring Cost and Reserve [Line Items]
Charge For Facility Closure	30.0
Scenario, Forecast [Member] | Ashland Works Hot End [Member] | Multiemployer Plan Withdrawal [Member]
Restructuring Cost and Reserve [Line Items]
Charge For Facility Closure	25.0
Scenario, Forecast [Member] | Ashland Works Hot End [Member] | Equipment Idling And Other Costs [Member]
Restructuring Cost and Reserve [Line Items]
Charge For Facility Closure	$ 5.0
Scenario, Forecast [Member] | Ashland Works Hot End [Member] | Facility Closing [Member]
Restructuring Cost and Reserve [Line Items]
Cash Charge For Facility Closure					10.0
Scenario, Forecast [Member] | Ashland Works Hot End [Member] | Accelerated Depreciation Due To Facility Closure [Member]
Restructuring Cost and Reserve [Line Items]
Cash Charge For Facility Closure					$ 4.0