Pension and Other Postretirement Benefits (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 31, 2019	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Payment for Pension Benefits				$ 19.5	$ 15.2
Net periodic benefit cost (income) [Abstract]
Net periodic benefit (income) expense				2.5	(16.2)
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Estimated pension plan contributions in current year		$ 43.5		43.5
Payment for Pension Benefits				19.5
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year		50.0		50.0
Defined Benefit Plans, Estimated Future Employer Contributions, Year Two		50.0		50.0
Net periodic benefit cost (income) [Abstract]
Service cost		0.7	$ 0.7	1.2	1.5
Interest cost		22.2	23.8	44.5	47.5
Expected return on assets		(28.0)	(38.8)	(56.0)	(77.6)
Amortization of prior service cost (credit)		0.8	0.9	1.5	1.9
Amortization of (gain) loss		(1.8)	4.0	(3.5)	7.9
Termination benefits—Ashland Works		0.0	0.0	9.7	0.0
Net periodic benefit (income) expense		(6.1)	(9.4)	(2.6)	(18.8)
Other Postretirement Benefits [Member]
Net periodic benefit cost (income) [Abstract]
Service cost		0.8	1.2	1.7	2.3
Interest cost		4.1	3.9	8.2	7.8
Amortization of prior service cost (credit)		(3.3)	(3.4)	(6.6)	(6.8)
Amortization of (gain) loss		(0.9)	(0.4)	(1.8)	(0.7)
Termination benefits—Ashland Works		0.0	0.0	3.6	0.0
Net periodic benefit (income) expense		$ 0.7	$ 1.3	$ 5.1	$ 2.6
Subsequent Event [Member] | Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Payment for Pension Benefits	$ 8.7