Pension and Other Postretirement Benefits - Benefit Costs (Details 5) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Recognized net actuarial loss (gain):
Pension and OPEB (income) expense			$ 17.8	$ (11.6)	$ (64.4)
Expense for defined contribution plans			29.0	31.1	24.9
Pension Plans, Defined Benefit [Member]
Components of pension and OPEB (income) expense:
Service cost			2.5	3.2	2.8
Interest cost			85.9	94.5	108.2
Expected return on plan assets			(109.7)	(148.8)	(149.9)
Amortization of prior service cost (credit)			3.6	3.8	4.7
Recognized net actuarial loss (gain):
Annual amortization			(7.6)	16.1	10.5
Settlement (gain) loss	$ 26.9	$ 14.5	26.9	14.5	0.0
Termination benefits—Ashland Works			9.7	0.0	0.0
Pension and OPEB (income) expense			11.3	(16.7)	(23.7)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of pension and OPEB (income) expense:
Service cost			3.5	4.5	4.7
Interest cost			16.4	15.6	17.3
Expected return on plan assets			0.0	0.0	0.0
Amortization of prior service cost (credit)			(13.2)	(13.6)	(58.5)
Recognized net actuarial loss (gain):
Annual amortization			(3.6)	(1.3)	(4.2)
Settlement (gain) loss			0.0	0.0	0.0
Termination benefits—Ashland Works			3.6	0.0	0.0
Pension and OPEB (income) expense			$ 6.7	$ 5.2	$ (40.7)