Environmental Obligations - Narrative (Details) $ in Millions				3 Months Ended		12 Months Ended
	Nov. 15, 2019 USD ($)	Sep. 28, 2017 USD ($)	Apr. 25, 2006 USD ($)	Dec. 31, 2019 USD ($) site	Dec. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) operable_unit site party	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 1998 USD ($)	Dec. 31, 2016 USD ($)
Environmental Exit Cost [Line Items]
Environmental reserves, excluding recoveries				$ 595.8	$ 529.4	$ 595.8	$ 529.4
Environmental loss contingencies, net of expected recoveries, in excess of accrual						150.0
Accrual for environmental loss contingencies				$ 585.8	521.5	585.8	521.5	$ 411.8		$ 360.4
Charges to expense for new losses						$ 138.8	178.2	$ 105.6
Number of operable units evaluated and proposed for CMAs | operable_unit						6
Number of sites FMC is named as a potentially responsible party | site				30		30
Number of sites FMC may be a potentially responsible party or equivalent | site				48		48
Non-judicial process, parties involved | party						70
Middleport Litigation
Environmental Exit Cost [Line Items]
Settlement payment						$ 22.2
Pending Litigation | Middleport Litigation
Environmental Exit Cost [Line Items]
Accrual for environmental loss contingencies					29.1		29.1
Charges to expense for new losses					106.3
Operable units | operable_unit						11
Cash outflow, maximum, after year 2021				$ 10.0		$ 10.0
Minimum | Pending Litigation | Middleport Litigation
Environmental Exit Cost [Line Items]
Cash outflow in 2020				20.0		20.0
Cash outflow in 2020				20.0		20.0
Maximum | Pending Litigation | Middleport Litigation
Environmental Exit Cost [Line Items]
Cash outflow in 2020				30.0		30.0
Cash outflow in 2020				30.0		30.0
Environmental Remediation Costs | Pending Litigation | Middleport Litigation
Environmental Exit Cost [Line Items]
Charges to expense for new losses					60.6
Implementation and Completion of Selected Remedy | Pending Litigation | Middleport Litigation
Environmental Exit Cost [Line Items]
Charges to expense for new losses					45.7
Environmental Clean-Up Costs | Pending Litigation | Middleport Litigation
Environmental Exit Cost [Line Items]
Accrual for environmental loss contingencies					31.1		31.1
Pocatello
Environmental Exit Cost [Line Items]
Accrual for environmental loss contingencies				107.5	33.1	107.5	33.1
Tribal permit fee		$ 1.5	$ 1.5						$ 1.5
Charges to expense for new losses	$ 72.8			$ 72.8		$ 72.8
Discount rate				2.25%		2.25%
Estimated year one fees				$ 29.5		$ 29.5
Estimated year two fees				1.5		1.5
Estimated year three fees				1.5		1.5
Estimated year four fees				1.5		1.5
Estimated year five fees				1.5		1.5
Expected aggregate undiscounted fees						103.0
Middleport
Environmental Exit Cost [Line Items]
Accrual for environmental loss contingencies				$ 159.4	$ 180.8	$ 159.4	$ 180.8