Income Taxes (Details 4) $ in Millions, $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 CAD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2021 USD ($)
Operating Loss Carryforwards [Line Items]
Tax Adjustments, Settlements, and Unusual Provisions		$ (24.0)		$ (10.0)	$ 0.0
Income Tax Examination, Interest Income, Net of Tax		23.0
Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions	$ 9.0
Income Tax Examination, Liability (Refund) Adjustment from Settlement with Taxing Authority, Income Tax Matter	68.0	68.0
Income Tax Examination, Liability (Refund) Adjustment from Settlement with Taxing Authority, Withholding Tax Matter	42.0	42.0
Income Tax Expense (Benefit)		(31.0)		(126.0)	(119.0)
Valuation allowance		0.0		0.0	4.0
Deferred tax assets, capital loss carryforwards	90.0	90.0
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities		23.0		44.0
Interest expense and penalties
Interest expense and penalties related to potential income taxes		(29.0)		4.0	$ 1.0
Accrued interest expense and penalties
Amount recognized in consolidated balance sheets for accrued interest and penalties related to income taxes	4.0	4.0		33.0
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	37.0	37.0
Terra Amended Tax Returns
Operating Loss Carryforwards [Line Items]
Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions	4.0
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities		19.0
Subsequent Event
Operating Loss Carryforwards [Line Items]
Income Tax Examination, Liability (Refund) Adjustment from Settlement with Taxing Authority, Withholding Tax Matter						$ 20.0
Canada Revenue Agency
Operating Loss Carryforwards [Line Items]
Tax assessment amount		137.0	$ 174
Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities	12.0
State Tax Authority
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards not expected to be realized	27.0	27.0
State Tax Authority | Tax Year 2033 to 2040
Operating Loss Carryforwards [Line Items]
Tax credit carryforward valuation allowance	69.0	69.0
Foreign Tax Authority
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards increase (decrease) valuation allowance	$ 99.0	99.0
Valuation allowance				(99.0)
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Income Tax Expense (Benefit)		18.0
Income Tax Examination, Interest Income		26.0
Accrued interest expense and penalties
Income Tax Examination, Interest Income		26.0
Point Lisas Nitrogen Limited (PLNL) | Foreign Tax Authority
Operating Loss Carryforwards [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount		$ 6.0
Interest Income | Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Tax Adjustments, Settlements, and Unusual Provisions				5.0
interest income, net [Member] | Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Tax Adjustments, Settlements, and Unusual Provisions				4.0
income tax benefit | Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Tax Adjustments, Settlements, and Unusual Provisions				$ 10.0