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The amount of cash, securities, or other assets held by a third-party trustee pursuant to the terms of an agreement which assets are available to be used by beneficiaries to that agreement only within the specific terms thereof and which agreement is expected to terminate more than one year from the balance sheet date (or operating cycle, if longer) at which time the assets held-in-trust will be released or forfeited.

+ References

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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The amount by which the fair value of an investment in debt and equity securities categorized as Available-for-sale is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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Marketable Securities Held in Trusts (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Aug. 31, 2016 USD ($)
Investments, Debt and Equity Securities [Abstract]
Amount deposited by Mosaic into the RCRA Trusts				$ 630.0
Number Of Decades Remaining For Trust	3
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities	$ 0.0
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	701.4		$ 696.3
Gross unrealized gains	16.9		27.5
Gross unrealized losses	(7.8)		(0.4)
Estimated fair value	710.5		723.4
Cash and Cash Equivalents \| Level 1
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	4.0		11.8
Gross unrealized gains	0.0		0.0
Gross unrealized losses	0.0		0.0
Estimated fair value	4.0		11.8
Corporate debt securities \| Level 2
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	199.4		193.3
Gross unrealized gains	8.3		14.0
Gross unrealized losses	(1.0)		0.0
Estimated fair value	206.7		207.3
Municipal bonds \| Level 2
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	197.2		190.5
Gross unrealized gains	7.4		8.8
Gross unrealized losses	(0.6)		(0.3)
Estimated fair value	204.0		199.0
U.S. government bonds \| Level 2
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	300.8		300.7
Gross unrealized gains	1.2		4.7
Gross unrealized losses	(6.2)		(0.1)
Estimated fair value	295.8		$ 305.3
Debt securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Gross realized losses	$ 0.3	$ 0.5