Marketable Securities Held in Trusts (Details) $ in Millions	9 Months Ended
Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	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	690.7	$ 666.5
Gross unrealized gains	28.4	13.4
Gross unrealized losses	(0.8)	(3.0)
Estimated fair value	718.3	676.9
Cash and Cash Equivalents \| Level 1
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	8.6	3.4
Gross unrealized gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Estimated fair value	8.6	3.4
Corporate debt securities \| Level 2
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	191.2	194.2
Gross unrealized gains	13.1	5.8
Gross unrealized losses	(0.2)	(0.1)
Estimated fair value	204.1	199.9
Municipal bonds \| Level 2
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	191.5	188.3
Gross unrealized gains	7.9	4.4
Gross unrealized losses	(0.6)	(0.4)
Estimated fair value	198.8	192.3
U.S. government bonds \| Level 2
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	299.4	280.6
Gross unrealized gains	7.4	3.2
Gross unrealized losses	0.0	(2.5)
Estimated fair value	$ 306.8	$ 281.3

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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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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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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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