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Marketable Securities Held in Trusts (Details) $ in Millions	Sep. 30, 2025 USD ($)	Dec. 31, 2024 USD ($)	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
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	$ 723.6	$ 712.3
Gross unrealized gains	15.2	1.5
Gross unrealized losses	(4.6)	(18.6)
Debt Securities, Available-for-sale	734.2	695.2
Cash and Cash Equivalents | Level 1
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	2.5	3.1
Gross unrealized gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Debt Securities, Available-for-sale	2.5	3.1
Corporate debt securities | Level 2
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	218.1	203.3
Gross unrealized gains	3.9	0.8
Gross unrealized losses	(3.1)	(6.8)
Debt Securities, Available-for-sale	218.9	197.3
Municipal bonds | Level 2
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	212.0	210.8
Gross unrealized gains	3.4	0.7
Gross unrealized losses	(1.5)	(4.0)
Debt Securities, Available-for-sale	213.9	207.5
U.S. government bonds | Level 2
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	291.0	295.1
Gross unrealized gains	7.9	0.0
Gross unrealized losses	0.0	(7.8)
Debt Securities, Available-for-sale	298.9	$ 287.3
Debt securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Debt Securities, Available-for-sale	$ 731.7

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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.

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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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