Marketable Securities Held in Trusts (Details) $ in Millions	Sep. 30, 2026 USD ($)	Dec. 31, 2025 USD ($)	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	$ 751.2	$ 740.6
Gross unrealized gains	0.0	7.3
Gross unrealized losses	(30.2)	(4.6)
Debt Securities, Available-for-sale	721.0	743.3
Cash and Cash Equivalents \| Level 1
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	5.4	4.8
Gross unrealized gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Debt Securities, Available-for-sale	5.4	4.8
Corporate debt securities \| Level 2
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	219.9	218.8
Gross unrealized gains	0.0	3.4
Gross unrealized losses	(8.8)	(2.8)
Debt Securities, Available-for-sale	211.1	219.4
Municipal bonds \| Level 2
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	214.7	208.3
Gross unrealized gains	0.0	3.9
Gross unrealized losses	(8.8)	(1.3)
Debt Securities, Available-for-sale	205.9	210.9
U.S. government bonds \| Level 2
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	311.2	308.7
Gross unrealized gains	0.0	0.0
Gross unrealized losses	(12.6)	(0.5)
Debt Securities, Available-for-sale	$ 298.6	$ 308.2

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