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Marketable Securities Held in Trusts (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	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		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	$ 705.9		705.9		$ 696.3
Gross unrealized gains	17.8		17.8		27.5
Gross unrealized losses	(5.1)		(5.1)		(0.4)
Estimated fair value	718.6		718.6		723.4
Cash and Cash Equivalents | Level 1
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	7.9		7.9		11.8
Gross unrealized gains	0.0		0.0		0.0
Gross unrealized losses	0.0		0.0		0.0
Estimated fair value	7.9		7.9		11.8
Corporate debt securities | Level 2
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	198.6		198.6		193.3
Gross unrealized gains	9.2		9.2		14.0
Gross unrealized losses	(0.2)		(0.2)		0.0
Estimated fair value	207.6		207.6		207.3
Municipal bonds | Level 2
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	197.2		197.2		190.5
Gross unrealized gains	8.1		8.1		8.8
Gross unrealized losses	(0.4)		(0.4)		(0.3)
Estimated fair value	204.9		204.9		199.0
U.S. government bonds | Level 2
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Amortized cost	302.2		302.2		300.7
Gross unrealized gains	0.5		0.5		4.7
Gross unrealized losses	(4.5)		(4.5)		(0.1)
Estimated fair value	298.2		298.2		$ 305.3
Debt securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis
Gross realized losses	$ 0.1	$ 0.7	$ 0.4	$ 0.9

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