Marketable Securities Held in Trusts (Details)	12 Months Ended
Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Aug. 31, 2016 USD ($)
Investments, Debt and Equity Securities [Abstract]
Amount deposited by Mosaic into the RCRA Trusts			$ 630,000,000
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	$ 633,000,000	$ 633,500,000
Gross Unrealized Gains	4,100,000	900,000
Gross Unrealized Losses	(7,700,000)	(9,300,000)
Fair Value	629,400,000	625,100,000
Number Of Decades Remaining For Trust			3
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities	0
Cash And Cash Equivalents \| Fair Value Inputs Level 1
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	4,000,000	1,200,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	4,000,000	1,200,000
Corporate Debt Securities \| Fair Value Inputs Level 2
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	180,800,000	186,100,000
Gross Unrealized Gains	300,000	400,000
Gross Unrealized Losses	(4,300,000)	(2,200,000)
Fair Value	176,800,000	184,300,000
Municipal Bonds \| Fair Value Inputs Level 2
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	186,100,000	184,500,000
Gross Unrealized Gains	500,000	500,000
Gross Unrealized Losses	(3,400,000)	(2,700,000)
Fair Value	183,200,000	182,300,000
U.S. Government Bonds \| Fair Value Inputs Level 2
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	262,100,000	261,700,000
Gross Unrealized Gains	3,300,000	0
Gross Unrealized Losses	0	(4,400,000)
Fair Value	$ 265,400,000	$ 257,300,000

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- Definition

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
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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).

+ References

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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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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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