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Marketable Securities Held in Trusts (Details) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 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 |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |  |
Amortized Cost | 696.3 | $ 666.5 |
Gross Unrealized Gains | 27.5 | 13.4 |
Gross Unrealized Losses | (0.4) | (3.0) |
Fair Value | 723.4 | 676.9 |
Cash And Cash Equivalents | Fair Value Inputs Level 1 |  |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |  |
Amortized Cost | 11.8 | 3.4 |
Gross Unrealized Gains | 0.0 | 0.0 |
Gross Unrealized Losses | 0.0 | 0.0 |
Fair Value | 11.8 | 3.4 |
Corporate Debt Securities | Fair Value Inputs Level 2 |  |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |  |
Amortized Cost | 193.3 | 194.2 |
Gross Unrealized Gains | 14.0 | 5.8 |
Gross Unrealized Losses | 0.0 | (0.1) |
Fair Value | 207.3 | 199.9 |
Municipal Bonds | Fair Value Inputs Level 2 |  |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |  |
Amortized Cost | 190.5 | 188.3 |
Gross Unrealized Gains | 8.8 | 4.4 |
Gross Unrealized Losses | (0.3) | (0.4) |
Fair Value | 199.0 | 192.3 |
U.S. Government Bonds | Fair Value Inputs Level 2 |  |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |  |
Amortized Cost | 300.7 | 280.6 |
Gross Unrealized Gains | 4.7 | 3.2 |
Gross Unrealized Losses | (0.1) | (2.5) |
Fair Value | $ 305.3 | $ 281.3 |

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