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Marketable Securities Held in Trusts (Details) $ in Millions | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2021 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 |  |
Other than Temporary Impairment Losses, Investments | $ 0.0 |  |
Available-for-sale Securities, Fair Value to Amortized Cost Basis |  |  |
Amortized cost | 708.8 | $ 696.3 |
Gross unrealized gains | 15.0 | 27.5 |
Gross unrealized losses | (5.6) | (0.4) |
Debt Securities, Available-for-sale | 718.2 | 723.4 |
Other than Temporary Impairment Losses, Investments | 0.0 |  |
Cash and Cash Equivalents | Level 1 |  |  |
Available-for-sale Securities, Fair Value to Amortized Cost Basis |  |  |
Amortized cost | 6.9 | 11.8 |
Gross unrealized gains | 0.0 | 0.0 |
Gross unrealized losses | 0.0 | 0.0 |
Debt Securities, Available-for-sale | 6.9 | 11.8 |
Corporate debt securities | Level 2 |  |  |
Available-for-sale Securities, Fair Value to Amortized Cost Basis |  |  |
Amortized cost | 198.7 | 193.3 |
Gross unrealized gains | 8.0 | 14.0 |
Gross unrealized losses | (0.4) | 0.0 |
Debt Securities, Available-for-sale | 206.3 | 207.3 |
Municipal bonds | Level 2 |  |  |
Available-for-sale Securities, Fair Value to Amortized Cost Basis |  |  |
Amortized cost | 196.2 | 190.5 |
Gross unrealized gains | 7.0 | 8.8 |
Gross unrealized losses | (0.6) | (0.3) |
Debt Securities, Available-for-sale | 202.6 | 199.0 |
U.S. government bonds | Level 2 |  |  |
Available-for-sale Securities, Fair Value to Amortized Cost Basis |  |  |
Amortized cost | 307.0 | 300.7 |
Gross unrealized gains | 0.0 | 4.7 |
Gross unrealized losses | (4.6) | (0.1) |
Debt Securities, Available-for-sale | 302.4 | $ 305.3 |
Debt securities |  |  |
Available-for-sale Securities, Fair Value to Amortized Cost Basis |  |  |
Debt Securities, Available-for-sale | $ 711.3 |  |

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