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Marketable Securities Held in Trusts (Details) $ in Millions | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 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 |  |
Available-for-sale Securities, Fair Value to Amortized Cost Basis |  |  |
Amortized cost | 690.7 | $ 666.5 |
Gross unrealized gains | 28.4 | 13.4 |
Gross unrealized losses | (0.8) | (3.0) |
Estimated fair value | 718.3 | 676.9 |
Cash and Cash Equivalents | Level 1 |  |  |
Available-for-sale Securities, Fair Value to Amortized Cost Basis |  |  |
Amortized cost | 8.6 | 3.4 |
Gross unrealized gains | 0.0 | 0.0 |
Gross unrealized losses | 0.0 | 0.0 |
Estimated fair value | 8.6 | 3.4 |
Corporate debt securities | Level 2 |  |  |
Available-for-sale Securities, Fair Value to Amortized Cost Basis |  |  |
Amortized cost | 191.2 | 194.2 |
Gross unrealized gains | 13.1 | 5.8 |
Gross unrealized losses | (0.2) | (0.1) |
Estimated fair value | 204.1 | 199.9 |
Municipal bonds | Level 2 |  |  |
Available-for-sale Securities, Fair Value to Amortized Cost Basis |  |  |
Amortized cost | 191.5 | 188.3 |
Gross unrealized gains | 7.9 | 4.4 |
Gross unrealized losses | (0.6) | (0.4) |
Estimated fair value | 198.8 | 192.3 |
U.S. government bonds | Level 2 |  |  |
Available-for-sale Securities, Fair Value to Amortized Cost Basis |  |  |
Amortized cost | 299.4 | 280.6 |
Gross unrealized gains | 7.4 | 3.2 |
Gross unrealized losses | 0.0 | (2.5) |
Estimated fair value | $ 306.8 | $ 281.3 |

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