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Marketable Securities Held in Trusts (Details) $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2021 USD ($) | Mar. 31, 2020 USD ($) | 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 |  |  |
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities | $ 0.0 |  |  |
Available-for-sale Securities, Fair Value to Amortized Cost Basis |  |  |  |
Amortized cost | 701.4 |  | $ 696.3 |
Gross unrealized gains | 16.9 |  | 27.5 |
Gross unrealized losses | (7.8) |  | (0.4) |
Estimated fair value | 710.5 |  | 723.4 |
Cash and Cash Equivalents | Level 1 |  |  |  |
Available-for-sale Securities, Fair Value to Amortized Cost Basis |  |  |  |
Amortized cost | 4.0 |  | 11.8 |
Gross unrealized gains | 0.0 |  | 0.0 |
Gross unrealized losses | 0.0 |  | 0.0 |
Estimated fair value | 4.0 |  | 11.8 |
Corporate debt securities | Level 2 |  |  |  |
Available-for-sale Securities, Fair Value to Amortized Cost Basis |  |  |  |
Amortized cost | 199.4 |  | 193.3 |
Gross unrealized gains | 8.3 |  | 14.0 |
Gross unrealized losses | (1.0) |  | 0.0 |
Estimated fair value | 206.7 |  | 207.3 |
Municipal bonds | Level 2 |  |  |  |
Available-for-sale Securities, Fair Value to Amortized Cost Basis |  |  |  |
Amortized cost | 197.2 |  | 190.5 |
Gross unrealized gains | 7.4 |  | 8.8 |
Gross unrealized losses | (0.6) |  | (0.3) |
Estimated fair value | 204.0 |  | 199.0 |
U.S. government bonds | Level 2 |  |  |  |
Available-for-sale Securities, Fair Value to Amortized Cost Basis |  |  |  |
Amortized cost | 300.8 |  | 300.7 |
Gross unrealized gains | 1.2 |  | 4.7 |
Gross unrealized losses | (6.2) |  | (0.1) |
Estimated fair value | 295.8 |  | $ 305.3 |
Debt securities |  |  |  |
Available-for-sale Securities, Fair Value to Amortized Cost Basis |  |  |  |
Gross realized losses | $ 0.3 | $ 0.5 |  |

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