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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 -Paragraph 1 -Subparagraph (SX 210.4-08.(b)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
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