Marketable Securities Held in Trusts (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Aug. 31, 2016 USD ($)
Investments, Debt and Equity Securities [Abstract]
Amount deposited by Mosaic into the RCRA Trusts			$ 630,000,000
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	$ 633,500,000	$ 619,700,000
Gross Unrealized Gains	900,000	100,000
Gross Unrealized Losses	(9,300,000)	(11,200,000)
Fair Value	625,100,000	608,600,000
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
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	1,200,000	1,200,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	1,200,000	1,200,000
Corporate Debt Securities | Fair Value Inputs Level 2
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	186,100,000	180,200,000
Gross Unrealized Gains	400,000	0
Gross Unrealized Losses	(2,200,000)	(4,300,000)
Fair Value	184,300,000	175,900,000
Municipal Bonds | Fair Value Inputs Level 2
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	184,500,000	180,900,000
Gross Unrealized Gains	500,000	0
Gross Unrealized Losses	(2,700,000)	(6,600,000)
Fair Value	182,300,000	174,300,000
U.S. Government Bonds | Fair Value Inputs Level 2
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	261,700,000	257,400,000
Gross Unrealized Gains	0	100,000
Gross Unrealized Losses	(4,400,000)	(300,000)
Fair Value	$ 257,300,000	$ 257,200,000