Commitments and Contingent Liabilities (Details) $ in Millions	3 Months Ended	12 Months Ended			59 Months Ended
	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) payment shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018 USD ($) shares	Feb. 03, 2017 USD ($)	Feb. 03, 2014 USD ($)
Environmental remediation
ZAI PD Account Funding							$ 34.4
ZAI P D Account, Payment on Third Anniversary of Effective Date of Joint Plan						$ 30.0
ZAI P D Account, Maximum Number of Contingent Deferred Payments | payment		10
ZAI P D Account, Deferred Payments, Each Year for Twenty Years		$ 8.0
Period in Which ZAI PD Contingent Deferred Payments Will be Made		20 years
Minimum ZAI P D Account Assets for Condition in Relation to Contingent Obligation Payments		$ 10.0			$ 10.0
ZAI PD Claims paid by the Trust					15.0
ZAI PD Operating and Education Expenses paid by the Trust					6.0
ZAI PD Trust Balance		$ 30.0			30.0
ZAI PD Attorney's fees paid by the Trust					$ 15.0
Frequency of PD Trust Payments		6 months
Estimated annual expenses related to PD Trust		$ 0.2
Number of Shares Issuable under Warrant | shares		77,372,257			77,372,257
Estimated liability for environmental investigative and remediation costs		$ 126.4	$ 70.3		$ 126.4
Pre-tax charges for environmental matters		73.8	24.4	$ 28.7
Vermiculite and non vermiculite matters [Member]
Environmental remediation
Pre-tax charges for environmental matters		73.8	24.4	29.2
Vermiculite Related Matters [Member]
Environmental remediation
Estimated liability for environmental investigative and remediation costs		81.7	25.8		81.7
Pre-tax charges for environmental matters		70.2	9.5	$ 24.8
Operable Unit 3 [Member]
Environmental remediation
Pre-tax charges for environmental matters	$ 70.0
Non Vermiculite Related Matters [Member]
Environmental remediation
Estimated liability for environmental investigative and remediation costs		44.7	44.5		44.7
Pre-tax charges for environmental matters		$ 3.6	14.9
Accrual for Environmental Loss Contingencies, Charges to Expense for New Losses			$ 7.2
Environmental Exit Costs, Nature of Costs		P10Y
Minimum | Operable Unit 3 [Member]
Environmental remediation
Loss Contingency, Estimate of Possible Loss		$ 30.0			30.0
Minimum | Spillway [Member]
Environmental remediation
Loss Contingency, Estimate of Possible Loss		40.0			40.0
Maximum | Operable Unit 3 [Member]
Environmental remediation
Loss Contingency, Estimate of Possible Loss		170.0			170.0
Maximum | Spillway [Member]
Environmental remediation
Loss Contingency, Estimate of Possible Loss		45.0			45.0
Net Present Value [Member] | ZAI PD Trust [Member] | Minimum
Environmental remediation
Loss Contingency, Estimate of Possible Loss		0.0			0.0
Net Present Value [Member] | ZAI PD Trust [Member] | Maximum
Environmental remediation
Loss Contingency, Estimate of Possible Loss		$ 20.0			$ 20.0