Leases - Schedule of Partnership's Leases, Excluding Leases for Mineral Reserves - Lease Assets and Liabilities (Details) $ in Thousands	Mar. 31, 2019 USD ($)
Assets
Operating lease right-of-use assets	$ 6,186
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssetsNoncurrent
Finance lease right-of-use assets	$ 54,655
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:PropertyPlantAndEquipmentNet
Total lease right-of-use assets	$ 62,732
Liabilities
Operating lease liabilities, current	$ 3,144
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	felp:AccruedExpensesAndOtherCurrentLiabilities
Finance lease liabilities, current	$ 10,932
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	felp:LongTermDebtAndFinanceLeaseObligationsCurrent
Operating lease liabilities, Non-current	$ 3,042
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent
Total lease liabilities	$ 19,009
Affiliate
Assets
Operating lease right-of-use assets	$ 1,891
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssetsNoncurrent
Liabilities
Operating lease liabilities, current	$ 174
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	felp:AccruedExpensesAndOtherCurrentLiabilities
Operating lease liabilities, Non-current	$ 1,717
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent