Significant Accounting Policies - Cost of Products Sold (Details) - PEDMARK $ in Thousands	3 Months Ended
Mar. 31, 2024 USD ($)
Percentage of royalties	1.00%
Manufacturing costs capitalized to inventory	$ 2,064
Manufacturing costs capitalized to inventory, work-in-process	774
Manufacturing costs capitalized to inventory, finished goods	1,290,000
Manufacturing costs capitalized to inventory, cost of products sold	$ 550

Manufacturing costs related to PEDMARK and capitalized to inventory.

No definition available.

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Namespace Prefix:	fenc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Manufacturing costs related to PEDMARK and capitalized to inventory, amount reclassified as cost.of products sold.

No definition available.

Name:	fenc_ManufacturingCostsCapitalizedToInventoryCostOfProductsSold
Namespace Prefix:	fenc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Manufacturing costs related to PEDMARK and capitalized to inventory, amount classified as finished goods.

No definition available.

Name:	fenc_ManufacturingCostsCapitalizedToInventoryFinishedGoods
Namespace Prefix:	fenc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Manufacturing costs related to PEDMARK and capitalized to inventory, amount classified as work-in-process.

No definition available.

Name:	fenc_ManufacturingCostsCapitalizedToInventoryWorkInProcess
Namespace Prefix:	fenc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Represents the percentage of royalties to be received as consideration.

No definition available.

Name:	srt_ProductOrServiceAxis=fenc_PedmarkMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: