Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
Mar. 31, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Transfers between Level 1, Level 2 and Level 3	$ 0	$ 0

Amount of transfers of assets measured on a recurring basis out of Level 1, 2, or 3 of the fair value hierarchy into another level of the fair value hierarchy.

No definition available.

Name:	flxn_FairValueAssetTransferAmongLevelOneLevelTwoLevelThree
Namespace Prefix:	flxn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration