Equity (401k) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Equity [Abstract]
Matching contribution	$ 0.2	$ 0.2	$ 0.7	$ 0.4

Defined Contribution Plan, Employer Matching Contribution

No definition available.

Name:	emkr_DefinedContributionPlanEmployerMatchingContribution
Namespace Prefix:	emkr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration