Equity (401k) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Equity [Abstract]
Matching contribution	$ 0.1	$ 0.1	$ 0.3	$ 0.3

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