Discontinued Operations - Narrative (Details) - Discontinued Operation - Omidria Assets Disposal - USD ($) $ in Millions	Dec. 23, 2021	Mar. 31, 2022
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash proceeds on sale of OMIDRIA	$ 126.0
Percentage of royalty receivable	50.00%
Percentage of royalty receivable after January 1, 2025 or payment of milestone	30.00%
Milestone payment receivable	$ 200.0	$ 200.0
Milestone Period	4 years
Percent of reduction in royalty receivable	10.00%
Percentage of royalty receivable outside US	15.00%

Amount of milestone payment receivable form sale of royalty under asset purchase agreement.

No definition available.

Name:	omer_MilestonePaymentReceivableFromSaleOfRoyaltyUnderAssetPurchaseAgreement
Namespace Prefix:	omer_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Milestone Period

+ References

No definition available.

+ Details

Name:	omer_MilestonePeriod
Namespace Prefix:	omer_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of reduction in royalty receivable on breach of agreement term.

+ References

No definition available.

+ Details

Name:	omer_PercentageOfReductionInRoyaltyReceivableUponBreachOfAgreementTerm
Namespace Prefix:	omer_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of royalty receivable on sale of product rights.

+ References

No definition available.

+ Details

Name:	omer_PercentageOfRoyaltyReceivableOnSaleOfProductRights
Namespace Prefix:	omer_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of royalty receivable on sale of product rights after January 1, 2025 or the payment of the milestone.

+ References

No definition available.

+ Details

Name:	omer_PercentageOfRoyaltyReceivableOnSaleOfProductRightsAfterSpecifiedDateOrMilestone
Namespace Prefix:	omer_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of royalty receivable on sale of product rights in other countries.

+ References

No definition available.

+ Details

Name:	omer_PercentageOfRoyaltyReceivableOnSaleOfProductRightsInOtherCountries
Namespace Prefix:	omer_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash inflow associated with the amount received from the sale of a portion of the company's business, for example a segment, division, branch or other business, during the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3179-108585

+ Details

Name:	us-gaap_ProceedsFromDivestitureOfBusinesses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_DisposalGroupClassificationAxis=us-gaap_DiscontinuedOperationsDisposedOfBySaleMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=omer_OmidriaAssetsDisposalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: