Represents the number of outstanding shares under the capped call transactions.

No definition available.

Name:	omer_DebtInstrumentCappedCallTransactionsNumberOfSharesOutstanding
Namespace Prefix:	omer_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the denominator amount used in the calculation of convertible debt.

+ References

No definition available.

+ Details

Name:	omer_DebtInstrumentDenominatorAmount
Namespace Prefix:	omer_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of proceeds from debt issuance used to repurchase previously issued debt.

+ References

No definition available.

+ Details

Name:	omer_ProceedsFromDebtIssuanceUsedForRepurchase
Namespace Prefix:	omer_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The price per share of the conversion feature embedded in the debt instrument.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 5
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-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031898-161870

+ Details

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Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio applied to the conversion of debt instrument into equity with equity shares divided by debt principal amount.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
-Topic 835
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+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

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-Name Accounting Standards Codification
-Topic 210
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Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

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

- Definition

Face (par) amount of the original debt instrument that was repurchased.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentRepurchasedFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_DebtInstrumentAxis=omer_ConvertibleSeniorNotes2023Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Unsecured Convertible Senior Notes - 2023 Unsecured Convertible Senior Notes (Details) - 2023 Unsecured Convertible Senior Notes $ / shares in Units, shares in Millions	Aug. 14, 2020 USD ($)	Nov. 15, 2018 USD ($) $ / shares	Jun. 30, 2021 shares
Debt Instrument [Line Items]
Principal amount		$ 210,000,000.0
Interest rate (as a percent)		6.25%
Conversion rate		0.0520183
Value used in calculation of conversion rate		$ 1,000
Conversion price \| $ / shares		$ 19.22
Proceeds from 2026 debt issuance used to repurchase 2023 Notes	$ 125,600,000
Principal amount of debt repurchased	$ 115,000,000.0
Number of shares outstanding on the 2023 Capped Call \| shares			4.9
Minimum
Debt Instrument [Line Items]
Conversion price \| $ / shares		19.22
Maximum
Debt Instrument [Line Items]
Conversion price \| $ / shares		$ 28.84