Significant Accounting Policies - Chargebacks and Returns (Details)	12 Months Ended
Dec. 31, 2019
Accounting Policies [Abstract]
Period chargebacks are generally settled	28 days
Period for product return past its expiration or damaged	12 months

Represents the maximum period in which product is allowed for return past its expiration or if its damaged.

No definition available.

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