BASIS OF PRESENTATION AND ACCOUNTING POLICIES (Details 5) (Subsequent events)	Jul. 30, 2013 item
Subsequent events
Subsequent events
Number of separate agreements entered into by the entity to acquire all of the outstanding shares of Trius Therapeutics, Inc. and Optimer	2

Represents the number of separate agreements entered into by the entity to acquire the outstanding shares of the acquiree.

No definition available.

Name:	cbst_NumberOfAgreementsEnteredIntoByEntityToAcquireOutstandingSharesOfAcquiree
Namespace Prefix:	cbst_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant