To do this correctly, try using the SCOPE() statement...for example
CREATE MEMBER CURRENTCUBE.[Measures].[CalcCommittedValue]
AS
SCOPE([Document].[Document Status].[&CP]);
THIS = [Measures].[Committed Value];
END SCOPE;
The Scope statement will cause the value to be calculated whenever the Document Status of [CP] is present, and not otherwise. Very powerful statement, and should get you want you need.