Update records in table from CTE

I have the following CTE that will give me the DocTotal for the entire invoice.

 ;WITH CTE_DocTotal
 AS
 (
   SELECT SUM(Sale + VAT) AS DocTotal
   FROM PEDI_InvoiceDetail
   GROUP BY InvoiceNumber
 )

UPDATE PEDI_InvoiceDetail
SET DocTotal = CTE_DocTotal.DocTotal

Now with this result I want to enter into the column the DocTotal value inside PEDI_InvoiceDetail.

I know is not going to work and I know I am missing something, what is it?