Substract The Sum Of Two Doc Types In A Table

I need to subtract the values of the totals of two document types in a table. So far I have the following with the results as shown.

SELECT TOP 100 PERCENT { fn MONTHNAME(DOCDATE) } AS [MONTH], DOCID, SUM(DOCAMNT) AS TOTAL
FROM dbo.tblSales
WHERE (DOCID = 'RTD') OR
(DOCID = 'INV')
GROUP BY DOCID, { fn MONTHNAME(DOCDATE) }
ORDER BY DOCID DESC

This results in the following:
Month DOCID TOTAL
MAY RTD 165752.87
MAY INV 18149416.37

I need one value showing the difference of the totals.
Thanks.