Oct 18 (Reuters) - U.S. buyout firm Silver Lake will acquire Intercontinental Exchange Inc’s stake in securities settlement platform Euroclear Holding for 709 million euros ($821.7 million), the companies said on Monday. $1 = 0.8628 euros Reporting by Niket Nishant in Bengaluru; Editing by Shounak Dasgupta Our Standards: The Thomson Reuters Trust Principles.