Despite Buffett overseeing stakes in nearly four-dozen securities, the lion's share of Berkshire's invested assets have been put to work in seven top ideas.

It’s been a rocky week in the stock market. Warren Buffett’s Berkshire Hathaway has been buying. The Omaha, Neb., company boosted its positions in Occidental Petroleum, VeriSign and SiriusXM with stock purchases this Tuesday through Thursday,

Since Warren Buffett became the CEO of Berkshire Hathaway ( BRK.A 0.80%) ( BRK.B 0.95%) in 1965, its stock has delivered a compound annual return of 19.8%. That would have been enough to turn a $1,000 investment into $42.5 million, whereas the same investment in the S&P 500 would have grown to just $325,053 over the same period.