Billionaire Warren Buffett published his annual letter to shareholders of Berkshire Hathaway on Saturday. Berkshire's net worth grew by $65.3 billion in 2017, Buffett said, but $29 billion of that gain came from savings effected by the Republican tax plan passed in December. The new tax law lowered the nominal corporate tax rate from 35 to 21 percent. For individual investors, Buffett cautioned against going into debt to purchase stock because the market may drop.