As some investors wonder if a torrid rise in US equities is running out of steam, one of the market’s most important buyers is standing firm.

Corporate America announced plans to buy back $665 billion worth of shares in S&P 500 companies in the four months through April, the most ever to start a year, according to data from Birinyi Associates. Just last week, Apple announced the year’s biggest repurchase as it seeks to reassure investors during its leadership transition.