The _New York Times_ has an idiosyncratic style guide; they follow each letter with a period when the letters are pronounced individually. So, "I.B.M", "C.I.A."

The _New York Times_ has an idiosyncratic style guide; they follow each letter with a period when the letters are pronounced individually. So, "I.B.M", "C.I.A.", but "NATO". (Naturally, how IBM itself spells its name is of no interest.)