Issuance was bolstered by heart-rhythm device maker Medtronic Inc.'s $17 billion bond sale yesterday, the largest dollar-denominated offering in more than a year, according to data compiled by Bloomberg. Internet commerce company Alibaba Group Holding Ltd. sold $8 billion last month, helping push this year's volume past the previous high of $1.494 trillion set last year.

Issuance is booming as companies have seized on record-low borrowing costs, boosted by the Federal Reserve maintaining its pledge this year to keep benchmark interest rates near zero for a "considerable time." After providing a 0.34 percent gain to investors last year, returns on the securities have grown to 6.9 percent, according to the Bank of America Merrill Lynch U.S. Corporate and High Yield Index.

"This has been one of the best-performing safe assets for the past few years," Rob Lutts, Salem, Massachusetts-based chief investment officer of Cabot Money Management Inc., said in a telephone interview. "The money flying in is just further evidence that investors are buying into this story that this is a good place to be."