Indianapolis-based WellPoint Inc., the largest U.S. health insurer by enrollment, boosted a two-part offering of notes to $1 billion from $750 million, according to a person familiar with the offering.

The debt will be sold as soon as Monday, said the person, who declined to be identified because terms aren’t set. Proceeds from the sale will be used for working capital and general corporate purposes, including debt repayment, the company said in a prospectus that didn’t specify the size or timing of the sale.

WellPoint has about $1 billion of debt coming due in early 2011, which it may seek to refinance, Chief Financial Officer Wayne DeVeydt said in an earnings call on July 28.

The company plans to sell $700 million of 10-year notes that may yield 155 basis points more than similar-maturity Treasuries, the person said. WellPoint also may sell $300 million of 30-year bonds at a spread of 187.5 basis points over a benchmark Treasury due in February 2040, the person said. A basis point is 0.01 percentage point.

The notes may be rated Baa1 by Moody’s Investors Service and A- by Standard & Poor’s, the person said.