CHICAGO ( TheStreet) -- Aon Corp. ( AON) is offering $1.5 billion in unsecured debt to fund its acquisition of the human resources firm Hewitt Associates ( HEW)

Aon will sell $600 million worth of five-year notes, $600 million worth of 10-year notes and $300 million worth of 30-year notes. Like other corporations, Aon was able to seize on the low-interest-rate environment to offer notes at 3.5%, 5% and 6.25%, respectively. The pricing came at a spread of 200-to-250 basis points above Treasury bills with comparable maturities.

Aon, an insurance and risk-management firm, announced a $4.9 billion cash-and-stock deal to acquire Hewitt on July 12. Pending approval by regulators and shareholders, the Chicago-based companies expect the deal to close by mid-November. They expect the deal to save $355 million a year by 2013 and generate $1.5 billion in stock value on a discounted cash-flow basis. They expect earnings per share to rise 1.2% next year and another 5.8% in 2013.

Since the close before the deal's announcement, Aon shares have been relatively flat, while Hewitt's stock has shot up 40% to just below Aon's $50-per-share offer. Late Thursday afternoon, Aon shares were up 7 cents at $37.85 and Hewitt was up 4 cents at $49.40.

--Written by Lauren Tara LaCapra in New York.

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