CVS Health Corp. is in talks to buy Aetna Inc., people familiar with the matter told The Wall Street Journal, in a deal that could value the health insurer at upward of $66 billion.

CVS Health has made a proposal to buy Aetna for more than $200 per share, one of the people said. On Thursday afternoon, before The Wall Street Journal reported news of the talks, shares of Aetna were trading at $160.62 each.

CVS made the offer earlier this month, although the two companies have been in discussion about a potential deal for at least two months, Reuters reported, citing an unnamed source. There is no certainty that an agreement will be reached, the source added.

Aetna shares initially shot up more than 10% on the report, giving it a market value of some $58 billion.

In 2015, Aetna agreed to buy rival insurer Humana Inc. for $34 billion, but the deal fell apart amid regulatory opposition.