Shares of the Hartford, Conn., company rose 18.7% to $68.14. Trading in Hartford Financial shares had been halted.
Chubb, the Zurich insurer, was finished down 2.64% to $168.11.
The Connecticut company confirmed "that it has received an unsolicited, non-binding proposal from Chubb Limited to acquire The Hartford."
"The Hartford’s Board of Directors is carefully considering the proposal with the assistance of its financial and legal advisors," Hartford said in a statement. "The Board of Directors is committed to acting in the best interests of shareholders over the long term."
Hartford Financial's market capitalization was $23 billion as of Thursday, with its shares having doubled in the past year, Bloomberg noted.
Chubb said in a statement that on March 11 the company presented Hartford "with a proposal for a combination of our two companies that we believe would be strategically and financially compelling for both sets of shareholders and other constituencies. "
"This proposal would value The Hartford at $65 per share and represents a premium of 26% based on its unaffected 20-day volume weighted average share price of $51.70 as of March 10, 2021," the statement said. "The consideration represents a mix of stock with the majority in cash."
Chubb said Thursday that it had not yet received a response to the proposal but looked forward "to constructive, private discussions in order to expeditiously consummate a fair transaction that benefits all of our respective stakeholders."
Last month Hartford Financial beat Wall Street's fourth-quarter earnings expectations. The company posted revenue of $3.44 billion. Fourth-quarter core earnings rose 22% to $636 million.
Written premiums were $673 million compared with $714 million in the year-earlier quarter, due primarily to a reduction in auto as non-renewed premiums exceeded new business.
Hartford Chairman and CEO Christopher Swift said at the time that "we have been through one of the most turbulent years in recent history, which was shaped by the COVID-19 pandemic, the economic shutdown, social unrest and a significant number of catastrophe events."
Hartford Financial is the second-biggest provider of workers’ compensation insurance in the U.S., according to A.M. Best. It also has a mutual-fund arm with about $139 billion under management.
Chubb CEO Evan Greenberg built the business into a mammoth insurer by combining Ace Ltd., the company he ran, with Chubb Corp. in a nearly $30 billion deal in 2016.
The son of former American International Group CEO Maurice “Hank” Greenberg, he has transformed Chubb into a firm with huge footholds in both personal and commercial lines