DETROIT — General Motors on Friday said that it was acquiring the software firm Cruise Automation as part of its larger effort to develop fully autonomous vehicles.

G.M., the largest automaker in the United States, has been aggressively expanding its portfolio of new businesses dedicated to driverless cars and other personal mobility options like ride-hailing services.

The company has invested $500 million in a strategic alliance with the ride service Lyft, and established its own brand, called Maven, for its car-hailing initiatives. While G.M. did not disclose the terms of the latest deal, two people with knowledge of the acquisition said the automaker agreed to pay more than $1 billion in cash and stock for Cruise.

It would be a rich payday for Cruise and its investors, which include venture firms such as Spark Capital and Felicis Ventures. Cruise was valued at $90 million in a fund-raising round late last year before the acquisition, the people said.