Simon Property Group Inc., the largest U.S. mall owner, plans to spin off its strip-center business and smaller enclosed malls into a new real estate investment trust. The shares gained the most in two years.

The new company, referred to as SpinCo, will own all or part of 54 strip centers and 44 malls and is expected to generate net operating income of more than $400 million in its first year, Indianapolis-based Simon said in a statement Friday. SpinCo will operate 53 million square feet of retail space in 23 states.