Massachusetts Mutual Life Insurance Co. says it’s expanding its business in Massachusetts, while closing a large office in Connecticut.

MassMutual said Thursday it plans to spend nearly $300 million in Massachusetts and increase its workforce in the state by approximately 70 percent by the end of 2021.

According to the company, it means increasing the number of positions in Springfield from about 3,000 to 4,500 and adding another 1,000 jobs at a new campus in Boston.

The company says it will close its offices in Enfield, Connecticut by 2021, moving most of those 1,500 workers to Springfield.

The company says it plans to spend about $50 million upgrading its Springfield headquarters and another $240 million on the campus in Boston's Seaport district.