"We've had extremely steady growth for a good number of years," president and chief executive Robert Allen said.
Until now, as its name suggests, Teachers - based in Farmingville - has served school employees and their families in Suffolk. With the change, it joins Bethpage Federal Credit Union and Island Federal Credit Union as nonprofit financial institutions available to most Long Islanders.
Allen said that with the turmoil affecting many banks, the lower interest rates on loans and higher rates on accounts make credit unions more attractive options than ever.
Teachers has more than 200,000 members and assets of $3.6 billion. In 2009 deposits grew by 40 percent and loans by 23 percent, according to the credit union.
"Our growth in 2009 was extraordinary, even for us," Allen said.
Teachers has 20 branches, all in Suffolk, although Allen said the union hopes to open some in Nassau.