Aberdeen News: First Interstate BancSystems Inc. has entered an agreement with Great Western Bank that will combine the two companies in an all-stock transaction. Great Western Bank will merge into First Interstate BancSystems, according to a news release. First Interstate BancSystems, headquartered in Billings, Mont., is the parent company of First Interstate Bank, a company with $18.9 billion in assets. It has locations in six western states. Great Western, headquartered in Sioux Falls, S.D., is a $13 billion regional bank. It has 174 branches in Iowa, Arizona, Colorado, Kansas, Minnesota, Missouri, Nebraska, North Dakota and South Dakota.