Complex sentence: On March 5th 2013 the Dutch company Aegon fully divested the formerly Dutch owned American based The Transamerica Corporation which then became a fully American owned company, the once again/newley American owned company The Transamerica Corporation also retained the Transamerica Pyramid.
Split into simpler sentences: On March 5th 2013, the Dutch company Aegon fully divested the formerly Dutch-owned American-based The Transamerica Corporation, which then became a fully American-owned company. The once-again/newly American-owned company The Transamerica Corporation also retained the Transamerica Pyramid.