Complex sentence: Crane joined then Minneapolis based NRG Energy in 2003 as executive director which he remained until he left December 2015 rising to President, Chief executive officer from December 2012 to December 2015.
Split into simpler sentences: Crane joined then Minneapolis based NRG Energy in 2003. He was Executive Director from December 2003 to December 2015 and President, Chief executive officer from December 2012 to December 2015.