1949 - 1952
1953 - 1957
1957 - 1958
1958 - 1960
1960 - 1961
1961 - 1964
1964 - 1967
1967 - 1972
1972 - 1978
1978 - 1982
1982 - 1989
1989 - 1990
1990 - 1995
1995 - 1999
1999 - 2000
2000 - 2002
2002 - 2008
2008 - 2012
October 2012 - January 2015
February 2015 - October 2019
November 2019 - incumbent