CREATE TABLE table_18399 ("District" TEXT, "Incumbent" TEXT, "Party" TEXT, "First elected" FLOAT, "Result" TEXT, "Candidates" TEXT)

What was the district who had their first elected in 1966?
SELECT "District" FROM table_18399 WHERE "First elected" = '1966'
SELECT "District" FROM "table_18399" WHERE "First elected" = '1966'