Michinaga's total de facto rule over Japan can be seen from the fact that he was father to four (non-reigning) empresses, uncle to two emperors and grandfather to another three. Technically, he never formally took on the title of kampaku regent, but in reality his word was law even after he formally retired from public life in 1019, since he continued to direct the affairs of his son and successor, Yorimichi. Michinaga is popularly known as the Mido Kampaku, implying that he had usurped the full power of a kampaku without necessarily calling himself that.
Soon afterwards, emperors started to retire to a monastery early in life, put young sons on the throne and run the country from behind the scenes. They may well have gotten the idea from Michinaga. As it turned out, this tactic briefly allowed the emperors to wrest power back from the Fujiwara clan, only to see it fall to the Taira warrior clan instead.
Michinaga left a diary, Mido Kanpakuki (御堂関白記), that is one of our prime sources of information about Heian-era court life at its height. According to some, he also was the inspiration for Prince Genji, the hero of "The tale of Genji" (J. Genji monogatari) by Murasaki Shikibu, widely viewed as one of the world's first novels.