on top of the world
on top of the world or sitting on top of the world also (Southern) sitting on high cotton {adj. phr.}, {informal}
Feeling pleased and happy; feeling successful.
John was on top of the world when he found out that he got into college.
When Ruth won first prize in the contest, she felt as though she was sitting on top of the world.
The girls were sitting on high cotton because their basketball team had won the trophy.
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Source: A Dictionary of American Idioms