Student or Learner
It is a company that helped lift hundreds of thousands of American workers into the middle class. It transformed Detroit into the Silicon Valley of its day, a symbol of Americaís talent for innovation. It built celebrated cars, like Cadillacs, that became synonymous with luxury.
Is the following idiom used in this paragraph?
of sbís day during a particular period of time when sb lived: the best player of his day Bessie Smith was the Madonna of her day.
Yes. Although, in this case it is not a person, so rather than being the period of time when someone lived, it would be the time period during which the company was most prominent or successful.