It's a countable noun. Can you see the car in the street? Can you see how many details has the car? One, two, three, .... Do you think that car has 40 details? I'm not sure in it. Please count again them in detail. Especially, in details (So you can see the difference of using of the phrase "in detail(s)"?)

"Detail" itself is a countable noun, meaning a trait or attribute.... However, the expression is "in detail," which rather functions as an adverb (i.e. tell me in detail). See also: "in graphic detail," "in great detail," "down to every last detail," etc....

it´s a countable noun. You can say "in detail" or "in details", "in details" is the plural.