1
general::
With close attention to particulars; thoroughly. For example, She explained her theory in detail. It is Also put as go into detail, meaning "to investigate thoroughly," as in You know what I mean, so I needn't go into detail. The first expression dates from about 1600, the second from the late 1800s.
American Heritage Idioms
2
general::
Phrase(s): in detail
with lots of details; giving all the details. (Often used with an adjective showing the level of detail, such as great, enormous, etc., as in the second example.) • I explained the policy to the customer in detail. • We planned the entire project in great detail.
McGrawhill's American Idioms And Phrasal Verbs