'Detailing' has a far narrower definition:
detailing {noun} [singular form, non-count]:
small decorative features on a building, garment, or work of art.
- keystones and other masonry detailing
[ODE courtesy of Google]
(ODE labels this usage [noun], and although the plural is rare and hardly ever used with a numeral, this is probably the best designation. The act of detailing is another, separate sense.)
'Detail' is highly polysemous:
detail {noun}:
1 [non-count]: extended treatment of or [in-depth] attention to particular items
- explaining without going into detail
- giving careful attention to detail
2: a part of a whole: such as
a [count]: a small and subordinate part [= PARTICULAR]
- planned the wedding down to the smallest detail
also: a reproduction of such a part of a work of art
b [count though most often used without a numeral]: a part considered or requiring to be considered separately
from the whole
- discussing the details of the building's columns
c [non-count]: the small elements that collectively constitute a work
of art
- admire the great detail of his work
d [non-count] [photography]: the small elements of an image
corresponding to those of the subject
- strong lighting to achieve clarity of detail
3
a : selection of a person or group for a particular task (as in
military service)
b(1) [count]: the person or group selected
- a member of a security detail
- police details at the parade
(2) [non-count]: the task to be performed
- were assigned to kitchen detail
[Merriam-Webster; amended]
So investigating the details of a building's columns would include inspecting the detailing on the building's columns (as well as determining their age, composition, architect, height ...).
One could argue that definition 2c for detail matches the definition of detailing, but 'detailing' has a more tangible connotation.
