ido-default-file-method

ido-default-file-method is a variable defined in `ido.el'.
Its value is

raise-frame

Documentation:
How to visit a new file when using `ido-find-file'.
Possible values:
`selected-window' Show new file in selected window
`other-window' Show new file in another window (same frame)
`display' Display file in another window without selecting to it
`other-frame' Show new file in another frame
`maybe-frame' If a file is visible in another frame, prompt to ask if you
you want to see the file in the same window of the current
frame or in the other frame
`raise-frame' If a file is visible in another frame, raise that
frame; otherwise, visit the file in the same window

You can customize this variable.