I don't know what "I think it would be a good idea" means!
Mycroft Holmes: I have access to the top level of the MI5 archive.
Mary Morstan: [already looking at that archive via her phone] Yep, that's where I'm looking.
Mycroft Holmes: What do you think of MI5 security?
Mary Morstan: I think it would be a good idea.
I don't know exactly what the last line of Mary Morstan means.
Also, in that sentence, what does "it" refer to?
Please help me!