Excel documents that open in the same instance - drives me..

This has happened three times in my career using Office. With excel, just
like with PP and Word, I want each document/spreadsheet to open as it's own
instance on my desktop. That means if I open a SS and then open another, I do
not want both to appear in the same instance where I have to toggle back and
fourth. I constantly close out of the one I'm in and end up closing out of
all of them. I do not want t o X out of the one I'm viewing - I want it gone
and have the "other" spreadsheet (SS) still there.

There is a way to do this. There is a way to check "ungroup like documents"
or something like that. But for the life of me I cannot find the option to
fix this.

Please HELP!