One small trap: current never (in normal cases; yes, I know about the antiparallel diodes and how to deal with/avoid them...) actually flows between the input terminals, it flows to the rails. So you will not, except by sheer luck, get 10nA into the (+) terminal and 10nA out of the (-) terminal. You will get 10nA into the (+) terminal and 4nA into the (-) terminal, both returned to VEE.