Definition of differential input resistance
I found on Sedra's book that \$R_{id}\$ could be determinated by the following way:

1) Plug in a voltage source \$v_{id}\$ on the initial terminals;

2) Link the two input terminals of the differential amplifier.

As this image demonstrates:



And then just calculate \$R_{id}=\frac{v_{id}}{{i_I}}\$