Measuring Bandwidth of a 1e10 gain TIA
Because of the limitations Kevin mentions, I would suggest a multi-stage attenuator with lower value components : say 100k:10 ohms stepping 100mVrms down to 10uVrms, then 100K to your input. 

These low values should dwarf stray capacitances at your frequencies of interest.

Bandwidth is higher because the first stage has a source impedance of 10 ohms, and the whole network's source Z is 100,010 ohms, into your input capacitance, instead of 1G into the same capacitance. (If that's still not good enough, try a 3-stage network along the same lines).