It is defined as the ratio of the maximum current amplitude (15 mA) to the dark current.
15 (mA) / 100(pA) = 150E+6, that is 150 millions.
It is a huge dynamic range. However, the limitation in this case will be limited NOT by the dynamic range of the photodiode itself, but by the noise properties of the POWER SUPPLY. Therefore, we recommend to use very low noise power supplies. The best solution is using a battery, or rechargeable battery. They have very low level of noise, probably 1E-5 or so,. This value will determine the dynamic range.