The Fluke 87 (original series) manual actually states how much it is: "The input impedance of the function (400 mV range) is changed from 10 megohms to greater than 4000 megohms." But that's for the 87 not the 87V.
Googling turned up only one source for this datum. Fortunately, it's pretty reliable since it comes from one of Fluke's own engineers. Chuck Newcombe tells us that,
If you want to measure a high impedance dc source, and the anticipated voltage is less than 600 mV, the Fluke 87V has a power-up feature that can save the day. Just hold the 'Hz %' button while you turn the meter on to remove the 10 megohm divider from the circuit. Now, the dmm's loading effect is greatly reduced, with an input resistance of 1000 Megohms or more.So there. For the 87V high input impedance mode value is at least 1000 Megs.
Why this isn't in the specs section of the manual boggles the mind.