KNOW ALL PERSONS BY THESE PRESENTS, that the undersigned hereby
constitutes, designates and appoints Paul Ghaffari, David R. Stewart and William
Benack as his true and lawful attorneys-in-fact and agents, each with full power
of substitution and resubstitution and full power to act alone and without the
other, for the undersigned and in the undersigned's name, place and stead, in
any and all capacities, to execute, acknowledge, deliver and file any and all
filings required by the Securities Exchange Act of 1934, as amended, including
Sections 13 and 16 of such Act, and the rules and regulations thereunder, and
requisite documents in connection with such filings, including but not limited
to Forms 3, 4 and 5, Schedules 13D and 13G and Form 13H under such act and any
amendments thereto.

     This power of attorney shall be valid from the date hereof until revoked by
the undersigned.

     IN WITNESS WHEREOF, the undersigned has executed this instrument as of the
8th day of February, 2013.

                                    /s/ Paul G. Allen
                                    Paul G. Allen