Exhibit 99.1
                             JOINT FILING AGREEMENT

The undersigned hereby agree that the foregoing statement on Schedule 13G is
filed on behalf of each of the undersigned in accordance with the provisions of
Rule 13d-1(k) under the Securities Exchange Act of 1934, as amended, and that
all subsequent amendments to this statement on Schedule 13G may be filed on
behalf of each of the undersigned without the necessity of filing additional
joint filing agreements.

Dated:  December 21, 2007


SIGMA CAPITAL MANAGEMENT, LLC


By: /s/ Peter Nussbaum
   -------------------------------
Name:   Peter Nussbaum
Title:  Authorized Person


SIGMA CAPITAL ASSOCIATES, LLC


By: /s/ Peter Nussbaum
   -------------------------------
Name:   Peter Nussbaum
Title:  Authorized Person


STEVEN A. COHEN


By: /s/ Peter Nussbaum
   -------------------------------
Name:   Peter Nussbaum
Title:  Authorized Person