IIRC, the precision is actually platform dependent. Anyway, to the question: I think you are looking at the wrong details.
>>> print numpy.finfo(numpy.longdouble)
Machine parameters for float128
---------------------------------------------------------------------
precision= 18 resolution= 1e-18
machep= -63 eps= 1.08420217249e-19
negep = -64 epsneg= 5.42101086243e-20
minexp=-16382 tiny= 3.36210314311e-4932
maxexp= 16384 max= 1.18973149536e+4932
nexp = 15 min= -max
---------------------------------------------------------------------
eps
is the smallest positive number fulfilling 1.0 + eps != 1.0
, so if your answer is in the order of 1, then you have 18 significant decimals. Due to the nature of floating point arithmetic, this changes with the value of the number itself, but you will always get 18 significant figures (however many decimals that is).