Ideal LC circuit decays over time in LTspice
You need to set both Series and Parallel resistance to 0.
It makes no sense, but it works. Setting only the Series resistance will not solve OP's problem.

EDIT  


  For general component values LTspice will accept numbers that range in magnitude from as large as ± 1.798 x 10+308 down to as small as ± 2.225 x 10−308.  Values exceeding this range are interpreted as ± infinity or as zero.
  Source


If you set a voltage source to a voltage of {1.8*10^308}, it will plot zero volts.

It seems it also works the other way around (setting Parallel resistance to 0), but setting Parallel resistance to {2*10^308} also works. The latter makes more sense.