Why does IsLiteral return false for decimal?

The following program will print the fields and whether the are constant or not using IsLiteral

public static class Program
{
    public static void Main(string[] args)
    {
        foreach (var field in typeof(Program).GetFields())
        {
            System.Console.WriteLine(field.Name + " IsLiteral: " + field.IsLiteral);
        }

        System.Console.ReadLine();
    }

    public const decimal DecimalConstant = 99M;
    public const string StringConstant = "StringConstant";
    public const int IntConstant = 1;
    public const double DoubleConstant = 1D;
}

It works correctly for all types, except for decimal is will return false.

Can anyone explain this behavior? And is there a better way to see if a field in constant?