Why does Math.ceil return a double?

When I call Math.ceil(5.2) the return is the double 6.0. My natural inclination was to think that Math.ceil(double a) would return a long. From the documentation:

ceil(double a)

Returns the smallest (closest to negative infinity) double value that is not less than the argument and is equal to a mathematical integer.

But why return a double rather than a long when the result is an integer? I think understanding the reason behind it might help me understand Java a bit better. It also might help me figure out if I'll get myself into trouble by casting to a long, e.g. is

long b = (long)Math.ceil(a);

always what I think it should be? I fear there could be some boundary cases that are problematic.