Convert long NSString to NSNumber

Convert long NSString to NSNumber

I am unable to reliably convert longer NSString to NSNumber. Specifically,
I am converting MPMediaEntityPropertyPersistentID as a string to a
NSNumber Sometimes it works, usually it doesn't.
Conversion code:
NSString *keke = [jsonArray objectForKey:@"next"];
NSNumberFormatter * f = [[NSNumberFormatter alloc] init];
[f setNumberStyle:NSNumberFormatterDecimalStyle];
NSNumber *persistentIDasNumber = [f numberFromString:keke];
Here is an example of a successful string to number conversion:
String: 3813955856659208324
Number: 3813955856659208324
And here is an unsuccessful conversion:
String: 12790162104953153719
Number:1.279016210495315e+19
It's close but what is happening at the end? Is it too large?