This is defined in twittertypes.go
Other 'id' types are int64, so maybe this is an oversight?
This caused the error 'json: cannot unmarshal number into Go value of type string' when unmarshalling JSON returned by the Twitter REST API. The error was fixed by changing the type.
BTW, twitter also returns a 'in_reply_to_status_id_str' value if a string is needed, although this value is not set in this file.