Before attempting any decryption when a decrypter is available, it is recommended to back up the original encrypted files first and perform a test decryption on sample copies of the encrypted files in case something goes awry. If the files become damaged or corrupted, you will have a back up to try again. If in an enterprise environment, disconnect from the network first. There is no way of knowing what kind of decryption program the criminals are sending you...if it will work or if it may be coded with additional malware.
Most crypto malware ransomware is typically programmed to automatically remove itself...the malicious files responsible for the infection...after the encrypting is done since they are no longer needed. That explains why many security scanners do not find anything after the fact. The encrypted files do not contain malicious code so they are safe. Unfortunately, most victims do not realize they have been infected until the ransomware displays the ransom note and the files have already been encrypted. In some cases there may be no ransom note and discovery only occurs at a later time when attempting to open an encrypted file. As such, they don't know how long the malware was on the system before being alerted or if other malware was downloaded and installed along with the ransomware. If other malware was involved it could still be present so be sure to perform full scans with your anti-virus.
If your antivirus did not detect and remove anything, additional scans should be performed with other security programs like Emsisoft Anti-Malware, Malwarebytes 3.0, Zemana AntiMalware, RogueKiller Anti-malware and HitmanPro. You can also supplement your anti-virus or get a second opinion by performing an Online Virus Scan.
If you need individual assistance only with removing the malware infection, follow the instructions in the Malware Removal and Log Section Preparation Guide...all other questions or comments should be posted in the support topics. When you have done that, start a new topic and post your logs in the Virus, Trojan, Spyware, and Malware Removal Logs forum, NOT here, for assistance by the Malware Response Team.