Utterson is suspicious of Hyde, how does Jekyll allay his suspicions?

From the text:

My good Utterson,” said the doctor, “this is very good of you, this is downright good of you, and I cannot find words to thank you in. I believe you fully; I would trust you before any man alive, ay, before myself, if I could make the choice; but indeed it isn’t what you fancy; it is not as bad as that; and just to put your good heart at rest, I will tell you one thing: the moment I choose, I can be rid of Mr. Hyde.