I am trying not to make this up, but the usage of the apostrophe in modern (ok, in living memory) English to indicate possession is a before living memory instance of its regular usage as the marker of a contraction in spoken English compared to written English.

Back in the day, English had an honest-to-Goddess genitive case, with the -es ending. The pluralising -s is applied first: a cowes milk, the cowses barn.

The apostrophe replaces the e of -es for the singular and the whole of -es for the plural: a cow's milk; the cows' barn.

So what do you do with singular words which happen to end in s in the first place? Often but not always proper nouns, like bus or James or Idris.

You should see what you say. If you pronounce the possessive "Idris", write "Idris'". If you pronounce the possessive "Idrises", write "Idris's".