The IdP ID. For white listed IdPs it's a short domain name e.g.
google.com, aol.com, live.net and yahoo.com. If the "providerId" param
is set to OpenID OP identifer other than the whilte listed IdPs the OP
identifier is returned. If the "identifier" param is federated ID in
the createAuthUri request. The domain part of the federated ID is
returned.