You must specify the Multiline property when you constrcut the object. You can not set this afterwards.
From the wxWidgets documentation it mentions this specifically:
Note that alignment styles (wxTE_LEFT, wxTE_CENTRE and wxTE_RIGHT) can be changed dynamically after control creation on wxMSW and wxGTK. wxTE_READONLY, wxTE_PASSWORD and wrapping styles can be dynamically changed under wxGTK but not wxMSW. The other styles can be only set during control creation.
Instead of:
detail->SetWindowStyle(wxTE_MULTILINE);
this should work:
wxTextCtrl(this,wxID_ANY, "", wxDefaultPosition, wxDefaultSize, wxTE_MULTILINE);