Summer is a high season for travel to Europe. It is going to be tougher than in off-season months.

I would suggest finding availability on individual flights. Start with the TATL segment: from DTW you have flights to CDG and AMS. Then find the feeder flights between IST and CDG/AMS.

Start with the simple itineraries and then you can get more creative. You can find a flight to a city in Europe and then use other SkyTeam partners to get to IST (Alitalia, Aeroflot, CSA, Tarom, etc)

Does DL fly JFK-IST? If so, check that leg as well and find DTW-JFK options later.

Good luck!