You are taking the risk that you don't get to connect in DTW on the return, and are instead sent via MSP or MEM or straight to EWR on CO. It's a small risk, but if it happens, you are really screwed, stuck with a walk-up ticket back to Detroit.

If I were you I would pay the $100 or so to fly the whole NW trip plus NYC-DTW. It's still a lot less than $1000 total.

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"The minute a Bundy has good luck he immediately starts building up an equal amount of bad luck." -- Al Bundy, Married With Children