Although I'm having trouble finding link for double mileage, it is apparently going on. Double miles for all economy class and triple for all business class. I'm not sure double mile is for ICN-DTW-ICN sector only or all flights include DTW-many cities-DTW. Double mile is for Jun 2nd-Sep 30th, where June 2nd is when this nonstop resumes. Also I'm not quite sure at this point if double mileage is for Korean residence only. At least this must be for Korea-departure passengers. I'll update if I get link with t&c.

Cheapest fare that includes nonstop ICN-DTW-ICN roundtrip is roughly $1000, and only through travel agencies in Korea. Most mid-east part of US cities, but XNA (or more?) is excluded. It's 20% discount from 1,200k won, thus 960k, and taxes are 200k. However many websites has additional 60k-70k discount on top of it, so in the end $1000 a.i.. This L-class fare promotion ends end of this month (Mar 31st). Also this promotion is valid for the departures by July 10th.

All these should include nonstop ICN-DTW-ICN.