A cruise missile requires both detection and interception. The detection side is classified, and I don't have access to the information. I know this because I've asked the question and have not had access to the information. But I believe that around North America the detection of cruise missiles would be done by the airborne warning and control craft, which the United States owns but which Canada helps man. They're based in Oklahoma.
I believe the north warning system also has a very limited ability to detect against cruise missiles, but it is certainly optimized for that function. The north warning system along the 70th parallel was set up to detect ballistic missiles. Cruise missiles are much harder to detect, because they fly low to the ground.