My friends have been making fun of me because I'm insisting on trying to get the Scala plugin working without the no-longer-developed sbt plugin. And I think I've just done it.
Set 'Use sbt shell' so you can see what's going on. And you'll see while most of the proxy settings are transported to JVM arguments (-Dhttp.proxyHost -Dhttp.proxyUser -Dhttp.proxyPassword) -Dhttp.nonProxyHosts is not. You have to specify it in Settings/Build Tools/sbt/JVM Arguments. For whatever reason, it gets quotes around it, but they don't seem to hurt anything.
java -agentlib:jdwp=transport=dt_socket,address=localhost:55147,suspend=n,server=y -Xdebug -server -Xmx4096M -Dsbt.override.repos=true -Dsbt.repository.config=project\sbt.repositories "-Dhttp.nonProxyHosts=*.baml.com|*.bankofamerica.com|*.ml.com" -Dhttp.proxyHost=internalProxy -Dhttp.proxyPort=8080 -Dhttp.proxyUser=yourMother -Dhttp.proxyPassword=I♡pupπes -Dfile.encoding=UTF-8 -Didea.runid=2017.2 -Didea.managed=true -jar project\sbt-launch.jar idea-shell
You don't seem to be able to operate without a proxy. Something in IntelliJ insists on going out to bintray, and sbt itself seems very attached to typesafe.
I also had the garbage collector blow up the heap during import in 2017.3, so I set the Maximum heap size to 4096 MB.