IntelliJ IDEA and Android Studio (which is based on the open source version of IntelliJ IDEA) are fantastic IDEs for Kotlin (and other languages, such as Java, Scala, Groovy, and others). Both offer autocomplete, formatting and support for Gradle and Maven.

Both IDEs are very well documented, and you can read more about Kotlin support on their websites:

• IntelliJ IDEA (http://kotlinlang.org/docs/tutorials/getting-started.html)
• Android Studio (https://developer.android.com/kotlin/get-started.html)