JDK documentation in IntelliJ IDEA on Mac OS X

I had to combine the responses I got to this point and add a few extra details so I’m answering my own question.

1. Search for “Java developer” from Downloads for Apple Developers
2. Download and install “Java for Mac OS X 10.6 Update 9 Developer Package” (or later).
3. In IntelliJ, open File -> Project Structure.
4. Click on “SDKs” under “Platform Settings”.

5. Add the following paths under the “Documentation Paths” tab (the paths may be different based on which JDK you’re using):

   /Library/Java/JavaVirtualMachines/1.6.0_29-b11-402.jdk/Contents/Home/docs.jar!/docs/api  
   /Library/Java/JavaVirtualMachines/1.6.0_29-b11-402.jdk/Contents/Home/appledocs.jar!/appledoc/api
   

6. Add the following path under the Sourcepath tab:

   /Library/Java/JavaVirtualMachines/1.6.0_29-b11-402.jdk/Contents/Home/src.jar!/src
   

Thanks, @Yishai for the download location and @Gareth Davis for the location of the documentation after installing.

Update for 10.7/10.8 [Mountain] Lion:

Steps are the same except for these changes:

1. In step 2, download “Java for OS X 2012-005 Developer Package” (or later).

2. Use these paths instead in step 5.

   /Library/Java/JavaVirtualMachines/1.6.0_35-b10-428.jdk/Contents/Home/docs.jar!/docs/api  
   /Library/Java/JavaVirtualMachines/1.6.0_35-b10-428.jdk/Contents/Home/appledocs.jar!/appledoc/api
   

3. Use these paths instead in step 7.

   /Library/Java/JavaVirtualMachines/1.6.0_35-b10-428.jdk/Contents/Home/src.jar!/src