Formal:
* Developed
* Expanded
* Amplified
* Expounded
* Detailed
* Explicated
Informal:
* Spelled out
* Went into detail
* Explained in more detail
* Gave more information
* Broke down
Specific to writing:
* Expatiated (to write or speak at length)
* Discussed
* Argued
* Illustrated
The best synonym will depend on the specific meaning you are trying to convey.