Can anyone suggest any good ideas to document complex & massive(100’s of pages) set of business rules and calculations?

Specifically, I am eliciting requirements for a new application, the stakeholders are helpful and providing a lot of long and descriptive process steps, rules and sample spreadsheet calculations. It seems very editorial and not sure if they would be helpful for development. What I see is that most of the steps are mostly automated calculations based on certain conditions. Seems like ideal candidate to use decision trees and business rules engine, but I don’t have much experience with it.

Has anyone had any similar experiences or suggest any good ways to document such large amounts of calculations / business rules them. Any examples would be helpful.

K