Except in the context of software development, specifically, it's not generally just "an idea." It's the sum total of all unpublished and unannounced informatio

Except in the context of software development, specifically, it's not generally just "an idea." It's the sum total of all unpublished and unannounced information of unreleased products that you're being given access to as a developer. There may not even be any plans to file patents: you have to take affirmative steps to maintain trade secret protection, as well. But, among other things, trade secrets have to be reasonably specific, certainly more so than "everything you learn about our products in the course of employment", and, well, "secrets" to begin with (not merely "unannounced" or "unpublished"). Confidential business information like customer lists, on the other hand, tends to be protected even in the absence of a written agreement; a good contract lawyer can quickly pick out the potentially troubling bits of an employment-related contract from the mass of "mostly harmless" boilerplate that merely restates your obligations under various laws (often including amusingly obvious items like not stealing physical property from the company and returning company-owned equipment upon request). Finally, many companies have agreements with third parties that require measures to ensure employees and contractors given access to third-party confidential information (of various sorts) hold it in confidence.