In many cases, employee work product – such as reports, drawings, designs, or other written content – is subject to United States copyright law, and the default rule is that the work product belongs to the employer. This is known as the “works made for hire” doctrine.
In most circumstances, the copyright over a creative work initially vests in the “author” of the work. In the case of works made for hire, however, the law provides that:

[T]he employer or other person for whom the work was prepared is considered the author for purposes of this title, and, unless the parties have expressly agreed otherwise in a written instrument signed by them, owns all of the rights comprised in the copyright.

17 U.S.C. § 201.
The works made for hire doctrine is somewhat different from the rules that apply to inventions, which are subject to U.S. patent laws. In many states, there are laws that impose certain requirements when an employer requires an employee to assign ownership of inventions to the employer.
Takeaway: When U.S. copyright laws apply, employee work product that qualifies as “works made for hire” presumptively belongs to an employer, unless there is an express written agreement to the contrary.