As Buildersmate says, a design and a patent are two different things. If you are unclear about what is what, then consider a new shampoo. The formula for the shampoo would be patented; the name of the shampoo might be a trade mark; and the shape of the bottle is a design. All are different rights but rights to a patent have to be registered while the other two can be automatic. A book or piece of music is subject to the laws of copyright, also an automatic right.
Your contract of employment may give details of ownership of IP rights, but if not the usual situation is that if the idea relates to your area of work, whether you developed your idea in your own time or your employer's, then they have claim to the rights. If the idea does not directly relate to your work then you have a stronger claim to rights.
HOWEVER - if you want to make any claim to the rights for yourself, then get the non-disclosure agreement signed BEFORE you give any details. That way you are on safer ground as, if you tell them first then ask them to sign the agreement, they can say no if they think they can cash in on your idea.