Does my employer own every idea I conceptualize?

Yeah, I signed the contract, but it still doesn't justify why my employer should own every new idea I come up with. For goodness sake, I started thinking and conceptualizing way before I started working for him. Most industries apparently apply this slave clause and I'd like to know - how is this different from the bad old slave days when the slave would invent something and the slave master claims all the credit and the royalties for the slave's novel idea?

Yes, I don't mind surrendering a percentage of my intellectual property if I used my current employer's resources to enhance my concept, but if it's something that I work on at home after hours, then what business does my employer have claiming ownership of MY ideas?