I left the company one year ago. However, while working there I developed an Idea which seems patentable. I did not submitted that while working in that company. Now I have moved to other company and want to patent my idea. Whom should I approach for the patent - old employer or current employer? Please note - the people in my previous company know that I developed that idea while working with them.
Patent filing requires that you disclose names of everyone involved in the invention. If the novel and patent-able portion is not your sole invention, then you need to approach the co-inventors and get their signature.
You also need to check your employment agreement to see if your former employer has the rights to your invention. If yes, then when you file your application, you need to assign it to your former employer. If there is no written agreement, the law gets complicated, and you should check with an attorney.
Since the idea was developed prior to your new employment, your current employer can't claim any right to your invention.
Don't just check your employer agreement - if the language wasn't there it may be in the NDA (Non-Disclosure Agreement) which either accompanied your Employer Agreement or maybe your first visit when you interviewed there. If you don't have copies of these - the HR should be able to give them to you.
It would be quite surprising if the language was not there. It might look something like this: "any work performed or intellectual property conceived of while at Company A belongs solely to Company A"