WASHINGTON (CNN) -- After three months of negotiations, a bipartisan group of senators said Thursday they have crafted an immigration bill that would give the estimated 12 million undocumented immigrants a path to citizenship.

"The agreement we just reached is the best possible chance we will have to secure our borders, bring millions of people out of the shadows and into the sunshine of America," Sen. Edward Kennedy told reporters.

The Massachusetts Democrat serves on the Judiciary Committee, where he is the chairman of the Immigration Subcommittee.

According to a summary provided by Kennedy, undocumented workers who arrived in the United States before Jan 1, 2007 would be given immediate work authorization, granted a "Z" visa and placed on a path to permanent residence.

Heads of household would have to return to their home country within eight years; they would be guaranteed the right to return. In addition, applicants would have to pay a $5,000 penalty.

At the same time, the number of Border Patrol agents would be doubled, border fencing would be strengthened and employers who hire undocumented workers would be fined.

Once those enforcement provisions are put into place -- a process estimated by Department of Homeland Security Secretary Michael Chertoff to last 18 months -- a guest-worker program would be initiated under which 400,000 temporary workers per year would be granted a "Y" visa. The two-year visas would require they return home for a year, then allow them to re-enter for an additional two-years. The process be repeated twice more.