Although this idea was "swiftly condemned", I have to think this is what is known as "floating an idea" in Washington. Many of the provisions of the USA PATRIOT Act were also rejected separately by Congress on earlier submissions but were able to easily gain passage soon after 9/11. I wouldn't be too surprised if "lifetime detentions" either snuck into another bill or became a de facto process without Congressional approval. From Reuters:

WASHINGTON (Reuters) - A reported U.S. plan to keep some suspected terrorists imprisoned for a lifetime even if the government lacks evidence to charge them in courts was swiftly condemned on Sunday as a "bad idea" by a leading Republican senator.

The Pentagon and the CIA have asked the White House to decide on a more permanent approach for those it was unwilling to set free or turn over to U.S. or foreign courts, the Washington Post said in a report that cited intelligence, defense and diplomatic officials.

Some detentions could potentially last a lifetime, the newspaper said.