Pew is out with the detailed report of their 2014 survey on religion in America. Their last survey was in 2007, and the report compares all the current numbers to those. The two headline numbers are that 89% of Americans believe in God, down from 92%, and that 23% are religiously "unaffiliated," up from 16%. "Unaffiliated" just means that these people do not consider themselves members of any particular religious faith; 61% of the unaffiliated believe in God.
What's with the young Millennials who believe in Hell but don't consider religion very important in their lives? This strikes me as a major contradiction. Maybe they believe in Hell but think it is only for serial killers and evil dictators, so makes little difference to them. But still.