Condensed Consolidated Statements of Operations (Unaudited) (Parenthetical) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Interest income $ 30 $ 18 $ 81 $ 38