CONSOLIDATED BALANCE SHEETS (Parenthetical) - shares shares in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Treasury stock, shares	1	1
Common Stock | Class A Common Stock:
Common stock, shares issued	173	180	194	201
Common Stock | Class B Common Stock:
Common stock, shares issued	687	689	693	705