CONSOLIDATED BALANCE SHEETS (Parenthetical) - shares shares in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Treasury stock, shares	1	1
Class A common stock
Common stock, shares issued	176	180	185	194
Class B common stock
Common stock, shares issued	687	689	689	693