Consolidated Statements of Equity (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Stockholders' Equity [Abstract]
Cash dividends on Class A and Class B common stock (in usd per share)	$ 0.9775	$ 1.0650	$ 1.0450