Stockholders' Equity (Narrative) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Stockholders' Equity Note [Abstract]
Class B common stock repurchased under repurchase program (shares)	3.8
Value of shares repurchased	$ 200
Average price per share repurchased (in dollars per share)	$ 52.67
Remaining authorization under share repurchase program	$ 2,860
Dividends per common share (in dollars per share)	$ 0.18	$ 0.18
Dividends recorded on common stock	$ 69	$ 75
Tax provision on net actuarial gain (loss) and prior service costs related to pension and other postretirement benefit plans	$ 4	$ 8