Income Taxes (Schedule of Unrecognized Tax Benefits Roll Forward) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits – January 1	$ 1,557	$ 1,617	$ 1,312
Gross increases – tax positions in prior periods	3	17	38
Gross decreases – tax positions in prior periods	(44)	(348)	(25)
Gross increases – current-period tax positions	220	344	292
Settlements		(73)
Unrecognized tax benefits – December 31	$ 1,736	$ 1,557	$ 1,617