Income Taxes (Reconciliation of Unrecognized Tax Benefits) (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]
Gross unrecognized tax benefits, January 1	$ 1,443	$ 1,441	$ 1,455
Additions based on tax positions related to the current year	121	74	67
Additions based on tax positions related to prior years	319	72	98
Additions from acquired subsidiaries	0	13	0
Reductions for tax positions of prior years	(251)	(66)	(101)
Reductions due to expiration of statutes of limitations	(70)	(44)	(41)
Settlements with tax authorities	(65)	(47)	(37)
Gross unrecognized tax benefits, December 31	1,497	1,443	1,441
Unrecognized tax benefits positions paid	(688)	(340)	(305)
Liability for uncertain tax positions	$ 809	$ 1,103	$ 1,136