Integration and Restructuring Costs (Restructuring Activities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restructuring Reserve [Roll Forward]
Liability at beginning of period	$ 65	$ 93	$ 93
Expense Incurred	161	40	44
Cash Paid	(114)	(71)	(44)
Other	0	3
Liability at end of period	112	65	93
Severance
Restructuring Reserve [Roll Forward]
Liability at beginning of period	15	32
Expense Incurred	137	31
Cash Paid	(77)	(49)
Other	(2)	1
Liability at end of period	73	15	32
Future rent under non-cancelable leases and other costs
Restructuring Reserve [Roll Forward]
Liability at beginning of period	50	61
Expense Incurred	24	9
Cash Paid	(37)	(22)
Other	2	2
Liability at end of period	$ 39	$ 50	$ 61