Restructuring Charges (Schedule of Changes in Restructuring Reserves) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Restructuring Cost and Reserve [Line Items]
Balance at beginning of period		$ 15	$ 11	$ 15	$ 18
Restructuring charges		12	7	39	32
Payments		(17)	(6)	(44)	(33)
Adjustments for non-cash charges	[1]				(5)
Balance at end of period		10	12	10	12
Total restructuring charges incurred to date		177		$ 177
Expected completion date of restructuring activities				Dec. 31, 2018
Employee costs
Restructuring Cost and Reserve [Line Items]
Balance at beginning of period		4	6	$ 8	10
Restructuring charges		2	1	7	5
Payments		(3)	(1)	(12)	(9)
Adjustments for non-cash charges	[1]				0
Balance at end of period		3	6	3	6
Total restructuring charges incurred to date		69		69
Facilities costs
Restructuring Cost and Reserve [Line Items]
Balance at beginning of period		0	0	0	4
Restructuring charges		1	0	3	5
Payments		(1)	0	(3)	(9)
Adjustments for non-cash charges	[1]				0
Balance at end of period		0	0	0	0
Total restructuring charges incurred to date		20		20
Technology costs
Restructuring Cost and Reserve [Line Items]
Balance at beginning of period		11	3	6	0
Restructuring charges		8	5	28	17
Payments		(12)	(4)	(27)	(8)
Adjustments for non-cash charges	[1]				(5)
Balance at end of period		7	4	7	4
Total restructuring charges incurred to date		54		54
Other costs
Restructuring Cost and Reserve [Line Items]
Balance at beginning of period		0	2	1	4
Restructuring charges		1	1	1	5
Payments		(1)	(1)	(2)	(7)
Adjustments for non-cash charges	[1]				0
Balance at end of period		0	$ 2	0	$ 2
Total restructuring charges incurred to date		34		34
Minimum
Restructuring Cost and Reserve [Line Items]
Total expected restructuring charges	[2]	185		185
Minimum | Employee costs
Restructuring Cost and Reserve [Line Items]
Total expected restructuring charges	[2]	70		70
Minimum | Facilities costs
Restructuring Cost and Reserve [Line Items]
Total expected restructuring charges	[2]	20		20
Minimum | Technology costs
Restructuring Cost and Reserve [Line Items]
Total expected restructuring charges	[2]	60		60
Minimum | Other costs
Restructuring Cost and Reserve [Line Items]
Total expected restructuring charges	[2]	35		35
Maximum
Restructuring Cost and Reserve [Line Items]
Total expected restructuring charges	[2]	190		190
Maximum | Employee costs
Restructuring Cost and Reserve [Line Items]
Total expected restructuring charges	[2]	70		70
Maximum | Facilities costs
Restructuring Cost and Reserve [Line Items]
Total expected restructuring charges	[2]	20		20
Maximum | Technology costs
Restructuring Cost and Reserve [Line Items]
Total expected restructuring charges	[2]	65		65
Maximum | Other costs
Restructuring Cost and Reserve [Line Items]
Total expected restructuring charges	[2]	$ 35		$ 35

[1] Adjustments for non-cash charges primarily relate to impairments of previously capitalized software costs in Technology Costs.

[2] Restructuring activities are expected to be substantially completed by 12/31/2018.