Benefit Plans (Funded Status) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amounts recognized on the Consolidated Balance Sheets as of December 31:
Other assets	$ 2,261	$ 2,193
Pension Plans, Defined Benefit
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Balance at beginning of period	2,729	2,821
Service cost	0	0	$ 4
Interest cost	103	113	112
Participants' contributions	0	0
Plan amendments	0	0
Actuarial (gain) loss	99	68
Benefits paid	(170)	(173)
Foreign currency translation and other	10	(14)
Settlements	(22)	(86)
Balance at end of period	2,749	2,729	2,821
Change in plan assets:
Balance beginning of period	2,193	2,267
Actual return on plan assets	221	193
Company contributions	29	9
Participants' contributions	0	0
Benefits paid	(170)	(173)
Foreign currency translation and other	10	(15)
Settlements	(22)	(88)
Balance end of period	2,261	2,193	2,267
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status	(488)	(536)
Amounts recognized on the Consolidated Balance Sheets as of December 31:
Other assets	4	4
Other liabilities	(492)	(540)
Net amount recognized	(488)	(536)
Amounts recognized in Accumulated other comprehensive income, not yet recognized in net periodic cost (benefit):
Prior service credit	0	0
Net actuarial loss	987	999
Net amount recognized	987	999
Other Postretirement Benefits Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Balance at beginning of period	15	23
Service cost	0	0
Interest cost	0	0	1
Participants' contributions	4	4
Plan amendments	0	0
Actuarial (gain) loss	(1)	(6)
Benefits paid	(7)	(7)
Foreign currency translation and other	0	1
Settlements	0	0
Balance at end of period	11	15	23
Change in plan assets:
Balance beginning of period	0	0
Actual return on plan assets	0	0
Company contributions	3	3
Participants' contributions	4	4
Benefits paid	(7)	(7)
Foreign currency translation and other	0	0
Settlements	0	0
Balance end of period	0	0	$ 0
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status	(11)	(15)
Amounts recognized on the Consolidated Balance Sheets as of December 31:
Other assets	0	0
Other liabilities	(11)	(15)
Net amount recognized	(11)	(15)
Amounts recognized in Accumulated other comprehensive income, not yet recognized in net periodic cost (benefit):
Prior service credit	(2)	(4)
Net actuarial loss	(4)	(3)
Net amount recognized	$ (6)	$ (7)