Benefit Plans (Schedule of Components of Net Periodic Pension Cost) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Termination, settlement and curtailment		$ 27
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 75	76	$ 86
Interest cost	288	281	337
Expected return on plan assets	(601)	(553)	(539)
Amortization of net loss	197	171	234
Amortization of prior service cost (credit)	4	5	7
Termination, settlement and curtailment	86	34	8
Net periodic benefit cost	49	14	133
Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	16	17	18
Interest cost	76	77	100
Expected return on plan assets	0	0	0
Amortization of net loss	25	25	43
Amortization of prior service cost (credit)	(38)	(39)	(39)
Termination, settlement and curtailment	0	(2)	0
Net periodic benefit cost	$ 79	$ 78	$ 122