Employee Benefit Plans (Net Periodic Benefit Costs) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits
Net periodic benefit costs [Abstract]
Service costs	$ 58	$ 61	$ 118	$ 122
Interest costs	96	106	192	212
Expected return on plan assets	(134)	(128)	(267)	(258)
Amortization of net actuarial (gains) losses	44	48	88	97
Amortization of prior service costs (credit)	0	(1)	0	(1)
Net periodic benefit costs (credit)	64	86	131	172
Other Postretirement Benefits
Net periodic benefit costs [Abstract]
Service costs	2	2	3	3
Interest costs	15	19	26	38
Expected return on plan assets	(18)	(18)	(36)	(36)
Amortization of net actuarial (gains) losses	(9)	0	(17)	0
Amortization of prior service costs (credit)	(4)	(6)	(9)	(11)
Net periodic benefit costs (credit)	$ (14)	$ (3)	$ (33)	$ (6)