Employee Benefit Plans (Net Periodic Benefit Costs) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Benefits
Net periodic benefit costs [Abstract]
Service costs	$ 57	$ 58	$ 115	$ 118
Interest costs	104	96	208	192
Expected return on plan assets	(123)	(134)	(245)	(267)
Amortization of net actuarial (gains) losses	48	44	96	88
Amortization of prior service costs (credit)	(4)	0	(8)	0
Net periodic benefit costs (credit)	82	64	166	131
Other Postretirement Benefits
Net periodic benefit costs [Abstract]
Service costs	1	2	2	3
Interest costs	13	15	26	26
Expected return on plan assets	(18)	(18)	(34)	(36)
Amortization of net actuarial (gains) losses	(12)	(9)	(24)	(17)
Amortization of prior service costs (credit)	(2)	(4)	(5)	(9)
Net periodic benefit costs (credit)	$ (18)	$ (14)	$ (35)	$ (33)