Employee Benefit Plans (Net Periodic Benefit Costs) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits
Net periodic benefit costs [Abstract]
Service costs	$ 61	$ 73	$ 122	$ 145
Interest costs	109	109	217	220
Curtailment costs	0	0	0	0
Expected return on plan assets	(130)	(127)	(262)	(254)
Amortization of net actuarial (gains) losses	48	50	97	99
Amortization of prior service costs (credit)	(1)	0	(1)	0
Net periodic benefit costs	87	105	173	210
Other Postretirement Benefits
Net periodic benefit costs [Abstract]
Service costs	2	2	3	4
Interest costs	20	21	39	42
Curtailment costs	0	(3)	0	27
Expected return on plan assets	(18)	(19)	(36)	(37)
Amortization of net actuarial (gains) losses	0	2	0	4
Amortization of prior service costs (credit)	(6)	(1)	(11)	(4)
Net periodic benefit costs	$ (2)	$ 2	$ (5)	$ 36