Employee Benefit Plans (Net Periodic Benefit Costs) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Benefits
Net periodic benefit costs [Abstract]
Service costs	$ 62	$ 67	$ 184	$ 212
Interest costs	106	101	318	316
Curtailment costs	3	(1)	3	(1)
Expected return on plan assets	(129)	(138)	(387)	(388)
Amortization of net actuarial (gains) losses	46	45	143	143
Amortization of prior service costs (credit)	0	0	(1)	0
Net periodic benefit costs	88	74	260	282
Other Postretirement Benefits
Net periodic benefit costs [Abstract]
Service costs	1	4	4	8
Interest costs	19	20	57	62
Curtailment costs	2	(1)	2	15
Expected return on plan assets	(18)	(19)	(54)	(56)
Amortization of net actuarial (gains) losses	0	2	0	7
Amortization of prior service costs (credit)	(6)	(1)	(17)	(5)
Net periodic benefit costs	$ (2)	$ 5	$ (8)	$ 31