Employee Benefit Plans (Net Periodic Benefit Costs) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Benefits
Net periodic benefit costs [Abstract]
Service costs	$ 60	$ 61
Interest costs	96	106
Expected return on plan assets	(133)	(130)
Amortization of net actuarial (gains) losses	44	49
Amortization of prior service costs (credit)	0	0
Net periodic benefit costs (credit)	67	86
Other Postretirement Benefits
Net periodic benefit costs [Abstract]
Service costs	1	1
Interest costs	11	19
Expected return on plan assets	(18)	(18)
Amortization of net actuarial (gains) losses	(8)	0
Amortization of prior service costs (credit)	(5)	(5)
Net periodic benefit costs (credit)	$ (19)	$ (3)