Employee Benefit Plans (Net Periodic Benefit Costs) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Benefits
Net periodic benefit costs [Abstract]
Service costs	$ 58	$ 60
Interest costs	104	96
Expected return on plan assets	(122)	(133)
Amortization of net actuarial (gains) losses	48	44
Amortization of prior service costs (credit)	(4)	0
Net periodic benefit costs (credit)	84	67
Other Postretirement Benefits
Net periodic benefit costs [Abstract]
Service costs	1	1
Interest costs	13	11
Expected return on plan assets	(16)	(18)
Amortization of net actuarial (gains) losses	(12)	(8)
Amortization of prior service costs (credit)	(3)	(5)
Net periodic benefit costs (credit)	$ (17)	$ (19)