Employee Benefit Plans (Net Periodic Benefit Costs) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Benefits
Net periodic benefit costs [Abstract]
Service costs	$ 62	$ 58
Interest costs	89	104
Expected return on plan assets	(132)	(122)
Amortization of net actuarial (gains) losses	45	48
Amortization of prior service costs (credit)	(4)	(4)
Net periodic benefit costs (credit)	60	84
Other Postretirement Benefits
Net periodic benefit costs [Abstract]
Service costs	1	1
Interest costs	10	13
Expected return on plan assets	(15)	(16)
Amortization of net actuarial (gains) losses	(19)	(12)
Amortization of prior service costs (credit)	(1)	(3)
Net periodic benefit costs (credit)	$ (24)	$ (17)