Employee Benefit Plans (Net Periodic Benefit Costs) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension Benefits
Net periodic benefit costs [Abstract]
Service costs	$ 63	$ 62
Interest costs	78	89
Expected return on plan assets	(132)	(132)
Amortization of net actuarial (gains) losses	38	45
Amortization of prior service costs (credit)	(4)	(4)
Net periodic benefit costs (credit)	43	60
Other Postretirement Benefits
Net periodic benefit costs [Abstract]
Service costs	1	1
Interest costs	8	10
Expected return on plan assets	(14)	(15)
Amortization of net actuarial (gains) losses	(20)	(19)
Amortization of prior service costs (credit)	0	(1)
Net periodic benefit costs (credit)	$ (25)	$ (24)