Employee Benefit Plans (Net Periodic Benefit Costs) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Benefits
Net periodic benefit costs [Abstract]
Service costs	$ 61	$ 72
Interest costs	108	111
Curtailment costs	0	0
Expected return on plan assets	(132)	(127)
Amortization of net actuarial (gains) losses	49	49
Amortization of prior service costs (credit)	0	0
Net periodic benefit costs	86	105
Other Postretirement Benefits
Net periodic benefit costs [Abstract]
Service costs	1	2
Interest costs	19	21
Curtailment costs	0	30
Expected return on plan assets	(18)	(18)
Amortization of net actuarial (gains) losses	0	2
Amortization of prior service costs (credit)	(5)	(3)
Net periodic benefit costs	$ (3)	$ 34