Employee Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Plans [Member]
Components of net periodic benefit cost:
Service cost	$ 33	$ 31	$ 100	$ 92
Interest cost	22	21	68	64
Expected return on plan assets	(40)	(37)	(122)	(112)
Amortization of:
Net actuarial (gain) loss	16	13	49	40
Prior service credit	(5)	(5)	(14)	(15)
Special charges	2	3	7	3
Net periodic benefit cost (credit)	28	26	88	72
Employee Benefit Plans (Textual)
Contributions to benefit plans			132	104
Employer discretionary contributions	110
Expected employer contributions for current fiscal year	141		141
Other Postretirement Benefit Plans [Member]
Components of net periodic benefit cost:
Service cost	2	1	5	4
Interest cost	2	3	7	8
Expected return on plan assets	0	0	0	0
Amortization of:
Net actuarial (gain) loss	(1)	0	(2)	(2)
Prior service credit	(2)	(4)	(8)	(12)
Special charges	0	0	0	0
Net periodic benefit cost (credit)	$ 1	$ 0	2	(2)
Employee Benefit Plans (Textual)
Contributions to benefit plans			$ 15	$ 17