Employee Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Plans [Member]
Components of net periodic benefit cost:
Service cost	$ 33	$ 30	$ 67	$ 61
Interest cost	23	22	46	43
Expected return on plan assets	(41)	(38)	(82)	(75)
Amortization of:
Net actuarial (gain) loss	17	14	33	27
Prior service credit	(4)	(5)	(9)	(10)
Special charges	3	0	5	0
Net periodic benefit cost (credit)	31	23	60	46
Employee Benefit Plans (Textual)
Contributions to benefit plans			16	14
Increase in discretionary contributions during remainder of fiscal year	10		10
Expected employer contributions for current fiscal year	141		141
Expected employer discretionary contributions for current fiscal year	110		110
Other Postretirement Benefit Plans [Member]
Components of net periodic benefit cost:
Service cost	2	2	3	3
Interest cost	3	2	5	5
Expected return on plan assets	0	0	0	0
Amortization of:
Net actuarial (gain) loss	(1)	(1)	(1)	(2)
Prior service credit	(3)	(4)	(6)	(8)
Special charges	0	0	0	0
Net periodic benefit cost (credit)	$ 1	$ (1)	1	(2)
Employee Benefit Plans (Textual)
Contributions to benefit plans			$ 9	$ 13