Employee Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Sep. 30, 2017
Pension Plans [Member]
Components of net periodic benefit cost:
Service cost	$ 30	$ 28	$ 61	$ 56
Interest cost	22	21	43	42
Expected return on plan assets	(38)	(34)	(75)	(69)
Amortization of:
Net actuarial (gain) loss	14	12	27	24
Prior service credit	(5)	(5)	(10)	(10)
Net periodic benefit cost (credit)	23	22	46	43
Employee Benefit Plans (Textual)
Contributions to benefit plans			14	14
Expected contributions to pension plans in current year	108		108
Pension Plans [Member] | Forecast [Member]
Employee Benefit Plans (Textual)
Discretionary contributions to pension plans					$ 80
Other Postretirement Benefit Plans [Member]
Components of net periodic benefit cost:
Service cost	2	1	3	3
Interest cost	2	3	5	6
Expected return on plan assets	0	0	0	0
Amortization of:
Net actuarial (gain) loss	(1)	0	(2)	0
Prior service credit	(4)	(4)	(8)	(8)
Net periodic benefit cost (credit)	$ (1)	$ 0	(2)	1
Employee Benefit Plans (Textual)
Contributions to benefit plans			$ 13	$ 8