Employee Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Plans [Member]
Components of net periodic benefit cost:
Service cost	$ 34	$ 31
Interest cost	23	21
Expected return on plan assets	(41)	(37)
Amortization of:
Net actuarial (gain) loss	16	13
Prior service credit	(5)	(5)
Special charges	2	0
Net periodic benefit cost (credit)	29	23
Employee Benefit Plans (Textual)
Contributions to benefit plans	8	7
Other Postretirement Benefit Plans [Member]
Components of net periodic benefit cost:
Service cost	1	1
Interest cost	2	3
Expected return on plan assets	0	0
Amortization of:
Net actuarial (gain) loss	0	(1)
Prior service credit	(3)	(4)
Special charges	0	0
Net periodic benefit cost (credit)	0	(1)
Employee Benefit Plans (Textual)
Contributions to benefit plans	$ 4	$ 5