Pension and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Benefits [Member]
Components Of Net Periodic Cost Details [Abstract]
Service cost	$ 7	$ 8
Interest cost	48	54
Expected return on plan assets	(50)	(57)
Amortization of actuarial loss (gain)	25	21
Net periodic cost	30	26
Postretirement Benefit [Member]
Components Of Net Periodic Cost Details [Abstract]
Service cost	0	0
Interest cost	4	5
Expected return on plan assets	0	0
Amortization of actuarial loss (gain)	(5)	(5)
Net periodic cost	$ (1)	$ 0