Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension benefits
Components of net periodic cost
Service cost	$ 29	$ 28
Interest cost	66	71
Expected return on plan assets	(102)	(99)
Amortization of prior service credit	(14)	(14)
Amortization of net actuarial loss (gain)	47	43
Settlement loss	8	8
Net periodic postretirement credit	34	37
Postretirement benefits
Components of net periodic cost
Service cost	2	2
Interest cost	4	4
Amortization of prior service credit	(6)	(5)
Amortization of net actuarial loss (gain)	(6)	(8)
Net periodic postretirement credit	$ (6)	$ (7)