Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension benefits
Components of net periodic cost
Service cost	$ 28	$ 29
Interest cost	60	66
Expected return on plan assets	(105)	(102)
Amortization of prior service credit	(14)	(14)
Amortization of net actuarial loss (gain)	44	47
Settlement loss	7	8
Net periodic postretirement credit	20	34
Postretirement benefits
Components of net periodic cost
Service cost	2	2
Interest cost	3	4
Amortization of prior service credit	(5)	(6)
Amortization of net actuarial loss (gain)	(6)	(6)
Net periodic postretirement credit	$ (6)	$ (6)