Employee Benefits (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension plans
Components of net periodic (benefit) expense
Service cost	$ 14	$ 16
Interest cost	110	100
Expected return on plan assets	(156)	(173)
Recognition of prior service credit	(2)	(2)
Recognition of actuarial loss	57	44
Net periodic pension benefit expense (income)	23	(15)
Post-retirement benefit plans
Components of net periodic (benefit) expense
Service cost	4	4
Interest cost	27	24
Recognition of prior service credit	4	5
Net periodic pension benefit expense (income)	$ 35	$ 33