Employee Benefits (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension plans
Components of net periodic (benefit) expense
Service cost	$ 16	$ 17
Interest cost	100	101
Expected return on plan assets	(173)	(166)
Recognition of prior service (credit) cost	(2)	(2)
Recognition of actuarial loss	44	51
Net periodic benefit (income) expense	(15)	1
Voluntary contributions to plan by employer	100
Post-retirement benefit plans
Components of net periodic (benefit) expense
Service cost	4	4
Interest cost	24	25
Expected return on plan assets	0	0
Recognition of prior service (credit) cost	5	5
Net periodic benefit (income) expense	$ 33	$ 34