Employee Benefits (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension plans
Components of net periodic (benefit) expense
Service cost	$ 17	$ 16	$ 49	$ 47
Interest cost	99	102	296	308
Expected return on plan assets	(171)	(166)	(513)	(499)
Recognition of prior service credit	(2)	(2)	(6)	(6)
Recognition of actuarial loss	45	51	135	154
Net periodic pension benefit (income) expense	(12)	1	(39)	4
Voluntary contributions to plan by employer	400		500
Post-retirement benefit plans
Components of net periodic (benefit) expense
Service cost	4	4	13	13
Interest cost	24	25	73	75
Expected return on plan assets	(1)	0	(1)	(1)
Recognition of prior service credit	5	5	15	15
Net periodic pension benefit (income) expense	$ 32	$ 34	$ 100	$ 102