Employee Benefits (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension plans
Employee Benefits
Voluntary contributions to plan by employer	$ 100
Components of net periodic (benefit) expense
Service cost	16	$ 16	$ 47	$ 48
Interest cost	102	107	308	321
Expected return on plan assets	(166)	(183)	(499)	(550)
Recognition of prior service (credit) cost	(2)	(2)	(6)	(6)
Recognition of actuarial loss	51	44	154	131
Net periodic benefit (income) expense	1	(18)	4	(56)
Post-retirement benefit plans
Components of net periodic (benefit) expense
Service cost	4	4	13	14
Interest cost	25	29	75	84
Expected return on plan assets	0	(2)	(1)	(6)
Recognition of prior service (credit) cost	5	5	15	15
Net periodic benefit (income) expense	$ 34	$ 36	$ 102	$ 107