Employee Benefits (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension plans
Employee Benefits
Voluntary contributions to plan by employer	$ 100
Components of net periodic (benefit) expense
Service cost	16	$ 20	$ 48	$ 62
Interest cost	107	142	321	425
Expected return on plan assets	(183)	(224)	(550)	(673)
Recognition of prior service (credit) cost	(2)	1	(6)	4
Recognition of actuarial loss	44	40	131	120
Net periodic benefit (income) expense	(18)	(21)	(56)	(62)
Post-retirement benefit plans
Components of net periodic (benefit) expense
Service cost	4	6	14	18
Interest cost	29	35	84	105
Expected return on plan assets	(2)	(6)	(6)	(16)
Recognition of prior service (credit) cost	5	6	15	16
Net periodic benefit (income) expense	$ 36	$ 41	$ 107	$ 123