Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension benefits
Components of net periodic cost
Service cost	$ 28	$ 28	$ 56	$ 57
Interest cost	72	64	143	128
Expected return on plan assets	(100)	(106)	(199)	(212)
Amortization of prior service credit	(14)	(14)	(28)	(28)
Amortization of net actuarial loss (gain)	44	48	87	96
Settlement loss	8	6	16	12
Net periodic postretirement (credit) cost	38	26	75	53
Postretirement benefits
Components of net periodic cost
Service cost	3	3	5	6
Interest cost	5	5	9	11
Amortization of prior service credit	(6)	(5)	(11)	(11)
Amortization of net actuarial loss (gain)	(8)	(2)	(16)	(4)
Net periodic postretirement (credit) cost	$ (6)	$ 1	$ (13)	$ 2