Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension benefits
Components of net periodic cost
Service cost	$ 28	$ 28	$ 57	$ 56
Interest cost	66	72	132	143
Expected return on plan assets	(102)	(100)	(204)	(199)
Amortization of prior service credit	(14)	(14)	(28)	(28)
Amortization of net actuarial loss (gain)	47	44	94	87
Settlement loss	8	8	16	16
Net periodic postretirement credit	33	38	67	75
Postretirement benefits
Components of net periodic cost
Service cost	2	3	4	5
Interest cost	3	5	7	9
Amortization of prior service credit	(6)	(6)	(12)	(11)
Amortization of net actuarial loss (gain)	(6)	(8)	(12)	(16)
Net periodic postretirement credit	$ (7)	$ (6)	$ (13)	$ (13)