Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension benefits
Components of net periodic cost
Service cost	$ 28	$ 28	$ 56	$ 57
Interest cost	61	66	121	132
Expected return on plan assets	(106)	(102)	(211)	(204)
Amortization of prior service credit	(14)	(14)	(28)	(28)
Amortization of net actuarial loss (gain)	45	47	89	94
Settlement loss	7	8	14	16
Net periodic postretirement credit	21	33	41	67
Postretirement benefits
Components of net periodic cost
Service cost	2	2	4	4
Interest cost	4	3	7	7
Amortization of prior service credit	(6)	(6)	(11)	(12)
Amortization of net actuarial loss (gain)	(5)	(6)	(11)	(12)
Net periodic postretirement credit	$ (5)	$ (7)	$ (11)	$ (13)