Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Components of net periodic cost
Remeasurement gains and losses	$ 15	$ 14
Pension benefits
Components of net periodic cost
Service cost	28	28
Interest cost	65	61
Expected return on plan assets	(93)	(113)
Amortization of prior service credit	(14)	(14)
Costs and expenses	(14)	(38)
Remeasurement of projected benefit obligation	387	(190)
Remeasurement gains and losses	(4)	22
Remeasurement of plan assets	(391)	212
Total net cost (benefit)	(18)	(16)
Postretirement benefits
Components of net periodic cost
Service cost	2	2
Interest cost	4	4
Amortization of prior service credit	(1)	(5)
Costs and expenses	5	1
Remeasurement of projected benefit obligation	19	(8)
Remeasurement gains and losses	19	(8)
Remeasurement of plan assets	0	0
Total net cost (benefit)	$ 24	$ (7)