Employee Benefits (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Sep. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension plans
Components of net periodic (benefit) expense
Service cost			$ 16	$ 14	$ 32	$ 31
Interest cost			98	105	197	206
Expected return on plan assets			(171)	(167)	(342)	(333)
Recognition of prior service (credit) cost			(2)	(2)	(4)	(4)
Recognition of actuarial loss			46	52	90	103
Net periodic benefit (income) expense			(13)	2	(27)	3
Voluntary contributions to plan by employer	$ 100
Pension plans | Scenario, Forecast
Components of net periodic (benefit) expense
Voluntary contributions to plan by employer		$ 400
Post-retirement benefit plans
Components of net periodic (benefit) expense
Service cost			5	5	9	9
Interest cost			25	25	49	50
Expected return on plan assets			0	(1)	0	(1)
Recognition of prior service (credit) cost			5	5	10	10
Net periodic benefit (income) expense			$ 35	$ 34	$ 68	$ 68