Employee Benefits (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension plans
Components of net periodic (benefit) expense
Service cost		$ 14	$ 15	$ 31	$ 32
Interest cost		105	107	206	214
Expected return on plan assets		(167)	(183)	(333)	(367)
Recognition of prior service (credit) cost		(2)	(2)	(4)	(4)
Recognition of actuarial loss		52	45	103	87
Net periodic benefit (income) expense		2	(18)	3	(38)
Post-retirement benefit plans
Components of net periodic (benefit) expense
Service cost		5	5	9	10
Interest cost		25	27	50	55
Expected return on plan assets		(1)	(2)	(1)	(4)
Recognition of prior service (credit) cost		5	5	10	10
Net periodic benefit (income) expense		$ 34	$ 35	$ 68	$ 71
Scenario, Forecast | Pension plans
Employee Benefits
Voluntary contributions to plan by employer	$ 100