Employee Benefit Plans - Schedule of Pension Plans with a Benefit Obligation in Excess of Plan Assets (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017
Plans with projected benefit obligation in excess of plan assets
Plans with projected benefit obligation in excess of plan assets, benefit obligation	$ 2,045	$ 2,381
Plans with projected benefit obligation in excess of plan assets, plan assets	1,749	2,081
Plans with accumulated benefit obligation in excess of plan assets
Plans with accumulated benefit obligation in excess of plan assets, benefit obligation	2,034	2,368
Plans with accumulated benefit obligation in excess of plan assets, plan assets	$ 1,741	$ 2,072