Employee Benefit Plans - Schedule of Fair Values of Plan Assets by Investment Category (Details) - Pension benefits - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 1,792	$ 2,081	$ 2,307
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	759	1,154
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	563	497
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	16	$ 23
Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	466	414
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	100	13
Cash and cash equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	100	13
Cash and cash equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash and cash equivalents | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash and cash equivalents | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. government securities and futures
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	74	339
U.S. government securities and futures | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(3)	207
U.S. government securities and futures | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	77	132
U.S. government securities and futures | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. government securities and futures | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	92	86
Other government securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	0
Other government securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	91	86
Other government securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other government securities | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Corporate debt instruments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,056	1,197
Corporate debt instruments | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	661	934
Corporate debt instruments | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	395	263
Corporate debt instruments | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Corporate debt instruments | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Insurance contracts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4
Insurance contracts | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0
Insurance contracts | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0
Insurance contracts | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4
Insurance contracts | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0
International equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		16
International equity | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0
International equity | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		16
International equity | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0
International equity | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0
Private equity and hedge funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	60	414
Private equity and hedge funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Private equity and hedge funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Private equity and hedge funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Private equity and hedge funds | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	60	414
Common collective trusts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	406
Common collective trusts | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0
Common collective trusts | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0
Common collective trusts | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0
Common collective trusts | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 406
Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		16
Other investments | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0
Other investments | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0
Other investments | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		16
Other investments | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 0