Retirement Plans Summary of Changes in Fair Value of Assets Measured Using Significant Unobservable Inputs (Level 3) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
U.S. Pension Plans
Retirement Plans [Line Items]
Fair value of plan assets at beginning of year		$ 2,606
Fair value of plan assets at end of year	$ 2,890
U.S. Pension Plans | Real Estate
Retirement Plans [Line Items]
Fair value of plan assets at beginning of year	2,947
Fair value of plan assets at end of year		2,947
Non-U.S. Pension
Retirement Plans [Line Items]
Fair value of plan assets at beginning of year	2,484	1,177
Fair value of plan assets at end of year	2,152	2,484
Non-U.S. Pension | Hedge Funds
Retirement Plans [Line Items]
Fair value of plan assets at beginning of year	169
Fair value of plan assets at end of year	112	169
Non-U.S. Pension | Real Estate
Retirement Plans [Line Items]
Fair value of plan assets at beginning of year	11
Fair value of plan assets at end of year	24	11
Fair Value, Inputs, Level 3 | U.S. Pension Plans
Retirement Plans [Line Items]
Fair value of plan assets at end of year	0
Fair Value, Inputs, Level 3 | U.S. Pension Plans | Real Estate
Retirement Plans [Line Items]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year		0
Fair Value, Inputs, Level 3 | Non-U.S. Pension
Retirement Plans [Line Items]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Fair Value, Inputs, Level 3 | Non-U.S. Pension | Hedge Funds
Retirement Plans [Line Items]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Fair Value, Inputs, Level 3 | Non-U.S. Pension | Real Estate
Retirement Plans [Line Items]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	$ 0	$ 0