Financial Instruments - Marketable Securities in a Continuous Unrealized Loss Position (Details) - USD ($) $ in Millions	Mar. 31, 2018	Sep. 30, 2017
Fair Value of Marketable Securities
Continuous Unrealized Losses, Less than 12 Months	$ 159,198	$ 101,986
Continuous Unrealized Losses, 12 Months or Greater	37,266	8,290
Continuous Unrealized Losses, Total	196,464	110,276
Unrealized Losses
Continuous Unrealized Losses, Less than 12 Months	(2,633)	(596)
Continuous Unrealized Losses, 12 Months or Greater	(1,072)	(186)
Continuous Unrealized Losses, Total	$ (3,705)	$ (782)