Comprehensive Income - Change in AOCI by Component (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Apr. 01, 2017	Mar. 31, 2018	Apr. 01, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances as of September 30, 2017			$ 134,047
Total other comprehensive income/(loss)	$ (1,945)	$ (667)	(2,636)	$ (1,056)
Balances as of March 31, 2018	126,878		126,878
Cumulative Foreign Currency Translation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances as of September 30, 2017			(354)
Other comprehensive income/(loss) before reclassifications			296
Amounts reclassified from AOCI			0
Tax effect			7
Total other comprehensive income/(loss)			303
Cumulative effect of change in accounting principle			(176)
Balances as of March 31, 2018	(227)		(227)
Unrealized Gains/Losses on Derivative Instruments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances as of September 30, 2017			(124)
Other comprehensive income/(loss) before reclassifications			191
Amounts reclassified from AOCI			(161)
Tax effect			(74)
Total other comprehensive income/(loss)			(44)
Cumulative effect of change in accounting principle			29
Balances as of March 31, 2018	(139)		(139)
Unrealized Gains/Losses on Marketable Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances as of September 30, 2017			328
Other comprehensive income/(loss) before reclassifications			(3,854)
Amounts reclassified from AOCI			(80)
Tax effect			1,039
Total other comprehensive income/(loss)			(2,895)
Cumulative effect of change in accounting principle			(131)
Balances as of March 31, 2018	(2,698)		(2,698)
Total AOCI
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances as of September 30, 2017			(150)
Other comprehensive income/(loss) before reclassifications			(3,367)
Amounts reclassified from AOCI			(241)
Tax effect			972
Total other comprehensive income/(loss)			(2,636)
Cumulative effect of change in accounting principle			(278)
Balances as of March 31, 2018	$ (3,064)		$ (3,064)