Comprehensive Income - Change in Accumulated Other Comprehensive Income by Component (Detail) $ in Millions	3 Months Ended
	Dec. 31, 2016 USD ($)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 128,249
Other comprehensive income/(loss) before reclassifications	(1,552)
Amounts reclassified from AOCI	548
Tax effect	615
Other comprehensive income/(loss)	(389)
Ending balance	132,390
Cumulative Foreign Currency Translation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(578)
Other comprehensive income/(loss) before reclassifications	(451)
Amounts reclassified from AOCI	0
Tax effect	76
Other comprehensive income/(loss)	(375)
Ending balance	(953)
Unrealized Gains/Losses on Derivative Instruments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	38
Other comprehensive income/(loss) before reclassifications	1,696
Amounts reclassified from AOCI	517
Tax effect	(439)
Other comprehensive income/(loss)	1,774
Ending balance	1,812
Unrealized Gains/Losses on Marketable Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	1,174
Other comprehensive income/(loss) before reclassifications	(2,797)
Amounts reclassified from AOCI	31
Tax effect	978
Other comprehensive income/(loss)	(1,788)
Ending balance	(614)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	634
Ending balance	$ 245