Benefit Plans - Restricted Stock Units Activity and Related Information (Details) - Restricted stock units $ / shares in Units, shares in Thousands, $ in Millions	6 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares
Number of Restricted Stock Units
Beginning balance (in shares) | shares	97,571
RSUs granted (in shares) | shares	40,184
RSUs vested (in shares) | shares	(22,348)
RSUs canceled (in shares) | shares	(3,023)
Ending balance (in shares) | shares	112,384
Weighted-Average Grant Date Fair Value Per RSU
Beginning balance (in dollars per share) | $ / shares	$ 110.33
RSUs granted (in dollars per share) | $ / shares	159.25
RSUs vested (in dollars per share) | $ / shares	103.66
RSUs canceled (in dollars per share) | $ / shares	123.28
Ending balance (in dollars per share) | $ / shares	$ 128.80
Aggregate Fair Value
Aggregate fair value of restricted stock units | $	$ 18,856