Shareholders' Equity - Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Jun. 30, 2023
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 23,799	$ 25,191	$ 25,191
Other comprehensive income (loss) before reclassifications	(97)	338
Increase (decrease) due to amounts reclassified from accumulated other comprehensive income	82	101
Other comprehensive income (loss)	(15)	439
Ending balance	24,433	24,750
AFS transferred to HTM	(492)		(530)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(2,354)	(3,711)	(3,711)
Other comprehensive income (loss)	(15)	439
Ending balance	(2,369)	(3,272)
Net Unrealized Gains (Losses) on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(131)	(359)	(359)
Other comprehensive income (loss) before reclassifications	34	14
Increase (decrease) due to amounts reclassified from accumulated other comprehensive income	(147)	37
Other comprehensive income (loss)	(113)	51
Ending balance	(244)	(308)
Net Unrealized Gains (Losses) on Investment Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(947)	(1,817)	(1,817)
Other comprehensive income (loss) before reclassifications	(30)	194
Increase (decrease) due to amounts reclassified from accumulated other comprehensive income	228	52
Other comprehensive income (loss)	198	246
Ending balance	(749)	(1,571)
Net Unrealized Losses on Retirement Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(145)	(143)	(143)
Other comprehensive income (loss) before reclassifications	6	0
Increase (decrease) due to amounts reclassified from accumulated other comprehensive income	1	12
Other comprehensive income (loss)	7	12
Ending balance	(138)	(131)
Foreign Currency Translation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,400)	(1,751)	(1,751)
Other comprehensive income (loss) before reclassifications	(292)	172
Increase (decrease) due to amounts reclassified from accumulated other comprehensive income	0	0
Other comprehensive income (loss)	(292)	172
Ending balance	(1,692)	(1,579)
Net Unrealized Gains (Losses) on Hedges of Net Investments in Non-U.S. Subsidiaries
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	269	359	$ 359
Other comprehensive income (loss) before reclassifications	185	(42)
Increase (decrease) due to amounts reclassified from accumulated other comprehensive income	0	0
Other comprehensive income (loss)	185	(42)
Ending balance	$ 454	$ 317