Comprehensive income - Accumulated Other Comprehensive Income (Loss), Net (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 28,525	$ 28,424	$ 29,027	$ 26,957
Total other comprehensive income (loss)	62	524	441	432
Ending balance	28,728	28,876	28,728	28,876
Investment Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	61	(179)	(153)	(187)
Total other comprehensive income (loss)	60	230	274	238
Ending balance	121	51	121	51
Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	63	(246)	(101)	(151)
Total other comprehensive income (loss)	5	291	169	196
Ending balance	68	45	68	45
Defined Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	95	(118)	98	(115)
Total other comprehensive income (loss)	(2)	(1)	(5)	(4)
Ending balance	93	(119)	93	(119)
Other
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(4)	(8)	(8)	(6)
Total other comprehensive income (loss)	(1)	4	3	2
Ending balance	(5)	(4)	(5)	(4)
Accumulated Other Comprehensive Income (Loss), Net
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	215	(551)	(164)	(459)
Total other comprehensive income (loss)	62	524	441	432
Ending balance	$ 277	$ (27)	$ 277	$ (27)