INVESTMENT SECURITIES AND OTHER SECURITIES - Contractual Maturities (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Available-for-sale securities, amortized cost:
Under 1 year	$ 2,101	$ 3,620
After 1 year through 5 years	5,628	5,993
After 5 years through 10 years	1,921	1,857
After 10 years	18,952	19,273
Total available-for-sale securities	28,602	30,743
Available-for-sale securities, fair value:
Under 1 year	2,096	3,624
After 1 year through 5 years	5,637	5,844
After 5 years through 10 years	1,796	1,732
After 10 years	16,556	16,073
Total available-for-sale securities	26,085	27,273
Held-to-maturity securities, amortized cost:
Under 1 year	404	255
After 1 year through 5 years	1,967	1,818
After 5 years through 10 years	146	65
After 10 years	13,080	14,230
Total held-to-maturity securities	15,597	16,368
Held-to-maturity securities, fair value:
Under 1 year	404	256
After 1 year through 5 years	1,981	1,796
After 5 years through 10 years	134	60
After 10 years	11,350	11,974
Total held-to-maturity securities	$ 13,869	$ 14,086