INVESTMENT SECURITIES - Securities Classified as Current and Long-Term (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Investment Securities, Available-for-sale Amortized Cost to Fair Value
Amortized Cost	$ 13,501	$ 12,437
Gross Unrealized Gains	305	520
Gross Unrealized Losses	(165)	(6)
Fair Value	13,641	12,951
Common stock	730	815
Total investment securities	14,371	13,766
U.S. Treasury and agency obligations
Investment Securities, Available-for-sale Amortized Cost to Fair Value
Amortized Cost	660	616
Gross Unrealized Gains	2	1
Gross Unrealized Losses	(11)	(1)
Fair Value	651	616
Mortgage-backed securities
Investment Securities, Available-for-sale Amortized Cost to Fair Value
Amortized Cost	3,809	3,115
Gross Unrealized Gains	84	140
Gross Unrealized Losses	(57)	(1)
Fair Value	3,836	3,254
Tax-exempt municipal securities
Investment Securities, Available-for-sale Amortized Cost to Fair Value
Amortized Cost	1,137	1,393
Gross Unrealized Gains	35	54
Gross Unrealized Losses	(2)	0
Fair Value	1,170	1,447
Residential
Investment Securities, Available-for-sale Amortized Cost to Fair Value
Amortized Cost	93	17
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	0
Fair Value	92	17
Commercial
Investment Securities, Available-for-sale Amortized Cost to Fair Value
Amortized Cost	1,284	1,260
Gross Unrealized Gains	35	59
Gross Unrealized Losses	(12)	(1)
Fair Value	1,307	1,318
Asset-backed securities
Investment Securities, Available-for-sale Amortized Cost to Fair Value
Amortized Cost	1,423	1,364
Gross Unrealized Gains	10	10
Gross Unrealized Losses	(1)	(2)
Fair Value	1,432	1,372
Corporate debt securities
Investment Securities, Available-for-sale Amortized Cost to Fair Value
Amortized Cost	5,095	4,672
Gross Unrealized Gains	139	256
Gross Unrealized Losses	(81)	(1)
Fair Value	$ 5,153	$ 4,927

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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