INVESTMENT SECURITIES - Gross Unrealized Losses and Fair Values of Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fair Value
Less than 12 months	$ 6,493	$ 3,999
12 months or more	2,877	2,308
Total	9,370	6,307
Gross Unrealized Losses
Less than 12 months	(141)	(30)
12 months or more	(132)	(58)
Total	(273)	(88)
U.S. Treasury and agency obligations | U.S. Treasury and other U.S. government corporations and agencies
Fair Value
Less than 12 months	476	273
12 months or more	143	130
Total	619	403
Gross Unrealized Losses
Less than 12 months	(1)	(1)
12 months or more	(4)	(1)
Total	(5)	(2)
Mortgage-backed securities | U.S. Treasury and other U.S. government corporations and agencies
Fair Value
Less than 12 months	1,661	581
12 months or more	642	672
Total	2,303	1,253
Gross Unrealized Losses
Less than 12 months	(52)	(2)
12 months or more	(37)	(17)
Total	(89)	(19)
Tax-exempt municipal securities
Fair Value
Less than 12 months	1,578	1,590
12 months or more	1,120	661
Total	2,698	2,251
Gross Unrealized Losses
Less than 12 months	(29)	(16)
12 months or more	(36)	(12)
Total	(65)	(28)
Residential mortgage-backed securities
Fair Value
Less than 12 months	56	20
12 months or more	1	3
Total	57	23
Gross Unrealized Losses
Less than 12 months	(1)	0
12 months or more	0	0
Total	(1)	0
Commercial mortgage-backed securities
Fair Value
Less than 12 months	407	131
12 months or more	74	28
Total	481	159
Gross Unrealized Losses
Less than 12 months	(13)	(1)
12 months or more	(5)	(1)
Total	(18)	(2)
Asset-backed securities
Fair Value
Less than 12 months	480	107
12 months or more	15	10
Total	495	117
Gross Unrealized Losses
Less than 12 months	(2)	0
12 months or more	0	0
Total	(2)	0
Corporate debt securities
Fair Value
Less than 12 months	1,835	1,297
12 months or more	882	804
Total	2,717	2,101
Gross Unrealized Losses
Less than 12 months	(43)	(10)
12 months or more	(50)	(27)
Total	$ (93)	$ (37)