INVESTMENT SECURITIES - Gross Unrealized Losses and Fair Values of Securities (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fair Value
Less than 12 months	$ 7,122	$ 3,999
12 months or more	1,718	2,308
Total	8,840	6,307
Gross Unrealized Losses
Less than 12 months	(154)	(30)
12 months or more	(82)	(58)
Total	(236)	(88)
U.S. Treasury and agency obligations | U.S. Treasury and other U.S. government corporations and agencies
Fair Value
Less than 12 months	437	273
12 months or more	114	130
Total	551	403
Gross Unrealized Losses
Less than 12 months	(2)	(1)
12 months or more	(2)	(1)
Total	(4)	(2)
Mortgage-backed securities | U.S. Treasury and other U.S. government corporations and agencies
Fair Value
Less than 12 months	1,596	581
12 months or more	583	672
Total	2,179	1,253
Gross Unrealized Losses
Less than 12 months	(40)	(2)
12 months or more	(29)	(17)
Total	(69)	(19)
Tax-exempt municipal securities
Fair Value
Less than 12 months	2,245	1,590
12 months or more	456	661
Total	2,701	2,251
Gross Unrealized Losses
Less than 12 months	(34)	(16)
12 months or more	(15)	(12)
Total	(49)	(28)
Residential mortgage-backed securities
Fair Value
Less than 12 months	16	20
12 months or more	1	3
Total	17	23
Gross Unrealized Losses
Less than 12 months	0	0
12 months or more	0	0
Total	0	0
Commercial mortgage-backed securities
Fair Value
Less than 12 months	457	131
12 months or more	27	28
Total	484	159
Gross Unrealized Losses
Less than 12 months	(14)	(1)
12 months or more	(2)	(1)
Total	(16)	(2)
Asset-backed securities
Fair Value
Less than 12 months	334	107
12 months or more	4	10
Total	338	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	2,037	1,297
12 months or more	533	804
Total	2,570	2,101
Gross Unrealized Losses
Less than 12 months	(62)	(10)
12 months or more	(34)	(27)
Total	$ (96)	$ (37)