INVESTMENT SECURITIES - Gross Unrealized Losses and Fair Values of Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Less than 12 months, Fair Value	$ 7,322	$ 3,999
Less than 12 months, Gross Unrealized Losses	(142)	(30)
12 months or more, Fair Value	2,187	2,308
12 months or more, Gross Unrealized Losses	(107)	(58)
Total, Fair Value	9,509	6,307
Total, Gross Unrealized Losses	(249)	(88)
Debt Securities | Tax-exempt municipal securities
Investment [Line Items]
Less than 12 months, Fair Value	2,409	1,590
Less than 12 months, Gross Unrealized Losses	(41)	(16)
12 months or more, Fair Value	621	661
12 months or more, Gross Unrealized Losses	(20)	(12)
Total, Fair Value	3,030	2,251
Total, Gross Unrealized Losses	(61)	(28)
Debt Securities | Residential Mortgage-Backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	18	20
Less than 12 months, Gross Unrealized Losses	0	0
12 months or more, Fair Value	3	3
12 months or more, Gross Unrealized Losses	(1)	0
Total, Fair Value	21	23
Total, Gross Unrealized Losses	(1)	0
Debt Securities | Commercial Mortgage-Backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	451	131
Less than 12 months, Gross Unrealized Losses	(10)	(1)
12 months or more, Fair Value	28	28
12 months or more, Gross Unrealized Losses	(1)	(1)
Total, Fair Value	479	159
Total, Gross Unrealized Losses	(11)	(2)
Debt Securities | Asset-backed securities
Investment [Line Items]
Less than 12 months, Fair Value	162	107
Less than 12 months, Gross Unrealized Losses	(1)	0
12 months or more, Fair Value	7	10
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	169	117
Total, Gross Unrealized Losses	(1)	0
Debt Securities | Corporate debt securities
Investment [Line Items]
Less than 12 months, Fair Value	2,431	1,297
Less than 12 months, Gross Unrealized Losses	(60)	(10)
12 months or more, Fair Value	761	804
12 months or more, Gross Unrealized Losses	(54)	(27)
Total, Fair Value	3,192	2,101
Total, Gross Unrealized Losses	(114)	(37)
Debt Securities | U.S. Treasury and other U.S. government corporations and agencies | U.S. Treasury and agency obligations
Investment [Line Items]
Less than 12 months, Fair Value	362	273
Less than 12 months, Gross Unrealized Losses	(2)	(1)
12 months or more, Fair Value	129	130
12 months or more, Gross Unrealized Losses	(3)	(1)
Total, Fair Value	491	403
Total, Gross Unrealized Losses	(5)	(2)
Debt Securities | U.S. Treasury and other U.S. government corporations and agencies | Mortgage-backed securities
Investment [Line Items]
Less than 12 months, Fair Value	1,489	581
Less than 12 months, Gross Unrealized Losses	(28)	(2)
12 months or more, Fair Value	638	672
12 months or more, Gross Unrealized Losses	(28)	(17)
Total, Fair Value	2,127	1,253
Total, Gross Unrealized Losses	$ (56)	$ (19)