INVESTMENT SECURITIES - Schedule of Gross Unrealized Losses and Fair Values of Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Less than 12 months, Fair Value	$ 3,999	$ 6,758
Less than 12 months, Gross Unrealized Losses	(30)	(187)
12 months or more, Fair Value	2,308	209
12 months or more, Gross Unrealized Losses	(58)	(9)
Total, Fair Value	6,307	6,967
Total, Gross Unrealized Losses	(88)	(196)
Debt Securities | Tax-exempt municipal securities
Investment [Line Items]
Less than 12 months, Fair Value	1,590	2,756
Less than 12 months, Gross Unrealized Losses	(16)	(67)
12 months or more, Fair Value	661	43
12 months or more, Gross Unrealized Losses	(12)	(1)
Total, Fair Value	2,251	2,799
Total, Gross Unrealized Losses	(28)	(68)
Debt Securities | Residential Mortgage-backed securities
Investment [Line Items]
Less than 12 months, Fair Value	20	0
Less than 12 months, Gross Unrealized Losses	0	0
12 months or more, Fair Value	3	4
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	23	4
Total, Gross Unrealized Losses	0	0
Debt Securities | Commercial Mortgage-backed securities
Investment [Line Items]
Less than 12 months, Fair Value	131	182
Less than 12 months, Gross Unrealized Losses	(1)	(3)
12 months or more, Fair Value	28	24
12 months or more, Gross Unrealized Losses	(1)	(1)
Total, Fair Value	159	206
Total, Gross Unrealized Losses	(2)	(4)
Debt Securities | Asset-backed securities
Investment [Line Items]
Less than 12 months, Fair Value	107	51
Less than 12 months, Gross Unrealized Losses	0	0
12 months or more, Fair Value	10	63
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	117	114
Total, Gross Unrealized Losses	0	0
Debt Securities | Corporate debt securities
Investment [Line Items]
Less than 12 months, Fair Value	1,297	1,544
Less than 12 months, Gross Unrealized Losses	(10)	(71)
12 months or more, Fair Value	804	69
12 months or more, Gross Unrealized Losses	(27)	(7)
Total, Fair Value	2,101	1,613
Total, Gross Unrealized Losses	(37)	(78)
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	273	697
Less than 12 months, Gross Unrealized Losses	(1)	(15)
12 months or more, Fair Value	130	3
12 months or more, Gross Unrealized Losses	(1)	0
Total, Fair Value	403	700
Total, Gross Unrealized Losses	(2)	(15)
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	581	1,528
Less than 12 months, Gross Unrealized Losses	(2)	(31)
12 months or more, Fair Value	672	3
12 months or more, Gross Unrealized Losses	(17)	0
Total, Fair Value	1,253	1,531
Total, Gross Unrealized Losses	$ (19)	$ (31)