INVESTMENT SECURITIES - Gross Unrealized Losses and Fair Values of Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Investment [Line Items]
Less than 12 months, Fair Value	$ 4,006	$ 6,758
Less than 12 months, Gross Unrealized Losses	(48)	(187)
12 months or more, Fair Value	1,152	209
12 months or more, Gross Unrealized Losses	(34)	(9)
Total, Fair Value	5,158	6,967
Total, Gross Unrealized Losses	(82)	(196)
Debt Securities | Tax-exempt municipal securities
Investment [Line Items]
Less than 12 months, Fair Value	1,320	2,756
Less than 12 months, Gross Unrealized Losses	(9)	(67)
12 months or more, Fair Value	460	43
12 months or more, Gross Unrealized Losses	(7)	(1)
Total, Fair Value	1,780	2,799
Total, Gross Unrealized Losses	(16)	(68)
Debt Securities | Residential Mortgage-Backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	0	0
Less than 12 months, Gross Unrealized Losses	0	0
12 months or more, Fair Value	4	4
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	4	4
Total, Gross Unrealized Losses	0	0
Debt Securities | Commercial Mortgage-Backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	115	182
Less than 12 months, Gross Unrealized Losses	(2)	(3)
12 months or more, Fair Value	0	24
12 months or more, Gross Unrealized Losses	0	(1)
Total, Fair Value	115	206
Total, Gross Unrealized Losses	(2)	(4)
Debt Securities | Asset-backed securities
Investment [Line Items]
Less than 12 months, Fair Value	64	51
Less than 12 months, Gross Unrealized Losses	0	0
12 months or more, Fair Value	0	63
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	64	114
Total, Gross Unrealized Losses	0	0
Debt Securities | Corporate debt securities
Investment [Line Items]
Less than 12 months, Fair Value	999	1,544
Less than 12 months, Gross Unrealized Losses	(15)	(71)
12 months or more, Fair Value	469	69
12 months or more, Gross Unrealized Losses	(22)	(7)
Total, Fair Value	1,468	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	522	697
Less than 12 months, Gross Unrealized Losses	(5)	(15)
12 months or more, Fair Value	137	3
12 months or more, Gross Unrealized Losses	(3)	0
Total, Fair Value	659	700
Total, Gross Unrealized Losses	(8)	(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	986	1,528
Less than 12 months, Gross Unrealized Losses	(17)	(31)
12 months or more, Fair Value	82	3
12 months or more, Gross Unrealized Losses	(2)	0
Total, Fair Value	1,068	1,531
Total, Gross Unrealized Losses	$ (19)	$ (31)