PARENT COMPANY FINANCIALS - Condensed Cash Flow Statements (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
OPERATING ACTIVITIES
Net income		$ 1,652	$ 1,045	$ 840
Deferred income taxes		(136)	153	249
Increase (decrease) in other liabilities		(119)	(59)	(161)
Decrease (increase) in other assets		(502)	(274)	(467)
Net cash provided by operating activities		1,883	1,490	1,229
INVESTING ACTIVITIES
Net cash used in investing activities		(3,984)	(11,326)	(5,905)
FINANCING ACTIVITIES
Proceeds from issuance of long-term borrowed funds		15,363	15,144	6,750
Repayments of long-term borrowed funds		(12,751)	(8,429)	(766)
Treasury stock purchased		(820)	(430)	(500)
Net proceeds from issuance of preferred stock		0	0	247
Dividends declared and paid to common stockholders		(322)	(241)	(214)
Dividends declared and paid to preferred stockholders		(14)	(14)	(7)
Net cash provided by financing activities		1,429	10,455	4,485
(Decrease) increase in cash and cash equivalents	[1]	(672)	619	(191)
Cash and cash equivalents at beginning of period	[1]	3,704	3,085	3,276
Cash and cash equivalents at end of period	[1]	3,032	3,704	3,085
Parent company
OPERATING ACTIVITIES
Net income		1,652	1,045	840
Deferred income taxes		(11)	5	49
Gain on sales of assets		(1)	(3)	(3)
Equity in undistributed earnings of subsidiaries		(665)	(526)	(546)
Increase (decrease) in other liabilities		99	(19)	(48)
Decrease (increase) in other assets		5	35	(16)
Other operating, net		(1)	(4)	3
Total adjustments		(574)	(512)	(561)
Net cash provided by operating activities		1,078	533	279
INVESTING ACTIVITIES
Proceeds from sales of securities available for sale		0	0	8
Investments in and advances to subsidiaries		(230)	(40)	(215)
Repayment of investments in and advances to subsidiaries		167	588	376
Other investing, net		(1)	(2)	0
Net cash used in investing activities		(64)	546	169
FINANCING ACTIVITIES
Proceeds from issuance of long-term borrowed funds		0	349	1,000
Repayments of long-term borrowed funds		0	(625)	(750)
Proceeds from issuance of common stock		34	22	27
Treasury stock purchased		(820)	(430)	(500)
Net proceeds from issuance of preferred stock		0	0	247
Dividends declared and paid to common stockholders		(322)	(241)	(214)
Dividends declared and paid to preferred stockholders		(14)	(14)	(7)
Net cash provided by financing activities		(1,122)	(939)	(197)
(Decrease) increase in cash and cash equivalents		(108)	140	251
Cash and cash equivalents at beginning of period		671	531	280
Cash and cash equivalents at end of period		$ 563	$ 671	$ 531

[1] Cash and cash equivalents includes cash and due from banks and interest-bearing cash and due from banks as reflected on the Consolidated Balance Sheets.