Condensed Financial Information of the Parent Company - Condensed Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Cash and due from banks	$ 671		$ 677	$ 607	$ 653
Short-term investments	4,447		2,775
Securities available for sale	18,139		20,212
Other investments	726		738
Total loans	85,528		85,180
Goodwill	2,538	$ 2,464	2,446	1,060
Other intangible assets	416		384
Corporate-owned life insurance	4,132		4,068
Derivative assets	669		803
Accrued income and other assets	3,568		3,864
Total assets	137,698		136,453
Accrued expense and other liabilities	1,803		1,796
Derivative liabilities	291		636
Total long-term debt	14,333		12,384
Total liabilities	122,673		121,213
SHAREHOLDERS' EQUITY	15,023		15,240
Total liabilities and equity	137,698		136,453
Key [Member]
Condensed Financial Statements, Captions [Line Items]
Cash and due from banks	2,257		3,286	$ 2,608	$ 2,207
Short-term investments	22		70
Securities available for sale	10		10
Other investments	29		25
Total loans	281		281
Total investment in subsidiaries	16,054		15,232
Goodwill	167		167
Corporate-owned life insurance	208		207
Derivative assets	29		53
Accrued income and other assets	353		451
Total assets	19,410		19,782
Accrued expense and other liabilities	466		553
Derivative liabilities	0		2
Long term debt due to Subsidiaries	480		492
Total long-term debt	3,921		3,987
Total liabilities	4,387		4,542
SHAREHOLDERS' EQUITY	15,023		15,240
Total liabilities and equity	19,410		19,782
Key [Member] | Banks [Member]
Condensed Financial Statements, Captions [Line Items]
Total loans	250		250
Investment in subsidiaries	15,169		14,564
Key [Member] | Nonbank Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Total loans	31		31
Investment in subsidiaries	885		668
Key [Member] | Unaffiliated Companies [Member]
Condensed Financial Statements, Captions [Line Items]
Total long-term debt	$ 3,441		$ 3,495