Loans and Allowance for Loan Losses - Pre- and Post-Modification Outstanding Recorded Investment Amounts of Troubled Debt Restructurings (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Pre- and Post - modification recorded investments
Financing Receivable, Modifications, Recorded Investment		$ 606		$ 606		$ 577
Pre-Modification Outstanding Recorded Investment	[1]	64	$ 28	171	$ 196
Post-Modification Outstanding Recorded Investment	[2]	64	28	171	196
Commercial
Pre- and Post - modification recorded investments
Financing Receivable, Modifications, Recorded Investment		384		384		337
Pre-Modification Outstanding Recorded Investment	[1]	59	22	163	180
Post-Modification Outstanding Recorded Investment	[2]	59	22	163	180
Commercial | Commercial and industrial
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment		59	20	163	116
Post-Modification Outstanding Recorded Investment		59	20	163	116
Commercial | Commercial mortgage
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment			2		3
Post-Modification Outstanding Recorded Investment			2		3
Commercial | Construction
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment			0		61
Post-Modification Outstanding Recorded Investment			0		61
Consumer
Pre- and Post - modification recorded investments
Financing Receivable, Modifications, Recorded Investment		222		222		240
Pre-Modification Outstanding Recorded Investment	[1]	5	6	8	16
Post-Modification Outstanding Recorded Investment	[2]	5	6	8	16
Consumer | Residential mortgage
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment		4	6	6	14
Post-Modification Outstanding Recorded Investment		4	6	6	14
Consumer | Home equity and other consumer loans
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment			0		2
Post-Modification Outstanding Recorded Investment			$ 0		$ 2
Construction | Commercial Loans Portfolio Segment [Member]
Pre- and Post - modification recorded investments
Financing Receivable, Modifications, Recorded Investment		$ 78		$ 78		$ 128

[1] Represents the recorded investment in the loan immediately prior to the restructuring event.

[2] Represents the recorded investment in the loan immediately following the restructuring event. It includes the effect of paydowns that were required as part of the restructuring terms.