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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	[1]	$ 28	$ 149	$ 196	$ 342
Post-Modification Outstanding Recorded Investment	[2]	28	149	196	341
Commercial
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	[1]	22	145	180	329
Post-Modification Outstanding Recorded Investment	[2]	22	145	180	328
Commercial | Commercial and industrial
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	[1]	20	144	116	320
Post-Modification Outstanding Recorded Investment	[2]	20	144	116	319
Commercial | Commercial mortgage
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	[1]	2	1	3	9
Post-Modification Outstanding Recorded Investment	[2]	2	1	3	9
Commercial | Construction
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	[1]	0		61
Post-Modification Outstanding Recorded Investment	[2]	0		61
Consumer
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	[1]	6	4	16	13
Post-Modification Outstanding Recorded Investment	[2]	6	4	16	13
Consumer | Residential mortgage
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	[1]	6	3	14	9
Post-Modification Outstanding Recorded Investment	[2]	6	3	14	9
Consumer | Home equity and other consumer loans
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	[1]	0	1	2	4
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 1	$ 2	$ 4

[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.