Bank Premises & Equipment - Bank Premises and Equipment (Parenthetical) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Bank Premises And Equipment [Line Items]
Land and improvements	$ 640	$ 586
Branches and undeveloped parcels of land
Bank Premises And Equipment [Line Items]
Land and improvements	$ 60	$ 55

Amount before accumulated deprecation and depletion of additions or improvements to real estate held for productive use. Examples include, but are not limited to, walkways, driveways, fences, and parking lots.

No definition available.

Name:	us-gaap_LandImprovements
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant