Significant Group Concentrations of Credit Risk Significant Group Concentrations of Credit Risk (Commercial Real Estate Exposure) (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		$ 49,173	$ 49,088
Real estate construction loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		2,961	2,869
Commercial mortgage loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		9,159	8,931
Total commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		12,120	11,800
Total unused commitments		3,018	3,046
Commercial Real Estate business line | Real estate construction loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	[1]	2,630	2,485
Commercial Real Estate business line | Commercial mortgage loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	[1]	1,831	2,018
Other business lines | Real estate construction loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	[2]	331	384
Other business lines | Commercial mortgage loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	[2]	$ 7,328	$ 6,913

[1] Primarily loans to real estate developers.

[2] Primarily loans secured by owner-occupied real estate.