Significant Group Concentrations of Credit Risk Significant Group Concentrations of Credit Risk (Commercial Real Estate Exposure) (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		$ 50,369	$ 50,163
Real estate construction loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		3,455	3,077
Commercial mortgage loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		9,559	9,106
Total commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		13,014	12,183
Total unused commitments		3,557	3,146
Commercial Real Estate business line | Real estate construction loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	[1]	3,044	2,687
Commercial Real Estate business line | Commercial mortgage loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	[1]	2,176	1,743
Other business lines | Real estate construction loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	[2]	411	390
Other business lines | Commercial mortgage loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	[2]	$ 7,383	$ 7,363

[1] Primarily loans to real estate developers.

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