Significant Group Concentrations of Credit Risk Significant Group Concentrations of Credit Risk (Commercial Real Estate Exposure) (Details) - USD ($) $ in Millions		Dec. 31, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		$ 49,084	$ 48,593
Real estate construction loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		2,001	1,955
Commercial mortgage loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		8,977	8,604
Total commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		10,978	10,559
Total unused commitments		3,063	2,335
Commercial Real Estate business line | Real estate construction loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	[1]	1,681	1,606
Commercial Real Estate business line | Commercial mortgage loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	[1]	2,104	1,790
Other business lines | Real estate construction loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	[2]	320	349
Other business lines | Commercial mortgage loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	[2]	$ 6,873	$ 6,814

[1] Primarily loans to real estate developers.

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