Significant Group Concentrations of Credit Risk Significant Group Concentrations of Credit Risk (Commercial Real Estate Exposure) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 48,593		$ 45,470
Real estate construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,955		1,762
Commercial mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	8,604		8,787
Total commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	10,559		10,549
Total unused commitments	2,335		1,780
Commercial Real Estate business line | Real estate construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,606	[1]	1,447	[1]
Commercial Real Estate business line | Commercial mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,790	[1]	1,678	[1]
Other business lines | Real estate construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	349	[2]	315	[2]
Other business lines | Commercial mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 6,814	[2]	$ 7,109	[2]

[1] Primarily loans to real estate developers.

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