Loans (Schedule of Finance Receivables by Segment, Based on Obligor Location) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 28,505.3	$ 29,535.9	$ 29,897.0
Domestic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	26,591.2	27,414.3
Foreign
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,914.1	2,121.6
Commercial Banking
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	22,692.6	22,562.3	22,780.9
Commercial Banking | Domestic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	20,778.5	20,440.7
Commercial Banking | Foreign
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,914.1	2,121.6
Consumer Banking
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	5,812.7	6,973.6	$ 7,116.1
Consumer Banking | Domestic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	5,812.7	6,973.6
Consumer Banking | Foreign
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 0.0	$ 0.0