Loans (Commercial Loans - By Risk Rating) (Parenthetical) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 38,530.4	$ 30,998.9	$ 31,247.0
PCI Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		1,277.6
Commercial Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	30,696.8	25,019.5
Commercial Loans | Non-Criticized Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		22,784.8
Commercial Loans | PCI Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		27.8
Commercial Loans | PCI Loans | Non-Criticized Loans | Real Estate Finance
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		20.7
Commercial Loans | PCI Loans | Criticized Loans | Real Estate Finance
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		$ 7.1
Commercial Loans | Other Assets
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrued interest receivable	$ 59.3