Fair Value (Quantitative Information about Level 3 Fair Value Measurements-Recurring) (Details) $ in Millions	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value - assets	$ 6,401.2	$ 6,249.7
Estimated fair value liabilities	(164.4)	(100.8)
Level 3 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value - assets	67.2	67.4
Estimated fair value liabilities	(71.9)	(69.6)
Level 3 | Derivative Assets — Non-Qualifying
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value - assets	1.5	$ 0.3
Level 3 | FDIC True-up Liability | Discount Rate | Discounted Cash Flow
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liability measurement input		2.2
Level 3 | FDIC True-up Liability | Weighted Avg. | Discount Rate | Discounted Cash Flow
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liability measurement input		2.2
Level 3 | FDIC True-up Liability | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value liabilities	(69.3)	$ (68.8)
Level 3 | Derivative Liabilities — Non-Qualifying
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value liabilities	$ (2.6)	$ (0.8)
Level 3 | Debt Securities — AFS | Minimum | Discount Rate | Discounted Cash Flow
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities — AFS measurement input	6.0	6.0
Level 3 | Debt Securities — AFS | Maximum | Discount Rate | Discounted Cash Flow
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities — AFS measurement input	6.2	6.2
Level 3 | Debt Securities — AFS | Weighted Avg. | Discount Rate | Discounted Cash Flow
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities — AFS measurement input	6.1	6.0
Level 3 | Debt Securities — AFS | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value - assets	$ 65.7	$ 67.1
Level 3 | Derivative Assets — Non-Qualifying | Minimum | Borrower Rate | Internal Valuation Model
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets — non qualifying, measurement input	2.6	2.8
Level 3 | Derivative Assets — Non-Qualifying | Maximum | Borrower Rate | Internal Valuation Model
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets — non qualifying, measurement input	4.8	5.0
Level 3 | Derivative Assets — Non-Qualifying | Weighted Avg. | Borrower Rate | Internal Valuation Model
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets — non qualifying, measurement input	3.2	3.6