Variable Interest Entities (Assets and Liabilities in Unconsolidated VIEs) (Details) - Unconsolidated Variable Interest Entities (VIEs) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Partnership Investment
Variable Interest Entity [Line Items]
Total Assets	$ 390.2	$ 361.1
Total Liabilities	119.1	120.1
Maximum loss exposure	390.2	361.1
Commitments to tax credit investments | Partnership Investment
Variable Interest Entity [Line Items]
Total Liabilities	119.1	120.1
Debt Securities
Variable Interest Entity [Line Items]
Total Assets	5,550.4	5,338.6
Maximum loss exposure	5,550.4	5,338.6
Debt Securities | Agency securities
Variable Interest Entity [Line Items]
Total Assets	5,550.4	5,338.6
Equity securities
Variable Interest Entity [Line Items]
Total Assets	102.9	84.0
Equity securities | Tax credit equity investments
Variable Interest Entity [Line Items]
Total Assets	$ 287.3	$ 277.1