Borrowings (Assets and Liabilities in Unconsolidated VIEs) (Details) - Unconsolidated VIEs - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt securities
Variable Interest Entity [Line Items]
Total Assets	$ 5,269.0	$ 2,921.9
Maximum loss exposure	5,269.0	2,921.9
Debt securities | Agency securities
Variable Interest Entity [Line Items]
Total Assets	4,950.2	2,152.9
Debt securities | Non agency securities — Other servicer
Variable Interest Entity [Line Items]
Total Assets	318.8	769.0
Equity securities
Variable Interest Entity [Line Items]
Total Assets	38.6	11.4
Equity securities | Tax credit equity investments
Variable Interest Entity [Line Items]
Total Assets	198.8	167.7
Partnership Investment
Variable Interest Entity [Line Items]
Total Assets	237.4	179.1
Total Liabilities	66.6	62.3
Maximum loss exposure	237.4	179.1
Partnership Investment | Commitments to tax credit investments
Variable Interest Entity [Line Items]
Total Liabilities	$ 66.6	$ 62.3