Borrowings (Assets and Liabilities in Unconsolidated VIEs) (Details) - Unconsolidated Variable Interest Entities (VIEs) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Debt securities
Variable Interest Entity [Line Items]
Total Assets	$ 4,218.9	$ 2,921.9
Maximum loss exposure	4,218.9	2,921.9
Debt securities | Agency securities
Variable Interest Entity [Line Items]
Total Assets	3,560.2	2,152.9
Debt securities | Non agency securities — Other servicer
Variable Interest Entity [Line Items]
Total Assets	658.7	769.0
Equity securities
Variable Interest Entity [Line Items]
Total Assets	23.8	11.4
Equity securities | Tax credit equity investments
Variable Interest Entity [Line Items]
Total Assets	220.2	167.7
Partnership Investment
Variable Interest Entity [Line Items]
Total Assets	244.0	179.1
Total Liabilities	104.8	62.3
Maximum loss exposure	244.0	179.1
Partnership Investment | Commitments to tax credit investments
Variable Interest Entity [Line Items]
Total Liabilities	$ 104.8	$ 62.3