Commitments and Contingencies (Residential Whole Loan Purchase Commitments) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017
Other Commitments [Line Items]
Other loans held at carrying value excluded	$ 8.6
Other residential whole loans
Other Commitments [Line Items]
Other loans held at carrying value excluded	6.8
Residential whole loans
Other Commitments [Line Items]
Excluded amount, unsettled purchases, residential whole loans, at fair value	$ 1.8	$ 120.4