Mortgage Loans - Loan Commitments and Related Derivatives (Details) - USD ($) $ in Millions	Mar. 31, 2018	Sep. 30, 2017
Interest rate lock commitments [Member]
Derivative [Line Items]
Notional amounts of instruments	$ 719.9	$ 446.2
Best-efforts and mandatory commitments [Member]
Derivative [Line Items]
Notional amounts of instruments	58.6	26.9
Hedging Instruments related to IRLCs [Member]
Derivative [Line Items]
Notional amounts of instruments	$ 601.1	$ 389.3