The Fair Value Option For Certain Loans - Narrative (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Eligible item for the fair value option	$ (34.8)	$ 8.8
Loans greater than 90 days past due or in nonaccrual status	30.9	14.7
Loans greater than 90 days past due or in nonaccrual status, unpaid principal balance	34.7	17.0
Long-term loans and written loan commitments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Eligible item for the fair value option	0.2	0.9	$ 1.6
Fair value, option, aggregate differences, long-term debt instruments	$ 900.0	$ 1,010.0