The Fair Value Option For Certain Loans - Narrative (Details) - USD ($) $ in Millions	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Eligible item for the fair value option	$ 74.1	$ 47.8
Loans greater than 90 days past due or in nonaccrual status	9.4	1.5
Loans greater than 90 days past due or in nonaccrual status, unpaid principal balance	10.8	1.5
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	1.6	0.2	$ 0.0
Fair value, option, aggregate differences, long-term debt instruments	$ 1,060.0	$ 1,070.0