The Fair Value Option For Certain Loans - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Eligible item for the fair value option	$ 10.1	$ 74.1
Loans greater than 90 days past due or in nonaccrual status	8.8	9.4
Unpaid principal balance greater than 90 days past due or in nonaccrual status	10.6	10.8
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	0.5		$ 0.2
Total unpaid principal balance of long-term loans	$ 1,070.0	$ 1,060.0