Commitments, Contingencies and Guarantees (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Undisbursed loan commitments	$ 571.0	$ 578.2
Allowance for credit losses on lending-related commitments	$ 1.2	1.3
Consumer Price Index increase	2.10%
Standby Letters of Credit
Loss Contingencies [Line Items]
Face amount of off-balance sheet liability	$ 187.6	118.9
Commitments to Extend Credit
Loss Contingencies [Line Items]
Face amount of off-balance sheet liability	3,030.0	2,700.0
Performance guarantee
Loss Contingencies [Line Items]
Outstanding balance of loans	13.3	16.9
Maximum potential amount of undiscounted future payments	12.8	16.1
Recourse liability	$ 0.2	$ 0.2