Commitments, Contingencies and Guarantees - Commitments (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Loss Contingencies [Line Items]
Potential losses from standby letters of credit	$ 0	$ 0
Undisbursed loan commitments	600,700,000	571,000,000
Probable credit losses	1,700,000	1,200,000
Financial and Performance Standby Letters of Credit
Loss Contingencies [Line Items]
Letters of credit outstanding	126,100,000	187,600,000
Commitments to Extend Credit
Loss Contingencies [Line Items]
Loan commitments under financial guarantee	3,100,000,000	3,030,000,000
Performance Guarantee
Loss Contingencies [Line Items]
Recourse liability	$ 100,000	$ 200,000