Commitments, Contingencies and Guarantees - Commitments (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Loss Contingencies [Line Items]
Potential losses from standby letters of credit	$ 0
Undisbursed loan commitments	512,700,000	$ 493,300,000
Probable credit losses	1,700,000
Financial and Performance Standby Letters of Credit
Loss Contingencies [Line Items]
Letters of credit outstanding	125,700,000	119,000,000
Commitments to Extend Credit
Loss Contingencies [Line Items]
Loan commitments under financial guarantee	3,540,000,000	3,550,000,000
Performance Guarantee
Loss Contingencies [Line Items]
Recourse liability	100,000	100,000
SBA Guaranteed Notes | Performance Guarantee
Loss Contingencies [Line Items]
Recourse liability	$ 1,200,000	$ 1,200,000