Commitments, Contingencies and Guarantees - Commitments (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Loss Contingencies [Line Items]
Potential losses from standby letters of credit	$ 0	$ 0
Undisbursed loan commitments	492,400,000	571,000,000
Probable credit losses	1,500,000	1,200,000
Financial and Performance Standby Letters of Credit
Loss Contingencies [Line Items]
Letters of credit outstanding	189,700,000	187,600,000
Commitments to Extend Credit
Loss Contingencies [Line Items]
Loan commitments under financial guarantee	$ 3,030,000,000	$ 3,030,000,000