Commitments and Financial Instruments with Off-Balance-Sheet Risk - Changes in Allowance for Indemnifications Losses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in the allowance for indemnification losses
Provision for indemnifications	$ 186	$ 290	$ 274
Indemnification reserve for recourse provisions
Change in the allowance for indemnification losses
Allowance, beginning of period	2,303	2,363	2,089
Provision for indemnifications	186	290	274
Payments		(350)
Allowance, end of period	$ 2,489	$ 2,303	$ 2,363