Contingent Liabilities and Commitments with Off-Balance Sheet Risk - Summary of Financial Instruments with Off-Balance-Sheet Risk (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Commitments to extend credit	$ 807,651	$ 532,082
Unfunded limited partner equity commitment	10,914	10,148
Secured
Standby letters of credit and financial guarantees written	12,913	10,776
Unsecured
Standby letters of credit and financial guarantees written	$ 681	$ 554