Related party transactions - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Unfunded commitments	$ 0	$ 2
Deposits from related parties	287,156	110,465
Aviation Time Sharing Agreement
Related Party Transaction [Line Items]
Payments to related party	208	176
Initial Public Offering
Related Party Transaction [Line Items]
Placement agent and other offering costs	671
Certain Executive Officers, Certain Management and Directors and Their Associates
Related Party Transaction [Line Items]
Unfunded commitments	15,000	4,672
Director
Related Party Transaction [Line Items]
Unamortized leasehold improvements	116		$ 137
Lease expense	516	504	$ 522
Amount of investment held by the company	$ 29	$ 200

For investments which are quantified by principal amount, the principle balance held at close of period.

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