Investment securities - Schedule of Amortized Cost and Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Amortized cost
Amortized cost, due in one year or less	$ 17,374	$ 1,148
Amortized costs, due in one to five years	33,022	11,553
Amortized cost, due in five to ten years	30,203	18,287
Amortized cost, due in over ten years	197,313	158,616
Amortized cost, total	277,912	189,604
Mortgage-backed securities - residential, amortized cost	439,843	487,101
Amortized cost	717,755	676,705
Fair value
Fair value, due in one year or less	17,473	1,152
Fair value, due in one to five years	33,515	11,676
Fair value, due in five to ten years	31,373	18,887
Fair value, due in over ten years	210,471	165,990
Fair value, total	292,832	197,705
Mortgage-backed securities - residential, fair value	454,606	490,676
Total debt securities, Fair value	$ 747,438	$ 688,381