Investment securities - Schedule of Amortized Cost and Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Amortized cost
Amortized cost, due in one year or less	$ 18,861	$ 1,148
Amortized costs, due in one to five years	29,353	11,553
Amortized cost, due in five to ten years	30,513	18,287
Amortized cost, due in over ten years	306,515	158,616
Amortized cost, total	385,242	189,604
Amortized cost	1,129,053	676,705
Fair value
Fair value, due in one year or less	19,009	1,152
Fair value, due in one to five years	29,811	11,676
Fair value, due in five to ten years	31,676	18,887
Fair value, due in over ten years	320,065	165,990
Fair value, total	400,561	197,705
Total debt securities, Fair value	1,160,521	688,381
Mortgage-backed securities - residential
Amortized cost
Amortized cost	723,333	474,144
Amortized cost	723,333	474,144
Fair value
Fair value	738,106	477,312
Total debt securities, Fair value	738,106	477,312
Mortgage-backed securities - commercial
Amortized cost
Amortized cost	20,478	12,957
Amortized cost	20,478	12,957
Fair value
Fair value	21,854	13,364
Total debt securities, Fair value	$ 21,854	$ 13,364