Investment securities - Schedule of Amortized Cost and Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Amortized cost
Due in one year or less	$ 36,486	$ 35,486
Due in one to five years	14,008	24,278
Due in five to ten years	36,838	40,038
Due in over ten years	264,378	257,721
Amortized cost, sub-total	351,710	357,523
Total debt securities	1,205,933	1,137,848
Fair value
Due in one year or less	36,660	35,662
Due in one to five years	21,113	24,684
Due in five to ten years	38,377	41,332
Due in over ten years	269,685	275,798
Fair value, sub-total	365,835	377,476
Total debt securities	1,225,178	1,172,400
Mortgage-backed securities - residential
Amortized cost
Mortgage-backed securities	834,335	760,099
Total debt securities	834,335	760,099
Fair value
Mortgage-backed securities	838,708	773,336
Total debt securities	838,708	773,336
Mortgage-backed securities - commercial
Amortized cost
Mortgage-backed securities	19,888	20,226
Total debt securities	19,888	20,226
Fair value
Mortgage-backed securities	20,635	21,588
Total debt securities	$ 20,635	$ 21,588