Investment securities - Schedule of Amortized Cost and Fair Value of Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Amortized cost
Due in one year or less	$ 21,851	$ 35,486
Due in one to five years	54,847	24,278
Due in five to ten years	45,714	40,038
Due in over ten years	255,077	257,721
Amortized cost, sub-total	377,489	357,523
Total debt securities	1,673,798	1,137,848
Fair value
Due in one year or less	21,884	35,662
Due in one to five years	55,307	24,684
Due in five to ten years	46,975	41,332
Due in over ten years	269,737	275,798
Fair value, sub-total	393,903	377,476
Total debt securities	1,678,525	1,172,400
Mortgage-backed securities - residential
Amortized cost
Mortgage-backed securities	1,281,285	760,099
Total debt securities	1,281,285	760,099
Fair value
Mortgage-backed securities	1,269,372	773,336
Total debt securities	1,269,372	773,336
Mortgage-backed securities - commercial
Amortized cost
Mortgage-backed securities	15,024	20,226
Total debt securities	15,024	20,226
Fair value
Mortgage-backed securities	15,250	21,588
Total debt securities	$ 15,250	$ 21,588