Investment securities - Schedule of Amortized Cost and Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Amortized cost
Amortized cost, due in one year or less	$ 17,075	$ 1,148
Amortized costs, due in one to five years	36,509	11,553
Amortized cost, due in five to ten years	27,580	18,287
Amortized cost, due in over ten years	173,357	158,616
Amortized cost, total	254,521	189,604
Mortgage-backed securities - residential, amortized cost	481,651	487,101
Amortized cost	736,172	676,705
Fair value
Fair value, due in one year or less	17,168	1,152
Fair value, due in one to five years	36,740	11,676
Fair value, due in five to ten years	28,112	18,887
Fair value, due in over ten years	182,539	165,990
Fair value, total	264,559	197,705
Mortgage-backed securities - residential, fair value	499,658	490,676
Total debt securities, Fair value	$ 764,217	$ 688,381