Investment securities - Schedule of Amortized Cost and Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Amortized cost
Amortized cost, due in one year or less	$ 1,148	$ 15,883
Amortized costs, due in one to five years	11,553	13,806
Amortized cost, due in five to ten years	18,287	18,539
Amortized cost, due in over ten years	158,616	99,151
Amortized cost, total	189,604	147,379
Mortgage-backed securities - residential, amortized cost	487,101	520,654
Amortized cost	676,705	668,033
Fair value
Fair value, due in one year or less	1,152	16,028
Fair value, due in one to five years	11,676	13,740
Fair value, due in five to ten years	18,887	18,387
Fair value, due in over ten years	165,990	98,963
Fair value, total	197,705	147,118
Mortgage-backed securities - residential, fair value	490,676	508,580
Total debt securities, Fair value	$ 688,381	$ 655,698