Investment securities - Schedule of Amortized Cost and Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Amortized cost
Amortized cost, due in one year or less	$ 5,292	$ 15,883
Amortized costs, due in one to five years	12,477	13,806
Amortized cost, due in five to ten years	15,412	18,539
Amortized cost, due in over ten years	141,334	99,151
Amortized cost, total	174,515	147,379
Mortgage-backed securities - residential, amortized cost	481,580	520,654
Amortized cost	656,095	668,033
Fair value
Fair value, due in one year or less	5,312	16,028
Fair value, due in one to five years	12,601	13,740
Fair value, due in five to ten years	15,978	18,387
Fair value, due in over ten years	149,340	98,963
Fair value, total	183,231	147,118
Mortgage-backed securities - residential, fair value	485,300	508,580
Total debt securities, Fair value	$ 668,531	$ 655,698