Investment securities - Schedule of Amortized Cost and Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Amortized cost
Amortized cost, due in one year or less	$ 6,436	$ 15,883
Amortized costs, due in one to five years	13,504	13,806
Amortized cost, due in five to ten years	15,948	18,539
Amortized cost, due in over ten years	115,566	99,151
Amortized cost, total	151,454	147,379
Mortgage-backed securities - residential, amortized cost	516,458	520,654
Amortized cost	667,912	668,033
Fair value
Fair value, due in one year or less	6,474	16,028
Fair value, due in one to five years	13,632	13,740
Fair value, due in five to ten years	16,404	18,387
Fair value, due in over ten years	121,200	98,963
Fair value, total	157,710	147,118
Mortgage-backed securities - residential, fair value	517,505	508,580
Total debt securities, Fair value	$ 675,215	$ 655,698